Hi folks, thanks for the join.

I need some help on HVAC electrics. I will be installing an eberspaecher water heater to preheat the engine and cabin. If you haven't heard of them, there worth looking into if you live somewhere especially chilly in the winter.

It's simply a small water boiler which draws fuel from the car's existing tank, and brings the temperature up to around 170f (80c) in minutes. The plumbing is straightforward, just a splice into the water pipe between the engine and the cabin heat exchanger. When the water temp is raised the heater's ECU sends an output to trigger the cabin fan.

My question relates to the cabin fan control. I have the blue backlit dual temp controller. My understanding is that the HVAC system is engaged when a relay closes at position ON at the ignition. I've read on another post here that the fan speed is controlled by a power MOSFET, which regulates current. However I saw a schematic somewhere showing there may be a feedback wire from the motor, it wasn't clear where the wire terminated, I'm assuming it's related to recording a fault code should the motor fail in some way.

Options

Do I set the HVAC controls manually the night before. Setting the fan to medium and blend doors to screen. The eberspaecher trigger would simply need to engage the HVAC relay.

Or. Insert an additional relay which completely bypasses the HVAC system, at the same time connects +12 and grd to the correct polarities of the motor.

I have some background in electronics, so it's not an obstacle to wire some replays and fuses using the correct cable gauge and tidy heat shrinking.

I'd be grateful for your thoughts.

Many thanks.

 

I have a .jpg of the schematic which shows feedback from the blower to the ECU, but I don't know how to upload it so you can see.