HVAC Temp Sensors

robwright · Jan 9, 2011

In case you didn't know it the three sensors that control your heating maybe on their last legs. To buy these they will charge you around £30 per sensor. In actual fact the sensor is just a basic NCT thermistor, which I think I have just sourced from RS for around 50p each. I have just ordered a job lot of 5, which will need soldering into the original housing (talking about the usual one that goes for the outside air temp) and the specs look good according to the manual. Will give them a try and if they are OK. If they are OK I will post the link. If you can't solder then post your old one to me and I'll do it for you [

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GPF · Jan 10, 2011

Nice one Rob,

And I think mine has recently gone! (No heat unless full up?)

zcacogp · Jan 10, 2011

Watching with interest ... the heating system on my S2 has been less than spectacular of late, and the sensors are top of my suspect list.

Oli.

barks944 · Jan 10, 2011

Do the sensors have the same temperature/resistance relationship?

robwright · Jan 10, 2011

I pulled the data sheets off of the RS website last night and the appear to be exactly the same as the figures given in the HVAC troubleshooting section of the manual [

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barks944 · Jan 10, 2011

Sweet

love it when thing like that work out. Although I guess it is some component standardisation.

robwright · Jan 11, 2011

Thermistors arrived tonight so I wasted no time checking them out. In the case of the outside temp sensor (which is usualy the culprit) you need to remove the plug and you will see that it is designed to split. As if the thermistor was somehow designed to be replaced [

] Then remove the old thermistor from the two contacts but paying attention to the length of leg to replace the new one with. Solder the new thermistor to the legs (polarity is not important as it is only a thermistor), trim off the excess leg then reassemble the plug. Then simply fit the plug back into the blower inlet. I took the car out for a little run to ensure both the engine and cabin were thoroughly warmed and hey presto full gradual control over the entire heat range. I had my outside sensor disconnected to make it think the outside air was super cold and hot air used to kick in around 18 degress or less. Now it is a nice warm flow when set to 21 [

] As they come in a pack of 5 I shall replace the cabin sensor and the mixing chamber sensor as a matter of course. Why not at 60 pence a throw!! For those of you wishing to tackle it please do as it is a doddle - the RS part code is 6974465. NTC thermistor, 3.5 mm, 10k, 5% (better than the wksp manual test spec which is 10%). Makers part number ND03N00103J. Will have 2 left when I am done. Will let you know when. First come first serve lol. Don't you just love sticking it to the man!!!

Direct link

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=searchProducts&searchTerm=6974465

zcacogp · Jan 11, 2011

Rob,

Thanks.

D'ya know, I have never worked out where the outside temperature sensor is on my S2. A plug, you say? Where? Somewhere in the scuttle ... that's about all I know.

Anyone care to/able to post a photo? Thanks!

Oli.

Diver944 · Jan 11, 2011

ORIGINAL: zcacogp

Anyone care to/able to post a photo? Thanks!

Oli.

Your wish is my command [

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Its the inch long black thing at the top of the grille here

zcacogp · Jan 11, 2011

Paul,

Superb - thanks!

I'm going past my local RS tomorrow, and need an excuse to drop in. I think you chaps have just given me one ... [

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Oli.

zcacogp · Jan 12, 2011

Dropped into RS. Out of stock of those items, but they will deliver them tomorrow before 4.00pm (and no delivery cost either!)

Rob, thanks for the tipoff. Will keep you posted.

Oli.

barks944 · Jan 12, 2011

How about a sticky in the tutorial section?

robwright · Jan 12, 2011

I was thinking the same. I did take some pics as I normally do for everything but due to the size of the items it was hard to get a photo that was in focus. If someone has a better camera then be my guest. Just make sure I get a mention in the footnote [

] Might do the other 2 on Friday as I am off and it's getting warmer. See how the pics from that turn out.

zcacogp · Jan 12, 2011

I'll have a shot (see what I did there?) when I do mine, if that's helpful.

Oli.

robwright · Jan 12, 2011

Nice one Oli. Worth posting I think as it is a doddle and such a money saver [

] Not beyond the means of most I think and a very common problem.

barks944 · Jan 12, 2011

This sort of info is going to be more and more useful as we run into the same problems as our cars get older. I think its a great idea to help each other out on this sort of thing and keep a record for new owners. Especially when people find a solution like this which has a very cheap easy fix

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robwright · Jan 12, 2011

My next project is a header tank cap which you can use for DIY pressure bleeding. If I get it right you should be able to do it with a foot pump, provided you don't put too much pressure in of course. I will of course have to butcher my old cap.

zcacogp · Jan 21, 2011

OK, I did it.

RS delivered the parts (next day, courier, for free - gotta love 'em*!) And I've done the job. And, frankly, it was so trivially easy I had completed it before I managed to get the camera out, so sorry for the lack of photos. Rob's phrase "doddle" covers it well.

Haven't tried it yet, but I did realise why the 1 inch long black sausage temperature sensor didn't look familiar to me; the sausage-shaped cover had dropped off the sensor on my car, leaving the thermistor component uncovered. Probably not a problem, it will work as well, but be a bit more vulnerable. Ah well ....

I haven't tested it yet but will do so tomorrow, and will keep this thread updated.

Out of interest, where are the other two sensors? I know about the one in the cabin, but the one in the mixing chamber is news to me. Does it often go and is it also worth replacing?

Thanks again Rob.

Oli.

robwright · Jan 21, 2011

Oli if you remove the trim from around binnacle and centre vent (about 8 screws) you will see a cable going to the centre of the void behind the vents you have just taken off. This should be clipped into position in the chamber. By pure coincidence I did mine today. This plug comes apart too revealing the sensor just soldered onto the end of the cable. The cabin sensor is a little more tricky. It is best to remove the glove box then remove the sniffer fan. 2 x 10mm nuts securing it to the body and a plug that probably hasn't budged for 20 years [

] Once you get it out you'll make sense of it mate. It will of course mean crawling upside down in the passenger footwell though [&:] While the centre vent was off I checked the operation of the flap in response to temperature inputs and all seemed pretty spot on.

zcacogp · Jan 21, 2011

Rob,

Thanks. I don't think I have seen any screws (let alone 8 of them!) around the binnacle and centre vent. I suspect that Clarks Garage is my friend ..

I know about the cabin temperature monitoring one, and also know of the joys of working upside-down in the passenger footwell to work on it! (Better than the drivers footwell tho' - no pedals to bang your head on!)

Thanks for your help. I'll probably do the remaining two just to be complete.

Oli.

ETA: Will add that last time I was rootling in the passenger footwell it was 'cos the footwell vent flap didn't seem to be working. The flap wasn't moving, and further investigation revealed that the servo actuator was jammed solid ... once dismantled, the motor itself was was full of rust. Not sure why, I suspect there was a leak somewhere once upon a time, but the rust certainly pre-dated my purchase of the car. Thankfully, a good clean-up of the armature and magnets, with some oil, brought the motor back to life and it now works just fine.