Webasto Airtop 3500 too HIGH voltage?

[Blue Ocean]

I have two Webasto Airtops on my boat - a 5000 and a 3500.
Both work perfectly as long as my engine is not running.
However, the 3500 will stop with an error code shortly after I start my engine and blink the error code for 'voltage too low or too high'.

Given the issue only occurs when the engine is running I assume it's a high voltage error but I can't figure out what to do about it.
The other heater never has this issue. Can it be that the control board of the 3500 is too sensitive?

My alternator will output 14.2V which is obviously higher than the battery voltage. Both heaters are placed far aft on opposite sides. The 3500 will have a somewhat longer wirerun and if anything it should see lower voltage than the other heater.

Ideas are welcome........

[fourlyons]

The problem is probably LOW voltage rather than high. When you start the engine the voltage in the system can drop pretty low due to the starter draw. A bigger starting battery bank and/or larger supply cables to the heater would help if that is causing the code. Checking the voltage at the heater while starting the engine should answer the question. You want a multimeter with a recording function, the drop may be too short lived to see visually.

[Blue Ocean]

Well, the issue also occurs if I turn on the heater when the engine is already running. Moreover, when the heater is running and I start the engine the error doesn't seem to happen until I put some revs on the engine - i.e. presumably when the alternator kicks in. That's why I tend to think the heater somehow thinks the voltage is too high.

The voltage appears normal even when the alternator is charging but I don't have a multimeter capable of recording any spikes. I might try to put a DC/DC converter on the power line to the heater to see if that cures the issue. I have a spare one of those.