automatic heat multi-fan dial controller - '87 930

[jwmarc]

Has anyone ever taken apart the Auto heat controller on the tunnel that controls the heater flaps and has the 3 speed/ defrost fan. I have a situation were I get the first two speeds but when I crank it to high speed defrost it runs for a couple of seconds and then the fan stops. Fuses are Ok, system resets quickly to the first two speeds but the high speed issue remains... Am i looking at a variable resistor that is failing or a dial potentiometer failure. I had a situation about a year ago where I kept blowing fuses and when I removed the knee well fans I found that the rats cage on one of the fans was dragging on the housing and froze the motor armature thus blowing a fuse.

Anyone have the full schematic for the control module?

Thanks

[flightlead404]

do some searches there are a couple of excellent threads

[16Volt]

Yes - search on my username.

[jwmarc]

Thanks 16v... I did a search and found two threads which explain.. Your post looks comprehensive ..just havent had time to review.

Cheers

[16Volt]

No worries I mostly remember rebuilding that thing. Let us know if you run into issues.

[jwmarc]

Your thoughts on the cut out in high speed mode while 1&2 speeds work fine? Are there any resistors that may be causing the hispeed cut out after 10-15 seconds. The system doesnt use IC's. I think all I saw were NO/NC series switches.

[1979-930]

Here is a great site with a lot of information specific to the Auto Heat

https://members.rennlist.com/imcarthur/911autoheat.htm

[16Volt]

Originally Posted by jwmarc

Your thoughts on the cut out in high speed mode while 1&2 speeds work fine? Are there any resistors that may be causing the hispeed cut out after 10-15 seconds. The system doesnt use IC's. I think all I saw were NO/NC series switches.

My first check would be the microswitches. Usually they are a pretty 'works or dont' though. You can get weirdness though since its also taking an input from the temp sensor in the interior as well as the flapper boxes.

[T77911S]

on hi the power for the fans comes from a relay.

if low and med work then the common switch that I think is the normal big failure item is working as it should, that is if you turn it to low, the fan does not come on until the flappers are open and the fans do NOT run when in the off mode.

this switch provides power to the hi speed relay. the other side of the relay goes back to the console and the switch provides a ground to energize the relay.
there is also a separate fuse for hi speed.
power for low speed is always there once the main switch has closed, so even if the relay is removed the fans should drop back to low speed. so you need to make sure that low and med work and when hi stops working the fan drops back to low.

on the hi speed relay with the switch set to hi you should have power on 30 and 85. ground should be 86.
use a test light to check for power.
then clip the light to 12v and probe pin 86 to test the ground.
if all this is good the relay is bad.


here is my thread.
16volt did a great wright up. more detailed than mine.

auto heat- foot well blowers part deux.