[Spruce]

in winter

1.starting temperature of your tank water is much lower before heating
2.you lose A LOT of heat from the hose with 100m on freezing ground

so it stands to reason to have it on full heat setting during the winter months

in summer

1.starting temperature of your tank water is much higher before heating
2.lots of heat is retained in the hose

so it stands to reason to turn it down in summer

i just leave mine on full heat setting all year round.....the mixer valves are rubbish anyway on these heaters IMO.....

Daz,
When you get your heater serviced please can you ask what terminal 7 in the plug does. It has a green wire.

From what I see from the wiring diagram and a note in the instruction manual, applying power (12v+) to that green cable changes the heat mode output on the Thermo Pro 90. In a vehicle application this cable would be connected to a positive on the ignition switch.

10.5
Setting control temperatures of Thermo Pro 90
When the signal “Engine on”/”Engine off” (Terminal D+) are connected to
the control unit connector X8, contact 7, different control thresholds are effective.

                                                           Nominal temperature on sensor     Regulating pause       Switch on again after regulating pause

“Engine on”                                                            65 °C                                                        75 °C                                               60 °C
“Engine off”                                                           80 °C                                                        90 °C                                                75 °C

If the terminal D+ signal is not applied, the temperatures are the same as those at “Engine off”.
The selected regulating pause temperature of the heater should be lower than the opening temperature of the radiator thermostat.

My heater gets up to 80 degrees plus.

Setting a diesel heater with a temperature/thermostatic control valve on the heat exchanger won't work with lower temps as the heater will switch off and then on to often during the day.
 A max of 65 degrees would probably work on an Ionic unit as they don't have a temperature control valve. The pump runs continually and the moment you stop water to the brush head with a tap/Univalve the hot water diverts to the tank via a pressure relief valve. (The pressure relief valve is set to 'blow off' at 65psi.)

With an Ionics unit you get what the units temperature is in the internal water jacket. If its 80 degrees then that's what the temp is of the water leaving your heater. If the switch is on then you get 65 degrees or less.

At least with a temperature control valve you can regulate that as DD says.

Each system has its pros and cons.

To late. Daz has had it serviced

[robert mitchell]

in winter

1.starting temperature of your tank water is much lower before heating
2.you lose A LOT of heat from the hose with 100m on freezing ground

so it stands to reason to have it on full heat setting during the winter months

in summer

1.starting temperature of your tank water is much higher before heating
2.lots of heat is retained in the hose

so it stands to reason to turn it down in summer

i just leave mine on full heat setting all year round.....the mixer valves are rubbish anyway on these heaters IMO.....

Daz,
When you get your heater serviced please can you ask what terminal 7 in the plug does. It has a green wire.

From what I see from the wiring diagram and a note in the instruction manual, applying power (12v+) to that green cable changes the heat mode output on the Thermo Pro 90. In a vehicle application this cable would be connected to a positive on the ignition switch.

10.5
Setting control temperatures of Thermo Pro 90
When the signal “Engine on”/”Engine off” (Terminal D+) are connected to
the control unit connector X8, contact 7, different control thresholds are effective.

                                                           Nominal temperature on sensor     Regulating pause       Switch on again after regulating pause

“Engine on”                                                            65 °C                                                        75 °C                                               60 °C
“Engine off”                                                           80 °C                                                        90 °C                                                75 °C

If the terminal D+ signal is not applied, the temperatures are the same as those at “Engine off”.
The selected regulating pause temperature of the heater should be lower than the opening temperature of the radiator thermostat.

My heater gets up to 80 degrees plus.

Setting a diesel heater with a temperature/thermostatic control valve on the heat exchanger won't work with lower temps as the heater will switch off and then on to often during the day.
 A max of 65 degrees would probably work on an Ionic unit as they don't have a temperature control valve. The pump runs continually and the moment you stop water to the brush head with a tap/Univalve the hot water diverts to the tank via a pressure relief valve. (The pressure relief valve is set to 'blow off' at 65psi.)

With an Ionics unit you get what the units temperature is in the internal water jacket. If its 80 degrees then that's what the temp is of the water leaving your heater. If the switch is on then you get 65 degrees or less.

At least with a temperature control valve you can regulate that as DD says.

Each system has its pros and cons.

Cheers spruce , i think i understand it now .

Currently trying to make sense of the wiring diagrams as i am going to fit the new thermo pro 90 in place of my old thermo 90 s .........but the wiring loom all has to be replaced as does the fuel pump as they are different on the new heater.

[robert mitchell]

You’ll still crack glass at 40 degrees if the glass is already damaged even new glass can have hairline cracks that start beneath the seals,the other cracking scenario is when the pane of glass has no flex within the unit say a painted in or a pane of glass that’s sealed in tight with putty.
The art to cleaning with hot water is to be quicker on the glass if you clean like some of these Idiots that stand there rinsing till the cows start moaning you’ll crack a lot of windows,with hot water you don’t need to constantly rinse that’s one of the benefits especially on regular stuff you are basically just colouring in windows in seconds and I mean seconds,some windows 3-4-5 windows on a bay are cleaned in and around 30-40 seconds if you know what you are doing.

You da man !

[NWH]

Yo da man Christ you’ll be banging on about coronavirus next like you’re other bully m8 on here get some hot water,scrubbing glass will be a distant memory.

[Spruce]

in winter

1.starting temperature of your tank water is much lower before heating
2.you lose A LOT of heat from the hose with 100m on freezing ground

so it stands to reason to have it on full heat setting during the winter months

in summer

1.starting temperature of your tank water is much higher before heating
2.lots of heat is retained in the hose

so it stands to reason to turn it down in summer

i just leave mine on full heat setting all year round.....the mixer valves are rubbish anyway on these heaters IMO.....

Daz,
When you get your heater serviced please can you ask what terminal 7 in the plug does. It has a green wire.

From what I see from the wiring diagram and a note in the instruction manual, applying power (12v+) to that green cable changes the heat mode output on the Thermo Pro 90. In a vehicle application this cable would be connected to a positive on the ignition switch.

10.5
Setting control temperatures of Thermo Pro 90
When the signal “Engine on”/”Engine off” (Terminal D+) are connected to
the control unit connector X8, contact 7, different control thresholds are effective.

                                                           Nominal temperature on sensor     Regulating pause       Switch on again after regulating pause

“Engine on”                                                            65 °C                                                        75 °C                                               60 °C
“Engine off”                                                           80 °C                                                        90 °C                                                75 °C

If the terminal D+ signal is not applied, the temperatures are the same as those at “Engine off”.
The selected regulating pause temperature of the heater should be lower than the opening temperature of the radiator thermostat.

My heater gets up to 80 degrees plus.

Setting a diesel heater with a temperature/thermostatic control valve on the heat exchanger won't work with lower temps as the heater will switch off and then on to often during the day.
 A max of 65 degrees would probably work on an Ionic unit as they don't have a temperature control valve. The pump runs continually and the moment you stop water to the brush head with a tap/Univalve the hot water diverts to the tank via a pressure relief valve. (The pressure relief valve is set to 'blow off' at 65psi.)

With an Ionics unit you get what the units temperature is in the internal water jacket. If its 80 degrees then that's what the temp is of the water leaving your heater. If the switch is on then you get 65 degrees or less.

At least with a temperature control valve you can regulate that as DD says.

Each system has its pros and cons.

Cheers spruce , i think i understand it now .

Currently trying to make sense of the wiring diagrams as i am going to fit the new thermo pro 90 in place of my old thermo 90 s .........but the wiring loom all has to be replaced as does the fuel pump as they are different on the new heater.

If you have a Thermo 90ST then the wiring looms are the same. You just have to replace the fuel pump and fuel pump supply cable. I've done that.

The Thermo 90S has the cables come from the bottom of the control panel and has a different connector, so yes, if that's the unit you have then you would need to replace the wiring harness as well.

[robert mitchell]

Yo da man Christ you’ll be banging on about coronavirus next like you’re other bully m8 on here get some hot water,scrubbing glass will be a distant memory.

I have had hot water for years you donut 🍩

[robert mitchell]

in winter

1.starting temperature of your tank water is much lower before heating
2.you lose A LOT of heat from the hose with 100m on freezing ground

so it stands to reason to have it on full heat setting during the winter months

in summer

1.starting temperature of your tank water is much higher before heating
2.lots of heat is retained in the hose

so it stands to reason to turn it down in summer

i just leave mine on full heat setting all year round.....the mixer valves are rubbish anyway on these heaters IMO.....

Daz,
When you get your heater serviced please can you ask what terminal 7 in the plug does. It has a green wire.

From what I see from the wiring diagram and a note in the instruction manual, applying power (12v+) to that green cable changes the heat mode output on the Thermo Pro 90. In a vehicle application this cable would be connected to a positive on the ignition switch.

10.5
Setting control temperatures of Thermo Pro 90
When the signal “Engine on”/”Engine off” (Terminal D+) are connected to
the control unit connector X8, contact 7, different control thresholds are effective.

                                                           Nominal temperature on sensor     Regulating pause       Switch on again after regulating pause

“Engine on”                                                            65 °C                                                        75 °C                                               60 °C
“Engine off”                                                           80 °C                                                        90 °C                                                75 °C

If the terminal D+ signal is not applied, the temperatures are the same as those at “Engine off”.
The selected regulating pause temperature of the heater should be lower than the opening temperature of the radiator thermostat.

My heater gets up to 80 degrees plus.

Setting a diesel heater with a temperature/thermostatic control valve on the heat exchanger won't work with lower temps as the heater will switch off and then on to often during the day.
 A max of 65 degrees would probably work on an Ionic unit as they don't have a temperature control valve. The pump runs continually and the moment you stop water to the brush head with a tap/Univalve the hot water diverts to the tank via a pressure relief valve. (The pressure relief valve is set to 'blow off' at 65psi.)

With an Ionics unit you get what the units temperature is in the internal water jacket. If its 80 degrees then that's what the temp is of the water leaving your heater. If the switch is on then you get 65 degrees or less.

At least with a temperature control valve you can regulate that as DD says.

Each system has its pros and cons.

Cheers spruce , i think i understand it now .

Currently trying to make sense of the wiring diagrams as i am going to fit the new thermo pro 90 in place of my old thermo 90 s .........but the wiring loom all has to be replaced as does the fuel pump as they are different on the new heater.

If you have a Thermo 90ST then the wiring looms are the same. You just have to replace the fuel pump and fuel pump supply cable. I've done that.

The Thermo 90S has the cables come from the bottom of the control panel and has a different connector, so yes, if that's the unit you have then you would need to replace the wiring harness as well.

Hi Spruce ,

No , I have the newest version which is the thermo 90 Pro , which is different again , although on first inspection the fuel pump wiring plugs look identical , the rest of the loom is very different though !

From what I have gathered so far I think I only have to connect the 12 pin plug to the battery ( through a fuse of course ) , plug in the fuel wiring and plug the on / off switch into the loom ........unless I’m missing something?

Obviously will change the exhaust and inlet and the fuel pump itself .

[Spruce]

in winter

1.starting temperature of your tank water is much lower before heating
2.you lose A LOT of heat from the hose with 100m on freezing ground

so it stands to reason to have it on full heat setting during the winter months

in summer

1.starting temperature of your tank water is much higher before heating
2.lots of heat is retained in the hose

so it stands to reason to turn it down in summer

i just leave mine on full heat setting all year round.....the mixer valves are rubbish anyway on these heaters IMO.....

Daz,
When you get your heater serviced please can you ask what terminal 7 in the plug does. It has a green wire.

From what I see from the wiring diagram and a note in the instruction manual, applying power (12v+) to that green cable changes the heat mode output on the Thermo Pro 90. In a vehicle application this cable would be connected to a positive on the ignition switch.

10.5
Setting control temperatures of Thermo Pro 90
When the signal “Engine on”/”Engine off” (Terminal D+) are connected to
the control unit connector X8, contact 7, different control thresholds are effective.

                                                           Nominal temperature on sensor     Regulating pause       Switch on again after regulating pause

“Engine on”                                                            65 °C                                                        75 °C                                               60 °C
“Engine off”                                                           80 °C                                                        90 °C                                                75 °C

If the terminal D+ signal is not applied, the temperatures are the same as those at “Engine off”.
The selected regulating pause temperature of the heater should be lower than the opening temperature of the radiator thermostat.

My heater gets up to 80 degrees plus.

Setting a diesel heater with a temperature/thermostatic control valve on the heat exchanger won't work with lower temps as the heater will switch off and then on to often during the day.
 A max of 65 degrees would probably work on an Ionic unit as they don't have a temperature control valve. The pump runs continually and the moment you stop water to the brush head with a tap/Univalve the hot water diverts to the tank via a pressure relief valve. (The pressure relief valve is set to 'blow off' at 65psi.)

With an Ionics unit you get what the units temperature is in the internal water jacket. If its 80 degrees then that's what the temp is of the water leaving your heater. If the switch is on then you get 65 degrees or less.

At least with a temperature control valve you can regulate that as DD says.

Each system has its pros and cons.

Cheers spruce , i think i understand it now .

Currently trying to make sense of the wiring diagrams as i am going to fit the new thermo pro 90 in place of my old thermo 90 s .........but the wiring loom all has to be replaced as does the fuel pump as they are different on the new heater.

If you have a Thermo 90ST then the wiring looms are the same. You just have to replace the fuel pump and fuel pump supply cable. I've done that.

The Thermo 90S has the cables come from the bottom of the control panel and has a different connector, so yes, if that's the unit you have then you would need to replace the wiring harness as well.

Hi Spruce ,

No , I have the newest version which is the thermo 90 Pro , which is different again , although on first inspection the fuel pump wiring plugs look identical , the rest of the loom is very different though !

From what I have gathered so far I think I only have to connect the 12 pin plug to the battery ( through a fuse of course ) , plug in the fuel wiring and plug the on / off switch into the loom ........unless I’m missing something?

Obviously will change the exhaust and inlet and the fuel pump itself .

I was able to replace a Thermo 90ST with a Thermo Pro 90 without changing the wiring harness. They tell you to change the fuel pump to a DP42 which has a different connector to the older DP30 pump. It also seems to be much quieter when running.  I bought a new harness with cable that had the correct plug for the new pump and purchased a separate plug for the control panel and fitted it on. I had to buy the pump from Turkey as no one had stock of them in the UK.

The Pro 90 does seem to have a different diagnostic plug but that won't stop the heater from working. For me the red cable was batt positive, brown batt negative, black positive from on/off switch and white the led info light. There are a couple of other wires that aren't used, grey, blue, green/white and violet.  They are used for other accessories when used as a engine preheater for example. (One of them will activate the fan inside the cabin when the coolant temperature reaches 30 degrees. Warm air is then directed to the windscreen if set up that way to defrost it.)

Depending on the application there are different wiring harness with wires and connectors for a timer and or thermostat. The Webasto supplier who works out of Turkey will add additional features to the loom to your requirements.

If you are going from a Thermo 90S to a Thermo Pro 90 then the wiring harness is different.

[robert mitchell]

in winter

1.starting temperature of your tank water is much lower before heating
2.you lose A LOT of heat from the hose with 100m on freezing ground

so it stands to reason to have it on full heat setting during the winter months

in summer

1.starting temperature of your tank water is much higher before heating
2.lots of heat is retained in the hose

so it stands to reason to turn it down in summer

i just leave mine on full heat setting all year round.....the mixer valves are rubbish anyway on these heaters IMO.....

Daz,
When you get your heater serviced please can you ask what terminal 7 in the plug does. It has a green wire.

From what I see from the wiring diagram and a note in the instruction manual, applying power (12v+) to that green cable changes the heat mode output on the Thermo Pro 90. In a vehicle application this cable would be connected to a positive on the ignition switch.

10.5
Setting control temperatures of Thermo Pro 90
When the signal “Engine on”/”Engine off” (Terminal D+) are connected to
the control unit connector X8, contact 7, different control thresholds are effective.

                                                           Nominal temperature on sensor     Regulating pause       Switch on again after regulating pause

“Engine on”                                                            65 °C                                                        75 °C                                               60 °C
“Engine off”                                                           80 °C                                                        90 °C                                                75 °C

If the terminal D+ signal is not applied, the temperatures are the same as those at “Engine off”.
The selected regulating pause temperature of the heater should be lower than the opening temperature of the radiator thermostat.

My heater gets up to 80 degrees plus.

Setting a diesel heater with a temperature/thermostatic control valve on the heat exchanger won't work with lower temps as the heater will switch off and then on to often during the day.
 A max of 65 degrees would probably work on an Ionic unit as they don't have a temperature control valve. The pump runs continually and the moment you stop water to the brush head with a tap/Univalve the hot water diverts to the tank via a pressure relief valve. (The pressure relief valve is set to 'blow off' at 65psi.)

With an Ionics unit you get what the units temperature is in the internal water jacket. If its 80 degrees then that's what the temp is of the water leaving your heater. If the switch is on then you get 65 degrees or less.

At least with a temperature control valve you can regulate that as DD says.

Each system has its pros and cons.

Cheers spruce , i think i understand it now .

Currently trying to make sense of the wiring diagrams as i am going to fit the new thermo pro 90 in place of my old thermo 90 s .........but the wiring loom all has to be replaced as does the fuel pump as they are different on the new heater.

If you have a Thermo 90ST then the wiring looms are the same. You just have to replace the fuel pump and fuel pump supply cable. I've done that.

The Thermo 90S has the cables come from the bottom of the control panel and has a different connector, so yes, if that's the unit you have then you would need to replace the wiring harness as well.

Hi Spruce ,

No , I have the newest version which is the thermo 90 Pro , which is different again , although on first inspection the fuel pump wiring plugs look identical , the rest of the loom is very different though !

From what I have gathered so far I think I only have to connect the 12 pin plug to the battery ( through a fuse of course ) , plug in the fuel wiring and plug the on / off switch into the loom ........unless I’m missing something?

Obviously will change the exhaust and inlet and the fuel pump itself .

I was able to replace a Thermo 90ST with a Thermo Pro 90 without changing the wiring harness. They tell you to change the fuel pump to a DP42 which has a different connector to the older DP30 pump. It also seems to be much quieter when running.  I bought a new harness with cable that had the correct plug for the new pump and purchased a separate plug for the control panel and fitted it on. I had to buy the pump from Turkey as no one had stock of them in the UK.

The Pro 90 does seem to have a different diagnostic plug but that won't stop the heater from working. For me the red cable was batt positive, brown batt negative, black positive from on/off switch and white the led info light. There are a couple of other wires that aren't used, grey, blue, green/white and violet.  They are used for other accessories when used as a engine preheater for example. (One of them will activate the fan inside the cabin when the coolant temperature reaches 30 degrees. Warm air is then directed to the windscreen if set up that way to defrost it.)

Depending on the application there are different wiring harness with wires and connectors for a timer and or thermostat. The Webasto supplier who works out of Turkey will add additional features to the loom to your requirements.

If you are going from a Thermo Pro 90 to a Thermo 90S then the wiring harness is different.

Cheers spruce ,

You were right about the fuel pump wiring , different plug at the pump end so replaced the loom  from pump to heater .

My Loom for the heater itself has just one 12 pin plug , a small four pin plug for a switch ( plug and play ) and another small four pin plug ( not sure what it does , maybe diagnostic) .

My switch hasn’t arrived yet , do you know if I can bridge 2 of the wires on the switch harness plug to test it ?

[robert mitchell]

My old one was a thermo 90 s not st .

[Spruce]

in winter

1.starting temperature of your tank water is much lower before heating
2.you lose A LOT of heat from the hose with 100m on freezing ground

so it stands to reason to have it on full heat setting during the winter months

in summer

1.starting temperature of your tank water is much higher before heating
2.lots of heat is retained in the hose

so it stands to reason to turn it down in summer

i just leave mine on full heat setting all year round.....the mixer valves are rubbish anyway on these heaters IMO.....

Daz,
When you get your heater serviced please can you ask what terminal 7 in the plug does. It has a green wire.

From what I see from the wiring diagram and a note in the instruction manual, applying power (12v+) to that green cable changes the heat mode output on the Thermo Pro 90. In a vehicle application this cable would be connected to a positive on the ignition switch.

10.5
Setting control temperatures of Thermo Pro 90
When the signal “Engine on”/”Engine off” (Terminal D+) are connected to
the control unit connector X8, contact 7, different control thresholds are effective.

                                                           Nominal temperature on sensor     Regulating pause       Switch on again after regulating pause

“Engine on”                                                            65 °C                                                        75 °C                                               60 °C
“Engine off”                                                           80 °C                                                        90 °C                                                75 °C

If the terminal D+ signal is not applied, the temperatures are the same as those at “Engine off”.
The selected regulating pause temperature of the heater should be lower than the opening temperature of the radiator thermostat.

My heater gets up to 80 degrees plus.

Setting a diesel heater with a temperature/thermostatic control valve on the heat exchanger won't work with lower temps as the heater will switch off and then on to often during the day.
 A max of 65 degrees would probably work on an Ionic unit as they don't have a temperature control valve. The pump runs continually and the moment you stop water to the brush head with a tap/Univalve the hot water diverts to the tank via a pressure relief valve. (The pressure relief valve is set to 'blow off' at 65psi.)

With an Ionics unit you get what the units temperature is in the internal water jacket. If its 80 degrees then that's what the temp is of the water leaving your heater. If the switch is on then you get 65 degrees or less.

At least with a temperature control valve you can regulate that as DD says.

Each system has its pros and cons.

Cheers spruce , i think i understand it now .

Currently trying to make sense of the wiring diagrams as i am going to fit the new thermo pro 90 in place of my old thermo 90 s .........but the wiring loom all has to be replaced as does the fuel pump as they are different on the new heater.

If you have a Thermo 90ST then the wiring looms are the same. You just have to replace the fuel pump and fuel pump supply cable. I've done that.

The Thermo 90S has the cables come from the bottom of the control panel and has a different connector, so yes, if that's the unit you have then you would need to replace the wiring harness as well.

Hi Spruce ,

No , I have the newest version which is the thermo 90 Pro , which is different again , although on first inspection the fuel pump wiring plugs look identical , the rest of the loom is very different though !

From what I have gathered so far I think I only have to connect the 12 pin plug to the battery ( through a fuse of course ) , plug in the fuel wiring and plug the on / off switch into the loom ........unless I’m missing something?

Obviously will change the exhaust and inlet and the fuel pump itself .

I was able to replace a Thermo 90ST with a Thermo Pro 90 without changing the wiring harness. They tell you to change the fuel pump to a DP42 which has a different connector to the older DP30 pump. It also seems to be much quieter when running.  I bought a new harness with cable that had the correct plug for the new pump and purchased a separate plug for the control panel and fitted it on. I had to buy the pump from Turkey as no one had stock of them in the UK.

The Pro 90 does seem to have a different diagnostic plug but that won't stop the heater from working. For me the red cable was batt positive, brown batt negative, black positive from on/off switch and white the led info light. There are a couple of other wires that aren't used, grey, blue, green/white and violet.  They are used for other accessories when used as a engine preheater for example. (One of them will activate the fan inside the cabin when the coolant temperature reaches 30 degrees. Warm air is then directed to the windscreen if set up that way to defrost it.)

Depending on the application there are different wiring harness with wires and connectors for a timer and or thermostat. The Webasto supplier who works out of Turkey will add additional features to the loom to your requirements.

If you are going from a Thermo Pro 90 to a Thermo 90S then the wiring harness is different.

Cheers spruce ,

You were right about the fuel pump wiring , different plug at the pump end so replaced the loom  from pump to heater .

My Loom for the heater itself has just one 12 pin plug , a small four pin plug for a switch ( plug and play ) and another small four pin plug ( not sure what it does , maybe diagnostic) .

My switch hasn’t arrived yet , do you know if I can bridge 2 of the wires on the switch harness plug to test it ?

I honestly don't know how they wire different plugs up.
The best thing is to wait intil it arrives and then see what you can from the different wiring diagrams for that heater.

As I've stated earlier, all I needed to get the heater in operation was those four wires, red, brown,  black and white.  If you have a Frostsat then the black wire connects to the blue from the frostat. White goes to the LED which lights up on starting/running and flashes up any fault codes. The other side of the LED goes to earth. Red is positive from the battery and brown negative.

There are some controllers that are timers as well. But I believe these wouldn't work for our application.

[robert mitchell]

Well that didn’t work , something else must need connecting other than power and the switch ......can’t for the life of me work out what though !

[robert mitchell]

Have had this up and running for a few months now , but the water now only gets to about 50c before hose ( just after heat exchanger ) where it used to get 80c with the old 90s .

Any ideas ?

[NWH]

New heat exchangers or burner then I’d do exchangers first though

[robert mitchell]

New heat exchangers or burner then I’d do exchangers first though

Don’t be daft , it was brand new a few months ago .

[NWH]

It hasn’t got flicked into winter mode has it.

[Spruce]

Have had this up and running for a few months now , but the water now only gets to about 50c before hose ( just after heat exchanger ) where it used to get 80c with the old 90s .

Any ideas ?

It would seem that the issue is in the heater itself. I would suggest a Webasto agent put it on his computer and try to diagnose the fault.
There is a water temperature sensor on the heater outlet which could be faulty.

There is apparently a low temperature Thermo Pro 90 12v reference number 9030723C. I can't find any specs on it.

[Splash & dash]

I think Nigel might be right the switch has been hit and put it into winter mode that reduces the water temperature , it’s easy dine it happens to ours now and again when switching on  the fill valve .

[NWH]

That’s what’s happened it’s in winter mode

[Spruce]

I think Nigel might be right the switch has been hit and put it into winter mode that reduces the water temperature , it’s easy dine it happens to ours now and again when switching on  the fill valve .

If it is the switch then there is also the possibility that the switch is faulty. If switching the switch doesn't do anything, then I would try removing one of the wires from the switch and see what happens.

I'm presuming the wire would be positive so would need to be made safe when disconnecting it.