Automatic heater control unit repaired -- 70 cents!

[logician]

Hi Kurt,

> I threw the old micro switch away so I'm not
> sure how it was wired but I think it was to
> the two outside terminals. Since the two
> stacked together are the same switch and the
> bottom one is wired to the two inside terminals
> (normally closed) then if the the top one is
> wired to the two outside terminals it should be
> normally open which doesn't make sense in your
> case.

Good observation. I'll have to double check everything and verify that the two switches do indeed have different behavior. So now I'm down to the conclusion that the factory built the servo wrong, and no one ever caught it or complained about it. Very strange.

Actually, the two switches do seem a little different. The fan switch doesn't have the detent feeling that the bottom switch has when the contact lever is pushed down. I suppose it is possible that the switch failed, but it seems unlikely that the failure mode would to reverse its on/off behavior!

> Having said that, I looked at the diagram and
> assumed the switch should power the relay when
> it's closed which is how I wired the new switch.
> Again, my system never worked since I bought
> the car so I'm not sure how it's supposed to
> work. Makes sense since the fans turn on when
> the flapper is wide open.

I agree with your conclusion here.

> If you have your switch hot wired, then your
> fan is running at low speed even when your
> rotary switch is turned off - may be OK now
> but won't help when it's time for A/C.

Yea, and its a little noisy too. Now that I understand how it should be wired, I will remove the workaround.

> I checked the micro switches and diodes and
> they are fine. If you pull back the black
> plastic cover around the gears, you can turn
> one of the larger gears with your finger to
> move the lever pretty quickly. I didn't remove
> the lever. My potentiometer has about the same
> readings so I assume it's OK.
>
> I seem to be back at square one.

It's probably worth the $180 investment at this point. The only remaining problem that I can see is that somehow the temperature input signal is not getting the the circuit board. If that signal is open, then it would behave exactly as you are observing.

Try checking the voltage on the circuit board for the input signal. I believe those are pins 6 and 8 on the circuit board. You should be able to probe them on the back of the board by the connector. Check that the voltage does indeed change when you change the rotary switch position. I haven't traced out how the temperature sensors are wired through the rotary switch, but it should be pretty simple.

If you order the board, let me know if you actually obtain it. I may need to get one at some point if my patched board fails again.

-Juan

[rokemester]

I'm resurrecting a 9 year old thread (and a very informative one) on the infamous automatic heating controller. I have an 87 911 Targa and I have the control box out of the car with a rubber cap on the cable arm to bring heat into the cabin as the weather is beginning to turn hear in Northeastern Ohio.

Any auto heat gurus still tuned in? I've found the temperature sensor in the heat duct has failed open, and I haven't been able to source a replacement. A microswitch in the servo controller is missing the tiny spring and not working. The servo motor works but is moving in only one direction. The heat relay in the engine compartment seems to be working as are the rear blower and two footwell blowers.

I'm really not sure I'm brave enought to get into the electronics of the circuit board like some brave members before me. I guess if I can't even source the sensor I'm in trouble and I may have heard that they are trying to get over $600 bucks for it even if I can find it!! Really?

[Shadetree930]

See post #63 for information on the microswich replacement.

Help with 930 Heating Problem (Footwell Blowers)

Actually, the entire thread is a good read for people with autoheat problems.

[rokemester]

This is an excellent thread thanks!

I'm hoping a faulty microswitch is the only component I nee to troubleshoot and repair. The microswitch you see is faulty, but if I can find a small "spring" piece I could be good to go. I looked around for that little guy but nothing. I'm not even sure that will clear up anything or everything.

[rokemester]

Has anyone been able to source a heat sensor for the heat duct? Does our host stock them? I haven't been able to find one.

[ant7]

hi All,
first of all please accept my apologies for bringing my own woes to this thread, but it seemed like a good chance to get some good advice as i am currently trying to get my heating system to work! The control unit works the the flaps in the heat exchangers fine, ie; opens and closes them correctly, however, i have no fans running, I have checked all three fans, ;engine bay, and both footwells and all three are working fine with auxilary power connected to them, I have taken the relay in the engine compartment out and examined it, no corosion, or burning present, And if i push the relay contacts together while the relay is connected i can get all three fans to work, I have removed and checked the resistance of the temp sensor in the cabin and it seems to be in tolerance, I have even shorted out the reed switch and it makes no diffrence, Any clues as to what might be going on her would be very welcome.
Anthony.

[pete3799]

I've spent hours trying to fix my auto heat. In the end i just disconnected the rod going to the servo motor and manually raise and lower the (1) red lever to control the heat. If i want the engine mounted blower on i turn the auto heat knob which turns the blower on.

[rokemester]

I'm jealous, I wish my controller was actuating the servo correctly. I don't think you (Ant7) are that far away from getting your system to work properly. I bet someone will have a suggestion. I'm getting closer to the point of the most recent post (Pete3799) where I can actuate the blowers but manually move the lever to bring in the heat. It's kind of awkward to reach into the controller box and manipulate the flapper lever. But I don't want to admit defeat quite yet. The system is over designed. A compromise would be to remove the sensor input from the circuit and just use the controller to electronically through the servo motor to open and close the flappers. I could live with that. Just don't know how to modify the circuit to make that happen.

[86 911 Targa]

Originally Posted by ant7

hi All,
first of all please accept my apologies for bringing my own woes to this thread, but it seemed like a good chance to get some good advice as i am currently trying to get my heating system to work! The control unit works the the flaps in the heat exchangers fine, ie; opens and closes them correctly, however, i have no fans running, I have checked all three fans, ;engine bay, and both footwells and all three are working fine with auxilary power connected to them, I have taken the relay in the engine compartment out and examined it, no corrosion, or burning present, And if i push the relay contacts together while the relay is connected i can get all three fans to work, I have removed and checked the resistance of the temp sensor in the cabin and it seems to be in tolerance, I have even shorted out the reed switch and it makes no diffrence, Any clues as to what might be going on her would be very welcome.
Anthony.

Anthony,

The relay contacts you are referring to are high current.

If the relay is activating, please check for corrosion due to arcing at the contacts.

Also, the reason for the reed switch, is to prevent exhaust fumes from entering the cabin.

Good luck,

Gerry

[86 911 Targa]

Here's are the input/output connections to the relay.

Check for ground at pin #2.

As an afterthought, you may want to re-flow the solder joints.

[Lorenfb]

"Any auto heat gurus still tuned in? I've found the temperature sensor in the heat duct has failed open, and I haven't been able to source a replacement. A microswitch in the servo controller is missing the tiny spring and not working. The servo motor works but is moving in only one direction. The heat relay in the engine compartment seems to be working as are the rear blower and two footwell blowers."

The heater control servo unit is a very simple unit. It just uses an op amp
(no dual flip-flop as in the previous post) and an H-Bridge to drive a motor
to move the heater linkage. The micro switches are used to limit the travel
of the heater linkage, i.e. to turn off the motor, and to turn on/off the blowers
as the linkage moves. The key components are the two temp sensors (cabin
and rear). If either is bad/open, the linkage moves to its limit. If the micro
switches lose their springs, the unit will not function.

"I have even shorted out the reed switch and it makes no diffrence,"

Do what? Where?

[rokemester]

That may explain why the servo motor moves in one direction and stops as the temperature sensor in the driver side heat flapper duct has failed open. The cabin sensor is giving me a reading so I'm pretty sure it's OK. The recent post on this thread (Shadetree930) gave me a good lead to replacing a micro switch that is missing a tiny spring, but I'm still wondering how the heck I'm going to get the system running without a temp sensor replacement. Thanks for continued interest and insight.

[ant7]

Originally Posted by 86 911 Targa

Anthony,

The relay contacts you are referring to are high current.

If the relay is activating, please check for corrosion due to arcing at the contacts.

Also, the reason for the reed switch, is to prevent exhaust fumes from entering the cabin.

Good luck,

Gerry

Hi Gerry,
I understand where your coming from on this, however, i have checked all voltages at the relay plug and most are present, including the earth connection, The voltages i am missing are the ones that activate the actual coils in both relays inside the main unit, that is why i explained that when i push these relay contacts together [as if the coil voltages were present] the fans work, Aditionaly, both relay coils check out as far as resistance is concerned, and do close with auxilery voltage applied.
Therefore, What i am getting at is, where in the chain might the voltage that actuates the coils be missing?
Again, any help would benefit quite a few of us it would seem.
Anthony.

[Joeaksa]

Great thread!

[rokemester]

Originally Posted by rokemester

That may explain why the servo motor moves in one direction and stops as the temperature sensor in the driver side heat flapper duct has failed open. The cabin sensor is giving me a reading so I'm pretty sure it's OK. The recent post on this thread (Shadetree930) gave me a good lead to replacing a micro switch that is missing a tiny spring, but I'm still wondering how the heck I'm going to get the system running without a temp sensor replacement. Thanks for continued interest and insight.

In case anyone else has an issue with their temp sensor in the heat control box here is a good thread to replace yourself. I included some pics. recently that hopefully clarify how to do it. Now that the car is put away for the winter I'm working to get the autoheat system operating properly...again!

Caloric Feeler

[john930]

Has anyone any information on a suitable replacement for the cabin sensor?

[intakexhaust]

Originally Posted by john930

Has anyone any information on a suitable replacement for the cabin sensor?

Subscribed -

[86 911 Targa]

Originally Posted by rokemester

Has anyone been able to source a heat sensor for the heat duct? Does our host stock them? I haven't been able to find one.

Hope this helps:

Caloric Feeler

Good luck,

Gerry

[rokemester]

Originally Posted by john930

Has anyone any information on a suitable replacement for the cabin sensor?

Check out this thread. I have PMd the poster regarding the digikey thermistor. If he get's back to me I'll post the part number.

Autoheat troubleshooting

[86 911 Targa]

Originally Posted by john930

Has anyone any information on a suitable replacement for the cabin sensor?

Call these folks:



Also, try DigiKey

Autoheat troubleshooting

The sensor resistance should be 1.9K Ohms @20C/68F.

Good luck,

Gerry