Eberspacher wiring using the 'external' thermostat

[pcatterall]

Eberspacher heaters and particularly the D2 and D4 Airtronic have been a bit of a retirement hobby for me.
I have very little experience, though, in setting them up with an external thermostat.
I am often asked about it though and having found an Eberspacher external thermostat decided to investigate.
In the 'normal' set up the red/slate lead provides the positive heat setting connection and the brown/white lead is the return for that.

The external thermostat uses the Eberspachers grey lead and on the diagram I have uses a spur from the brown white lead for the return.
The diagram does not indicate if the red/slate remains in place though I guess it does in order for the heat setting control to function?
I guess ( again) that the ECU senses the connection of the grey lead and starts to use the external thermostat rather than the 'internal' one ?
Can anyone offer an experienced based view on this before I start to test my theory?

During my examination of some circuit diagrams in respect of the above I came across one showing a switch between the red/slate and the brown/white leads and this was called 'fan only switch' This was interesting on 2 counts..... One was that I suppose we could wire in a switch which would just circulate air ( cool air if the air inlet duct was in a cool area).
Two because I have had several ECU failures where the ECU has reverted to a 'fan only' mode, in these situations I found that I could get the heater to start and heat by removing the red/slate and the brown/white leads from the circuit, there was of course then no control on the heat level but I now wonder if an external thermostat would allow these otherwise good ECUs to work?

Possibly some of you have had similar problems and can contribute some further info/

 

[David2452]

If it were a Webasto / Mikuni / Wallas I could answer definitively, but, I do know the Eberspacher marine kits with the right loom are simply plug and play for the remote sensor with no mods required to the loom so you are probably on the right track.

 

[Minchsailor]

I regret that I have had to spend rather longer than I would have wished studying the EberD4 wiring diagrams.

The 'room' thermostat sensor is a thermistor (a KTY81/210) with a resistance at 0C of 1630 +/-27 ohms, and at 100C a resistance of 3392 +/-74 ohms.

My understanding (willingly to be corrected) is that if the control unit 'sees' a room thermostat it uses this signal in preference to the inlet air temperature sensor.

The room sensor is connected to the grey and brown/white. Polarity is not important.

Eber circuit diagrams are not the most intuitive in the world and on mine there could be some confusion over grey or grey/red.

At the end of last season my heater worked, but would not control on the room thermostat. I've got some new thermistors from Farnell and will be trying them out shortly (boat is nowhere near where I live).

Nowhere on the documentation I have can I find a reference to a 'fan only switch'. I have the model 701 CU which can go to fan only, but not sure how....