FiveElements

My 89 car has heater problems. When I put the temp selector on 65• cool air comes though the center vents but lukewarm air out of the side door vents. Then if I put the temp on any other setting 68, 70, 75... etc the heater comes on so hot, it's too much. The temp sensor does not seem to notice its burning hot in the car. Even at 68• temp setting, super hot air just keeps coming out. Only when the temp slider is full over to the left do I hear the click of the vacuume relay and see the heater value close.

So far I have:
1) replaced the heater valve
2) replaced the recirculating flap vacuume valve
3) can hear the fan running behind the temp sniffer in the dash board
4) checked & repaired all vacuum leaks in the center console area

Is this a bad dash temp sensor? Is there a way to test or ohm out this sensor? Or is this a bad control board in the climate control pannel?

westija

Many threads about this around. If I remember it right, a likely culprit could be a bad outside sensor (in / under the left front wheel well).
I am not sure how to check it, but a search should bring it up
Does that help?

FiveElements

I have seen a bit about that, but the temp reading on my instrument pod does show the correct outside air temp. Wouldn't that be off if that sensor is failing?

Mrmerlin

here read this

https://rennlist.com/forums/928-foru...ix+HVAC+system

dr bob

Different sensors.

The outside air temp in the digital dash display uses a thermocouple in the bottom of the left side mirror. Run you fingers under the bottom of the mirror, and you'll feel the end of the little probe sticking slightly proud of the trim ring right of center.

Outside temp sense for the HVAC uses a thermistor in the alternator cooling duct, under the headlight in the forward wheelhouse area of the left front fender. The duct forms a loop attached to the front liner, and the sendor lives in a metal connecting sleeve. The connectors get tired with age and sometimes cause problems. When that connector or the thermistor itself fail, you'll see the HVAC symptom you describe.

Mrmerlin

oops your right Dr Bob, thanks so much for catching that my info was for up to 89

FiveElements

Thanks guys. I know where to go. I will ohm out & clean the connectors on the temp sensor in the wheel well area of the alternator tube.

FiveElements

Ok... I the outside temp sensor has a reading of 380 ohms. It seems to be okay. So I'm going to check the inside dash mounted temp sensor. Anyone know the correct readings on that sensor for 20ºC or 68ºF ?

Speedtoys

IIRC, the fail mode of the dash sensor is "all hot, all the time".

Alan

Not quite.

If the temp sensor loop has any issues on either sensor (dash/external) or if the head unit potentiometer has issues or if there are any connectivity issues between any of them them then you won't get temperature control - for most of the temp range you will indeed get full hot (water valve open and mixing flaps fully open).

However in the full cold position there is a microswitch that overrides the temp sensor loop and gives you full cold (water valve off and mixing flaps fully closed)

Technically when everything is working normally there is also a microswitch at the full hot end that gives you unconstrined heating also by overiding the temp sensor loop - e.g. it isn't trying to maintain a temperate its just full blast everything it can give you.

This is the same as you get when the temp sensor loop is broken (it can be a quite surprising amount of heat...). So if your engine is overheating - you can supplement the radiator heat extraction with this mode and the windows open to vent the heat - you'd get pretty toasty though... (much too much so in Phoenix)

Alan

Alan

Not quite.

If the temp sensor loop has any issues on either sensor (dash/external) or if the head unit potentiometer has issues or if there are any connectivity issues between any of them them then you won't get temperature control - for most of the temp range you will indeed get full hot (water valve open and mixing flaps fully open).

However in the full cold position there is a microswitch that overrides the temp sensor loop and gives you full cold (water valve off and mixing flaps fully closed)

Technically when everything is working normally there is also a microswitch at the full hot end that gives you unconstrined heating - e.g. it isn't trying to maintain a temperate its just full blast everything it can give you - same as you get when the temp sensor loop is broken (it can be a quite surprising amount of heat...)

If your engine is overheating - you can supplement the radiator heat extraction with this mode and the windows open to vent the heat - you'd get pretty toasty... (much too much so in Phoenix)

Alan

Calgary Ole

Check that the setting motor is working properly before taking the dash appart. Its beside the steering column and if is 'stuck' in one position, will give the symptoms you described

FiveElements

Do you have a picture of this setting motor? I don't know of anything like that on the steering column.

WallyP

The setting motor is on the side of the HVAC box inside the center console - roughly by your right calf (on LH drive versions). It is a black box a little bigger than a pack of cigarettes, with an arm sticking out of the top that moves the doors inside the HVAC box.

FiveElements

Ok... I pull this unit out, took it apart to see what is what. It may be bad, but how would I test it? It seems to control the heater mixing flap with a small motor. Nothing seems burt or fowled inside. Strange thing is when I put everything back together, my heater control temp correctly today for part of the day. Something intermittent is going on. Maybe loose or dirty connector contacts? I also retested my outside temp sensor and it was about 650 ohms at about 65º F outside. I'm starting to think the outside temp sensor is bad.