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CCU repair - temperature sensor / thermistor

N

nfearn

Member
One of the items which can fail on the CCU is the cabin temperature sensor - a thermistor device.

Mine has failed in this way and, rather than cough up £300 plus for a reconditioned unit, I am trying to find out what the thermistor resistances are across a range of temperatures so I can just solder in a replacement thermistor.

Does anyone have a defective CCU which I could remove the thermistor from to do some measurements? Or - long shot - does anyone know what the resistance readings should be across a range of temperatures between about 0 and 100 degrees?

I am working with Tore B on this mini-project, and he seems confident that he'll be able to source the relevant component (with a few spares for other owners with the same problem) once we've figured out the required specification.

regards
 
Hi Nick

Does the thermistor have any markings? If so, as well as the manufacturer it may show the resistance and temperature coefficient (negative or positive). Alternatively does the CCU have a manufacturer identified somewhere (or was Porsche themselves?) - if you contact them they may have a circuit diagram with component values that they could let you have. I'm sure I've seen posts (on Rennlist?) referring to companies in the US that that refurb CCU's so they must have circuit diagrams - perhaps track them down and they might at least be prepared to give you the thermistor values.

As you say once you've got the values for the thermistor it ought to be a simple taks to source a replacement.

Good luck.

Regards
Anton
 
Hi Anton

Unfortunately I don't have a thermistor to examine at the moment - mine apparently broke some years ago and was replaced by a bulb (to provide a resistance across the terminals).

If anyone does have one I can look at I can take it from there but I will also try to see if any of the rebuild companies can provide the specification.

Nick
 
It would be interesting to find out from Nick what effect the bulb had (I wonder who was the 'bright' spark who thought that one up!). However the bulb will have a pretty constant resistance value (once it's warmed up) - given that the applied voltage will be constant. So I imagine that the CCU would think that the temperature is constant - with it's estimate being whatever the bulb's resistance is. The result of this would be constant cooling (if the estimated temperature was too high) or constant heating (if too low) as the CCU tried to correct the temperature - which it would never be able to do as the bulb's resistance wouldn't change! Whereas the thermistor's resistance would go up, or down - depending on it's type, with temperature.

Kind regards
Anton
 
Mark

I've not really had the car long enough or in enough different temperature conditions to be able to tell with accuracy what the impact is.

Having a bulb (i.e. a resistance) across the terminals previously joined by the thermistor fools the ccu into thinking that the thermistor is "seeing" a certain temperature inside the cabin. I have no idea at this stage what that temperature is, but since I can get hot or cold air into the cabin by maipulating the ccu controls I imagine it must be somewhere in the mid range of the likely temperatures it expects to see.

The problem is of course that the resistance of the bulb doesn't change with temperature, so the system keeps pumping out air at a higher or lower temperature rather than acting in a thermostatic manner.

I also haven't had the car in cool enough temperatures to be able to tell if I can readily get warm air other than through the demist option. I must go out and try this sometime soon.

Nick
 
Hi Nick

If the bulb seems to be placing the temperature in the mid range that may give you a starting point if you can't get any info on the thermistor values. You could start with a thermistor that has the same resistive value as the bulb - measure the current the bulb is drawing & the voltage across it's pins/connectors. Of course you'd really need to know if the thermistor needs a positive (PTC) or negative (NTC) coefficient (does the resistance need to increase or decrease with temperature?) but I'm sure you could get one of each and test it out. A quick look at RS Spares shows that although they have differing resistance at 25degC the NTC & PTC value seem to be around 4 to 5% per degC. So as long as you got the 'start' resistance right you might be able to get it to work quite easily as it would only be a case of getting the right coefficient (PTC ot NTC). Cost wise one I looked at was 49p so on that basis getting a couple for testing (mix of NTC & PTC) wouldn't break the bank!

Ideally you need to find out the correct values, but, if you can't then a bit of 'experimenting' might get you a working CCU!

Kind regards
Anton
 
Anton

That's useful information - thanks.

I've been out in the car tonight and, aside from reminding me it needs new dampers, I've found that I can get both perfectly cold and warmer air with a useful degree of variation in the output air temperature.

Hence I believe the bulb that's been temporarily inserted into the circuit must mimic the thermistor resistance well enough to fool the CCU into thinking the cabin temperature is around 20 degrees C. This is not an urgent repair for me (those dampers come first) but when I get around to finding a solution I'll report back.

Nick
 
Nick, maybe try different bulbs with different resistance for summer and winter use ? To be honest, even when the CCU is working properly, the temperature sensor isn't that sensitive, depending on the amount of fluff it contains, so this could be a valuable mod.

Mark
 
Mark

I expect it would also be possible to wire in two resistors in parallel with a switch to provide summer and winter settings. Once I've nailed the most important issues (dampers and geometry now) I'll take the time to have a more in-depth look at this and will let the forum know the results.

regards
Nick
 
Perhaps this guy can help you Nick? Obviously deeply steeped in electronic skills.
http://privat.bluezone.no/toreb2000/ccurepair.html
btw having sorted the geometry out on mine last year I now regret not changing the tie-rods at the same time. See
http://www.porscheclubgbforum.com/tm.asp?m=554285&mpage=1&key=&#554285

 
ORIGINAL: Mears

Perhaps this guy can help you Nick? Obviously deeply steeped in electronic skills.
http://privat.bluezone.no/toreb2000/ccurepair.html
Click to expand...

I've been in touch with Tore (who's a very friendly and helpful person, by the way) but he doesn't have any information concerning the specification of the correct thermistor is. Not unreasonable of course, especially if you've never had reason to measure one for this application.

Hence I've agreed to try to deduce the values which can then be used to buy an off-the-shelf thermistor to replace defective ones.

Once we have a solution for this it'll go on Tore's website (and here, no doubt).

regards
Nick
 
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