Webasto hydronic issue

[Tricky]

Hi there!

So we have a Webasto S90 hydronic heater onboard that hasn't really work since we bought the boat. I've been working on it a little while and have gotten it from not starting to starting and running on the first go.
So the issue now is the heater turns on and heats up (I have a temp gauge on the coolant hose) once it gets to about ~120F. It shuts down and I get an error. On this model it's the Error 05 Flame detector interruption or flame detector short-circuit. I have replaced the flame detector sensor, so I don't think it's that, plus it fires up no problem, which it wouldn't do if the flame wasn't detected.
Does anyone have a better idea of what triggers that error or what else I can try?

Thanks!
Patrick

 

[mz4wheeler]

Sounds like a heat induced failure, which could be a number of things.
1) Circuit board: Once the board heats up, a bad solder joint fails.
2) Water pump: Windings short/open with heat. Or, doesn't pump, unit heats up, shuts down.

My guess would be the water pump. It is the most vulnerable component. You can test it (cold/hot) by jumpering the leads to 12VDC. You can also check it's resistance.

http://www.suremarine.com/manuals/webasto-repair-manuals/thermo90-repair.pdf

You can also get it bench tested.