Cayenne 955 Seat Heat debugging

[944sarethebest]

I am starting to debug the Cayenne seat heating system and I'm starting a post with some findings as there was limited information in forum searches and this could be a helpful comparison as we keep these running.

 

The seat heater is actual composed of 4 seat heating elements: a pad that heats the bottom (center), a coil that heats in the bottom bolster left and right, a pad that heats in the backrest (center), and an element that heats the side bolster.  The center seat pad also contains a thermistor (a resistor whose resistance changes based on the temperature), and it's role is to prevent overheating.

 

A common issue is that the seats begin to heat and then turn off after 2-6 minutes.  It is most common in the front seats and normally affects only one seat.  Apparently, I have read that this is likely due to either the thermistor or its thin leads failing.  Since the thermistor is attached to the seat cushion, they'll typically replace the whole seat, which is great if you're under warranty, not so much if you are not (>$1000).

 

I was working on a 2005 CTT that started to have this issue and I started debugging.  The connector that attaches to the bottom seat cushion heater pad includes 4 leads.  2 go to the seat heater elements, and 2 to the thermistor.   

heater power: brown/black and green/white
thermistor: brown/white and black/white

 

The heater is separate from the seat motion controls, and there is also a 2-wire connector that goes to the backrest. I'm attaching a diagram illustrating the circuit.

 

I located the 4pin connector to the bottom seat cushion and pulled it out, and checked resistances. The seat cushion heater elements measure at 2.5Ohms (this would be about 60W heater, seems reasonable) and the thermistor measures 6.8k ohms on an 84F day.  Assuming this is a standard 12k NTC thermistor, this indicates about 90-91F.    Since the thermistor in this case has not completely failed, it seems like it is not a simple case of the leads failing and I'll have more debugging to find more conclusive results.

One next idea is to temporarily bypass the thermistor with a 10-12k resistor. I wouldn't want to do this long term as it could cause problems, but I'll update as I go along.

 

2005 Cayenne Turbo

Edited October 7, 2020 by 944sarethebest