I have been dealing with an intermittent problem on the HydroHot diesel burner for past couple years.
The problem is the Diesel Burner Light going out and burner shutting down indicating a problem with the Webasto Burner Control Unit. Not to be confused with the HydroHot/AquaHot main control panel.
HydroHot/AquaHot Control Panel controls the overall operation of the complete system
The Webasto Control unit is what controls only the diesel burner. This maintains the temperature of the boiler fluid when diesel burner is on.
The intermittent problem was not a big deal in warm temps when we just switched on diesel burner when taking showers. The burner would work fine most of the time for several hours, but would never stay on all night. With us using the coach in colder temps and not putting in storage till later I really wanted to solve the problem. Given the nature of the problem it was hard to monitor and collect good information.
I happened to have a small data collection device that I was able to connect and program to collect data to my MQTT Servers. The device was connected to the diesel burner switch light (B3) and the Fuel Solenoid Circuit (C4). This allowed me to capture status of the light and when the burner had fuel flowing.
What the data showed me was the Control Unit would fault out when the burner was on but that was about it. Confirmed the random nature of the failure. Green Lines are Fuel Solenoid, 0=Off 1=On, Yellow Lines are the Diesel Burner Light, 2=On (Ignore some of the random spikes as I was altering program.)
With this information I looked closer at the burner schematic.
What this showed was the fault was limited to a small portion of the circuit between C4 and C6 on the Control Unit Plug. In this circuit is an Over Temp tStat(S4), Fuel Solenoid(L1) and Flame Sensor (R1).
Flame Sensor was replaced along with Control Unit within past 18 months so ruling those out for now.
Ignition Coil was tested 100x manual firings with zero issues. Fuel delivery tested, nozzle replaced along with pressure check and burns clean, no smoke.
Over Temp seemed like a logical place to explore deeper. These are supposed to trip and require a manual reset with button on end. Some research indicated these are a known failure point.
With the above information I disconnected OT sensor and placed jumper in its place. I also connected the OT sensor to the data collection device. Disclaimer and Warning: Bypassing Over Temp sensor is for testing only and should be watched closely as this is a safety device.
HydroHot has now ran for over 24 hours without faulting! And the data is showing that the OT sensor seems to have an intermittent issue as it is tripping for brief periods but resetting.
Cyan Lines are the Over Temp Sensor Up is tripped.
With this new found information a replacement Over Temp Thermostat (ELE-HLT-322) was ordered. Once in place I will repeat test and collect data.
I could have bypassed some of the data collection and just replaced parts till it worked but I wanted to dig deeper and actually replace parts that showed to be defective via testing.