Webasto Thermotop C glow plug resistance? - flame detect?

[wetfoot]

My TTC, bought new in November, cuts out after a while claiming a flame failure (which I don't believe).

Using the diagnostic software I can see that during a 20 minute combustion at full load, the resistance of the flame detect / glow plug fluctuates between about 0.513 and 0.519 ohm with no issues at all but then suddenly the flame detect flag drops from 1 to 0 (with the resistance still around 0.519 ohm) and the thing shuts down as it thinks there is no flame.

 

U tube videos showing TTC heaters being tested seem to show flame detect resistances nearer 0.865 ohm at full load, not just over 0.5 ohm like mine.

see

https://www.youtube.com/watch?v=lWL9ipWpVwA

https://www.youtube.com/watch?v=3gRwv_2nDNs

 

Is it likely that this almost new unit is faulty, with the flame detect resistance being too close to the cut off point? 

 

Anyone know what the 'correct' resistance should be during full load combustion when in flame detect mode?

 

I've heard of instances with other webasto hearers when the tolerance of the 1.0 ohm flame detect shunt resistor is a but too wide, being closer to 0.95 ohm, which in turn causes an error in the detect reading. Is this likely with a TTC? 

[Boredrider]

6.3.1 Components Testing
6.3.1.1 Glow Plug/Flame Sensor Resistance Check
When testing the glow plug/flame sensor with a digital
multimeter, the following readings should be obtained:
Resistance at 25 °C: 0.324 ... 0.360 Ohms
Test current: < 5 mA

Edited February 3, 2019 by Boredrider
speelchucker

[wetfoot]

2 hours ago, Boredrider said:

6.3.1 Components Testing
6.3.1.1 Glow Plug/Flame Sensor Resistance Check
When testing the glow plug/flame sensor with a digital
multimeter, the following readings should be obtained:
Resistance at 25 °C: 0.324 ... 0.360 Ohms
Test current: < 5 mA

Yes, I've read that but it says 0.324 ... 0.360 Ohms at 25 deg C, i.e. when it is cold.

 

The resistance of the glow plug / flame detect electrode increases with temperature, being around 1.6 ohm when glowing is turned on because the electrical heating makes it almost white hot.

When it is in flame detect mode the temperature will be lower than white hot but obviously much higher than 25 deg C, hence the resistance will be between 0.360 ohm (when cold) and 1.50 ohm (when white hot).

 

My question is, what is the normal resistance to be expected when its running hot at full load and detecting a flame - and consequently, at what lower resistance (due to lower temperature) does it decide that there is no flame?  I've heard 0.6 ohm mentioned as the flame detect threshold but cannot confirm this.  

 

Hoping someone here can give me the correct figure