Hi Folks,
I know the english winter is coming, but my 91 C2 seems to to have over compensated for the cooler weather.

Here's the story so far..........I've searched the archives and found some excellent posts on the topic. I've also done some diagnostics and figured now was the time to ask for some Rennlist wisdom.

I have found that of the five servos that control air flow around and into the cabin, three work ok. Looking from the front of the car(LHD), if I was to number them 1,2,3,4,5 left to right, then numbers 2 and 3 are not functioning. These are for temp mixing left(passenger side on LHD) and the fresh air flap(recirc). My tests were rotate the temp selector for red to blue and servos 2 and 4 should move, but 2 does not. The recirc servo is in the closed position(no fresh air), regardless of switch position on the control. I can get the servo 3 to move by plugging in the plug from servo 4, so the servo is fine, but the feed to it is at fault, why????

So I am in a position where I can test servo 2, by applying 12v to the right pins once i've figured it out from a wiring diagram. But what is up with servo 3. Is it the main controller, and do I need to test the air temp sensor? The rear blower fan works ok. What do I try next guys?

Any views and comments appreciated as always.

Thanks,

Taj.

 

Hi Taj,

>The recirc servo is in the closed position(no fresh air), regardless of switch position on the control.

I know it's obvious, and I don't mean to offend you, but I have to ask .... you did push the recirc button to test the recirc (a.k.a. fresh air) flap, right?

To test your servos, place 12V at pin #4 (positive), #5 (negative).

The servos have a total of 5 pins. #1/#2 are common between the servos = "sensor ground" or +5V (on some servos #1 = ground, some it is +5V). Since they're all common, it seems unlikely that they're the source of your problem.

Pins #3, 4 & 5 have unique feeds from the CCU. You might make sure you have continuity between your CCU and these pins on the recirc servo. Pin #3 = G20, #4 = K3, #5 = K16.

Best of luck!

 

Hi Randall,

Yes I used the button to test the recirc, the servo is now in the open position(set by using the plug from servo 4) and disconnected as I want fresh air, and if I connect the servo 3 up, it always goes to the closed position. I will test the servos and feed back to this thread with my findings.

cheers Taj.

 

Still have not tested servos yet, but remembered that fuse 39 was also blown, which has been replaced. Does this offer any further clues. Needless to say, my fear is that the CCU is faulty, and anything that points to an alternative is welcome.

 

Hi Taj,

Fuse #39 is for the A/C condenser blower (12V power). This may indicate that your condenser fan is binding, or had something (a rock?) stuck in the fan blades. Anyway, fuse #39/the condenser fan's relayed power supply circuit is completely separate from the CCU/servo control, so it's unlikely to be the source of your servo problems.

I think the best way to test the CCU is to swap your unit with a friend's CCU, and see if your recirc servo will work. A '91-94 964 CCU is compatible with a US '91 964, but I would need to verify in PET that this is true for a U.K. car. If your friend has an '89/90 964, a simple jumper should make its CCU compatible to your '91's CCU.

 

Hi Randall,

I still like the idea of testing the servos first, for resistance, as the CCU must read the servo's position through measuring resistance across it, and by applying 12v across #4, and #5.

I say this because it seems that faulty servos can cause faults in CCU's and I would hate to ruin another CCU due to faulty servos, and besides, i don't know anyone else with a 964 in person.

I don't know how the resistance of the servos vary, so I was going to use one of the working servos as a reference.

I don't know if you picked it up, but the recirc servo has moved, when driven from the loom plug that belongs to one of the servos that does work, so once again this points to the CCU telling recirc servo to close permanantly, regardless of the recirc switch position. If this was the only fault, it could be the switch, but the passenger cold/hot mixer servo is also not moving which once again points to the CCU :-(.

This is my logic so far, with the only other factors being the temp sensors in the left and right chambers, and in the CCU itself, but I have not read of any failures in these in the past.

Taj.

 

Hi Taj,

I've never studied exactly how a servo works, so I searched the web to see what I could learn. Here's the best link I could dig up:

<a href="http://www.seattlerobotics.org/guide/servos.html" target="_blank">How a Servo Works Link</a>

Pretty nice description, except ... the servo described here has only 3 wires. Our 964 servos use 5 wires. Since the shop manual describes powering the servo motors by placing 12V at pin #4 (positive) and pin #5 (negative), it's clear that 12V is used to run our 964's servo motors. Apparently, on the servo motor described above, the 5V power is used both for control and power??

Another anomaly. In the link above, the white wire is the control wire. Looking at the 964 wiring diagram, I can't see how the wire at servo pin #3 could possibly provide a control signal (the other 4 wires are power or ground). I'm guessing that #3 senses the voltage on the potentiometer, and feeds this back to the CCU. If this is the case, the control circuitry is internal to the CCU.

Will all this in mind, this is my (unverified) theory of how the 964's HVAC servos work:

-Pins #1 and #2 (+5V and sensor ground) provide the reference voltage for the potentiometer. As these pins are common to all the servos, they can't possibly provide any feedback to the CCU.

-Pin #3 senses the voltage on the potentiometer. The pick-up for #3 moves with the output shaft. As the pick-up moves along the potentiometer, more or less voltage is sensed at the CCU. At one extreme end of the pot., +5V is sent back to the CCU. At the other extreme end, 0V is sent back to the CCU.

-Pin #3 is independent for each servo. Thus, it can provide unique control/sensing of each servo.

-Control circuitry inside the CCU sends 12V (pin #4) to the motor, to reposition the servo motor, if necessary. Again, pins #4 & #5 are unique for each servo, so each motor can be controlled individually.

Wathca' think, Taj? Sound plausible? It will be interesting to see what you learn, as you dig deeper into the servo's operation.

 

Hi Randall,

I like your logic :-), voltage sensing rather than resistance, varying +5 to 0. The only other thing to consider is that the servos rotate in two directions, so the CCU must be alternating -ve and +ve feeds to pins #4 and #5, to get the servo to move in two directions. I will gat a chance to start trying things this weekend, and now feel I have the information to proceed on. If it turns out to be the CCU, then I will attempt a repair. Fingers crossed. I will feed back what I find.

Taj.

 

Hi Taj,

&gt;The only other thing to consider is that the servos rotate in two directions, so the CCU must be alternating -ve and +ve feeds to pins #4 and #5, to get the servo to move in two directions.

This makes sense, and--upon closer inspection--the shop manual supports this. Depending on which flap needs to be adjusted, it describes using both pins #4 and #5 as positive. Pages 87-9 & 87-10.

 

Hi Randall,

It's fixed!!!!!!

Thanks for your help on this one, its great to have the list and some great minds to bounce ideas/questions off.

Here's an account of the process.

First thing I did, was took the CCU out with a view to inspecting the two boards for dry solder joints and any other obvious damage. I also thought I had proved that servo 3 was ok, by using the loom for servo 4. Some of the components get really hot, and John Miles' site has an article that shows an IC that needed to be replaced. On taking it apart, I found the following, internal temp sensor wire dry joint, pin G13 dry joint(AC condensor fan), 2-3 other suspect dry joints. As a result, I figured resoldering both boards could do no harm.

On replacing the CCU and reconnecting, the faults were still present, servos 3 and 2 not working. So I turned my attention to the servos, which were not that simple to remove, but its a lot easier to work on if you get in the trunk of the car!! On removing the servos, I was able to connect them to the plugs of the servos that working and found that servo 2 was frozen in position. On taking both servos apart, the contacts that feed back the position of the servo to the CCU were dirty on both servos, so I cleaned them up, tested the motors which were fine and then tested them in situ, connected to their connectors. They worked, YES!!! I was in business. Put it all together again and everything for the moment works fine. I can only conclude that it was a mix of CCU faults and servo faults that contributed to my problems, well, here's to the next challenge............
Thanks again,

Taj. <img src="graemlins/wave.gif" border="0" alt="[byebye]" />

 

That's great news, Taj! Glad to hear it. That must be a relief, not having to replace the $CCU$.

Were you able to verify any of our theories of operation during your testing? Suppose not, as it looks like you didn't have to dig deeper to fix your problem.

<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

 

Hi Randall,

On the theory of how it all works, I can verify the pins that pin 3 is connected to a potentiometer that is connected to the output shaft and that this is where the problem was, i.e. contaminated pickups, so the control circuitry in the CCU, could not tell the position of the servo, and therefore could/did not rotate it. The factory manual shows varying resistance in the servo? or varying voltage? I'm not that well clued what all the sysmbols mean. The only other thing to say is that 12v to pin 4 and 5 does not test the sevro, it tests the motor in the servo. The servo may still not be feeding its position back to the CCU. I think the recirc servo was not working due to a fault in the CCU, and the cold/hot mix servo had a fault within it.

 

I have a slightly different problem, was wondering if anyone could recommend which of the above might be the best to try.

Both #2 and #4 servos are stuck in cold (and I mean stuck - won't move by hand, should they?) so I get only cold air no matter where the temp **** is set. All the other functions seem to operate fine.

Before I tear into it, I thought someone might make an educated guess on servo, wiring, or CCU as the culprit. I figured on starting with testing the servos via +12V, but was looking for other steps should that fail. Details are helpful- I'm a novice!

 

Kusch1991,
Seeing as no one else has replied, and as I have had these things apart recently i'll have a go at answering your questions and offering a way forward. You may want to chack that the cold/hot flaps move freely by popping the ball/socket joints apart off the servo and moving them by hand. The servos do not move by hand, but it is possible to test the motors in the servos by applying 12v to pin 4 and 5 as suggested by Randall. I would try this first to see if they move. If they do the motors are fine. The next thing I would do is take them out and open them up to clean the contacts that are connected to the output shaft, mine had black crud on them. Once clean and reassembled, I connected them up to the loom, without screwing them back into their brackets just to check that they move. If they do move when the temp **** is moved hot to cold then your are ok and everything should work when reassembled.

If the servos still do not move i would suspect the CCU, but you can test for this by finally checking your 2 and 4 servos which are in your hand and can be connected to the plugs of any of the other three servos that do work. If the servos work, then the CCU is not sending the right signals to servo 2 and 4, or there is a break in the wiring for servos 2 and 4.
Good luck, from one novice to another.

Taj.

 

Taj,

Thanks for the words. I have tested the flaps - they move smoothly - and in fact as an interim fix I tied them open so at least I'm not in a freezing, fogged up sports car on the way to work! This weekend I'll try the voltage - first could you describe EXACTLY where to connect the leads? Sounds from above that I would go ground on 5 and +12V on 4, or is it ground on 2 and +12 on 4, then reverse for 5? Like I said, "novice" with a voltmeter...