hwyrnr

1984 928s

HVAC only blows hot air.

My AC has always blown nice cold air would almost freeze you out been that was since I've owned the car for about a year now.

I noticed the issue after I removed the HVAC control unit to replace the light bulb and put new slider ***** on.

Once the car is warmed up and I just use the vent system with out AC it blows hot air, slide it to the cold setting still hot air.

I press the AC button I get hot air with the temp slider on the cold setting and hot setting.

The AC clutch does engage when the AC button is pressed to the on position and disengages after the button is pressed to be in the off position.

I looked up the test the vacuum solenoids here on Rennlist. I disconnected the vacuum tube that goes to the HVAC system by the brake booster and drew 10 psi of vacuum using a mightyvac. It held pressure until you turn the ignition to the on position then the pressure drops. so that seems to be working. I used the mightyvac on the heater valve thats under the air box and it moves with vacuum. Hooked it all back up and start the car and the heater valve moves. So it looks like that is working. I moved the slider from the hot setting to the cold setting and the valve did not move.

Is it suppose to move "close" when you slide it to the cold setting?

So I decided to pull the HVAC control back out of the dash and take a look at it again. Got it apart and every thin looked good. the soldier joints look intact, no loose wires, no apparent broken parts.

While I had the HVAC head apart I noticed two micro switches and a potentiometer. that are part of the temperature portion of the unit. I'm not the most knowledgeable person when it comes to using a voltmeter. and I'm not sure how to test a potentiometer. But with the two micro switches, I found the switch that is pressed when the temp slider is slid to max heat has continuity, and when it is not pressed does not have continuity. The micro switch that is triggered for max cold does not have continuity weather pressed on not pressed. (Picture attached)

So the max cold switch does not appear to be working. But I don't think that is the main problem since I should get cold air if the slider is at least on the cold side.



What is the likely hood the potentiometer has gone bad? and how do you test it. what do you set the voltmeter to?

Does anyone have an idea where to look next, or what to check/look for?

Help Please

Thanks James

Mrmerlin

the probability is hi that your work has disturbed a wire connection.
first to check are the wire connectors.
then maybe open up the HVAC controller and remove the E clip on the finger slider plate,
and clean the tracks and retension the fingers

Chalkboss

I found over time that my cold microswitch started to creep in it's housing, and thus was not depressing the switch when lever set to cold max. I used a bit of epoxy and glued it into position. It now works.

hwyrnr

Mrmerlin, I'm assuming you are taking about the tracks that select the vents the air will come out. My vent selector works fine with no issues.

Chalkboss, I manually pressed the micro-switch and heard the click it just doesn't have continuity.

Chalkboss

Does the lever depress the switch? My switch worked when manually pressed, just not when I slid the lever.

hwyrnr

yes the lever depress' the switch. It appears the switch is dead. I found a new switch online. but I dont think this is the problem. but not 100% sure

Mrmerlin

I have found that when messing with the HVAC , one or more of the the slider fingers can get a small piece of dirt in them and thus one part might not work as it should.
check the wire harnesses to the the A/C component maybe you didnt get the sensor wires connected IIRC their are 3 connectors 2 big and one smaller these come from the AC switch section ,
not including the wires to the HVAC head

SeanR

The switch is a simple work/no work switch and is an easy test with a continuity tester. Nothing tricky about that.

I'd start with seeing if the heater control valve is shutting when you have the slider all the way to full cold, contacting the switch. If it's not then you most likely have an issue on your vacuum system. Start there and let us know.

hwyrnr

SeanR, The switch when pressed does not have continuity. I have ordered a new switch.

So i'm clear on this, when that switch is depressed by the slider to the max cool position the heater control valve should fully close?

my next question is, and I assume the potentiometer also regulates how much the heater control valve is open or closed?

bureau13

I'm pretty sure (someone correct if wrong) the heater control valve is an open or closed device...no middle ground. How much hot air you get vs cold air based on slider temp should be controlled by the settings motor which opens and closes a flap to allow heater core air into the mix.

hacksaw

Has anyone done a comprehensive write up on rebuilding the HVAC head? I have seen the ones concerning the modification of the compressor actuating relay. Can't find anything more than that.

StratfordShark

[QUOTE=hwyrnr;13903892
my next question is, and I assume the potentiometer also regulates how much the heater control valve is open or closed?[/QUOTE]

Yes it does, but in conjunction with the cabin temp sensor and the external temp sensor.

Failure of any of these (if they or connections to them go open circuit) would result in hot air at all selections of the slider.

But if all was ok before, and you've had the head out to change illumination bulb, then the most likely problem is with the edge connectors to the head unit (or maybe you disturbed wiring/contacts to the slider inside the head). Worth checking those first before the more complicated stuff!

hwyrnr

Okay Stratfordshark, That gives me some help.

I ordered a new micro switch. I'm going to do some more testing and retest the vacuum stuff from a previous write up. I'm going to test the potentiometer. when I pull the head out. and check to make sure I didn't pull a connection loos while unplugging it.

I will post the micro switch part number when it comes in for someones future needs.

James

Mrmerlin

FWIW when removing the HVAC head/.
it slides out of the console paired with the lower AC switch component.

NOTE disconnect the harnesses from the AC switch panel before you try to slide this part out of the console,
or you run the risk of breaking wires

hwyrnr

I GOT IT FIXED. woohoo.

OK, it was the micro switch on the max cold setting. As I said before the micro switch had no continuity when depressed.

While it was out I went ahead and checked the potentiometer with the multimeter set at 2K OHM. The max cold "65" was .152, max hot "85" was 1.132, med setting "75" is suppose to be .642, I got a reading of .676 so close enough for me.

I ordered a new micro switch, soldiered it in and put it back together. I drove it around and every thing worked as it did before. When the slider was set to max cold the heater valve would shut as it should. AC worked great as well.

For anybody that may need the microswitch part number here it is.

BURGESS
F1T8-GP
SA 250 VAC

This part comes with a little arm that activates the switch that you can easily take off. I mention this because the actual stock part number is F1T8-VO but I was not able to locate one. The F1T8-GP is the exact same one just with the little activation arm.

Thanks guys for all the input.
James