Auto Heat problem

[Yahow lam]

Hi,

Hoping to get some help on my 1987 930 heating problem.

When ignition is on or engine is running I can turn the Auto Heat dial at any position I will have controllable heat like it suppose to.

It started a week ago notice this problem with ignition on or engine running, the Auto Heat dial at "0" at off position, as soon as I turn the head light switch on the servo motor will also turn on, fully opens the heat exchanger valves blowing uncontrollable very hot air out the vents, if the head light switch is off the servo motor will close the flappers and the hot air stop blowing, seems it totally bypass the dial switch between the seats.

Thank you,
Yahow

[Yahow lam]

OK, I have been searching hard for information on what cause the heat to come on when head light is turned on at the same time with the dial off at "0" at the heat control unit between the seats for the last two weeks with no luck at all, just replaced both ignition and head light switches with known working ones, but still have the same problem, just wondering how the headlight switch wiring is related to the heat control unit to cause it to turn on, any similar past experience and suggestion can share will be greatly appreciate!

Thanks,
Yahow

[Alan L]

What year car?
There must be a commonality in the loom between those functions. I will check my wiring diagrams tomorrow with luck - no access to them right now.
Hoping yours is 83 or earlier - I find the later diagrams much harder to follow.
I once had a problem - when I turned the interior fan on the glovebox light came on. Turns out they shared the same multi pin plug, and the pins were out of synch.
Alan

[Yahow lam]

Hi Alan,

My car is 1987. That will be great if you can help me out and give me some pointers where to check.

Thanks,
Yahow

[HenryCooper90]

Hello, To address the heating problem in your 1987 930, start by checking the wiring connections around the headlight switch and the dial switch between the seats. Look for loose or damaged wires, ensuring they are properly connected, and check for signs of corrosion or overheating. Test the functionality of the headlight switch and the dial switch to verify they are operating correctly and sending the appropriate signals to the servo motor. Inspect the servo motor responsible for controlling the heat exchanger valves, ensuring it functions properly and checking for any damage or wear. If you're unable to identify or resolve the issue, it's best to consult a qualified mechanic or Porsche specialist with expertise in diagnosing and repairing heating problems. Home Bargains Portal

[356911930]

^ this guy sounds like a scammer!

Rahl

[Alan L]

Sounds like an AI bot.
I have had a very quick look at the wiring diag, and it is very hard to follow.
The earlier ones are easier.
I am wondering if the two units share a common earth and it is suspect, so current is tracking down the other route to try and find a decent earth. Just not sure where the earth point for these units is right now.
Alan

[Yahow lam]

Hi Alan,
Thank you for getting back to me, I will start to look for ground points in the trunk and under the dash, if you can find out the exact location of the shared ground point for these two units later that will be really helpful, please let me know. Thank you!
Yahow

[Alan L]

If you have replaced the light switch you will be able to access those connections. You could piggy back a temporary ground from that switch (it must have a ground wire - usually brown). Loop in another wire to that point to any temporary ground point - a bolt or screw in the body.
See if that changes things. Just a long shot short cut. You are providing an extra ground to that circuit.
Alan

[Yahow lam]

Hi Alan, Thank you.
Yes, The headlight switch is still out dangling by all the wires easy to get to, I am going to give it a try and report back.
Yahow

[Yahow lam]

Hi Alan,

Just finished testing the headlight switch, don't see any brown ground wire at the switch. this is what I did and found with battery disconnected:

Light switch Off: terminals 57, 58a, 30, 58 and 56 have grounds. terminals N, 58L, 58R and 75 no grounds.

Light switch pull out one click On: terminals 58L, 58R, 57,58a, 30, 58 and 56 have grounds. terminals N and 75 no grounds.

Light switch pull all the way out On: all 9 terminals have grounds.

I hooked up the battery with the light switch pulled out to On position and start pulling the wires off the terminals at the light switch one at a time, all terminals will still turn the heater On when is disconnected, except when disconnected terminal 58a ( 3 wires go to that terminal one bigger Black/blue strip and one smaller Black/blue together with one small Blue/black for the instrument lights, it will NOT turn the heater On when light switch is in On position, I don't know what that means may be you can explain what the problem is. Thank you.

Yahow

[Yahow lam]

Hi Alan,

Your diagnostic was correct, found the brown ground wires at the big white connector at the back of the auto heat unit under the carpet was half way out, pushed it back in problem solved!

Thanks again for your help!
Yahow

[Alan L]

Good result.
Alan