neilh

Troubleshooting my 86 - heater is on full all the time, fan only runs on defrost position, heater valve checks out, holds vacuum. Discovered Po removed external temp sensor and the complete air duct to the alternator :-(

So - after reviewing the wiring diagram i'm concluding that an open circuit ( as in no external sensor) may cause the full heat condition - and that maybe by temporarily faking it out by putting a 1k Ohm variable resistor in its place while i get a replacement and setting it about half way may work.

Comments?
TIA

Leon Speed

Need to get a new temperature sensor to complete the circuit. In the meanwhile you can zip tie the heater valve closed to see if it really shuts off the heat. If the blower only runs at defrost, the resistor pack in the blower duct is faulty and it needs to be replaced.

karl ruiter

I once tried running mine without the external sensor. It was about 20 years ago, so I don't remember the details, but I do remember that I bought a sensor pretty quick.

neilh

I'll unplug the external sensor on my 81 and see what it does, I had the same problem on my 84, and it was not the resistor pack , but i don't remember what I did to fix it

Mrmerlin

check the vacuum pods as one is probably leaking if one leaks then the HCV wont shut off.
If the sensor isnt in the loop or otherwise disconnected you should still be able to shut off heat by placing the temp control to full cold this takes the sensor out of the loop..

Back to check for a vacuum leak as this will let the HCV stay open

If the fans not working its usually the blower relay,
also check that the harness is connected to the blower switch on the head unit

neilh

Thanks Stan. I'm leaning towards the vacuum leak, the car starts first time, runs and hunts for several minutes and wil try to stall out if given gas, then settles down and idles fine and all ok after that - the dealer that bought it at auction pulled the intakes and had them powder coated, and i'm guessing he screwed up something when they were put back on.
Neil

Mrmerlin

Nope the vacuum leak is in the pods under the console bet cha 5 bux

thats the first place to look

928gt.com , page 2
under tips and links ,
look for Wallys writeup on testing and HVAC operation

Speedtoys

Going out on a limb here..

If Temp2 has no good ground to the intake from the PC, then you have no cold enrichment from those sensors, and until warmed up..you wont run right at all.


??

WallyP

Faulty temp control string (outside temp sensor, inside temp sensor, temp control lever) results in binary control. Lever on full cold gives full cold, lever up just a little results in full heat.

If the blower works only on top speed in Heat/A.C., problem is usually resistor pack.
If the blower works only on Defrost, and not at all in Heat/A.C., problem is usually blower relay.
If the blower doesn't work at all, problem is usually blower fuse.

mr. merlin is right - check the ground on the Temp II sensor housing. Powder coating will electrically insulate it, causing poor running.

IcemanG17

try this test......hook up the vac line to the heater valve to your trusy mity-vac....it should show zero vacuum until you select the the heater...then it should show pretty close to the same reading you get from right off the booster....if the heater valve reading is significantly lower then you have lots of leaks...for example Sharky would show 22" at the heater valve and about 24" at the booster...everything worked great

Another option is your heater valve is failed internally...I had one do that...

Bill Ball

Stan, Wally and Iceman covered the likely suspects. Rule them out. If they check out OK, then look at the setting motor. If it does not move when you move the temperature slider (i.e., it's stuck in the heat position), the heater valve will open as soon as you turn on the climate control (move the bottom slider away from OFF), although it will closed when the climate control system is off. There area couple of transistors inside the setting motor that can fail. If you are electronically semi-skilled, they can be replaced. The setting motor unit is rather expensive. Hope that's not it.

neilh

Thanks Stan, Wally, Iceman and Bill - as soon as the rain lets up here i'll dive in and check out al these suggestions, and report back.
Neil

Mark SF

In my case I found that all heater functions worked fine when connecting the brake booster straight to the heater vacuum line via the one way valve. Then I tracked it down to a leak under the left fender. Must be either the vacuum cruise or the reservoir. So right now I have intake valve and heater only connected to the brake booster, both working.