catamount

Ok, my '84 928s has since I purchased has blown heat full time at full temp, as we know not a fun experience in 90+ deg summer heat, was nice this morning though. :-) No matter what the temp setting has been, I've gotten heat. On some occasions it's cooled down briefly only to return to full heat.

What have I looked at and or done? I replaced the heater control valve and nope didn't change a thing. Vacuum is good so I don't think that's the problem.

This morning with it blowing full temp I pushed the temp guide to the coldest setting, COOL AIR! Back to full heat, HOT AIR! Then back to coldest setting and again cool air.

Given that in the past I've not gotten similar results with the temp control, what do y'all think? I'm wondering if the head unit is bad and that's what needs to be replaced.

jcorenman

It could be an intermittent connection to either the outside or inside temp sensor, or (more likely) a balky temperature-regulator (the motor and flaps that control the mixing of hot and cold air).

Start with Wally's excellent description and troublshooting guide for the HVAC system: http://www.928gt.com, click on "928 Tips/Links", Wally World, HVAC.
Or try this direct link: https://www.928gt.com/t-wallyhvac.aspx

There is also some good troubleshooting info in the WSM, the sort of methodical step-by-step approach that we all hate

Cheers,

StratfordShark

+1 on Jim's suggestions.

Just want to add that in case of hvac the wsm is very detailed on test procedures.

It has the resistance specs of internal, external and temp slider. It shows where to measure the total string resistance on connector to setting motor. If this shows a problem then you can use the wsm to narrow down to the sensors Jim mentions. If within spec you can investigate whether setting motor is behaving (you can do this from start by peering up at how arm controlling flap behaves at various demanded temperatures).

I had same "all hot" problem which turned out to be the cabin sensor going out of spec and having too high resistance. The fan motor was also turning very sluggishly and jerkily. Replacing sensor and fan was complete fix.

Don Ashe

In addition to the previous advice, look at the 2 microswitches on the temp slider. One is mounted on the left of the slider, the other is actuated by the sector gear mechanism. They are marked MAX Heiz and Max AC on the wiring diagram.
You'll notice that they over-ride the settings of the temperature pot and it's voltage divider network when the slider is at extreme left and right positions. If max heat and max AC are working but the intermediate settings are not, the problem could very likely be in that part of the circuit, the most vulnerable of which is probably the pot.
It may be as simple as a shot of contact cleaner in the pot, or you may need to replace it or one of the resistors. (the pot is 2.2 k, I believe)
On mine, there was enough wear that the max heat and max cool were not always engaging, so that may explain why they don't always work.

HTH,
-don

WallyP

All of the suggestions are good - but the most common cause by far is a bad connection on the exterior temp sensor under the left headlamp.