Oilslick964

I've done everything I know to do with this and it still isn't right. Can anyone please help me?

1989 C4. heater would not heat. I removed the servos and cleaned/checked them. They were working fine when I run power to them. But when I change the heater controls at the CCU, only the right(passenger) side would change. I switched the motors and got the same thing. Only the right (passenger) side would change. After much tracing and checking wires, I concluded that it has to be in the CCU. If I run hot power to the servos, they both work. If I run hot power at the plug receptacle for the CCU, they both work. But when I use the CCU to change temperature, only the right (passenger) side works.
Even if I run the drivers side to full hot and then turn the key on, the drivers side will move back to cold.
So, a very helpful fellow Rennlister sold me a CCU. I plugged it in and guess what? Exactly the same thing. No change whatsoever.

I am at my wits end. Has anyone ever experienced this? Is is possible that both CCUs have the exact same fault? Is there anything I am overlooking? For now, I just ran both to warm and unplugged the drivers side so it will stay on warm. If I get too hot in the car, I'll just close the vent and roll down a window but I'd really like for this to be right. Please help me!!!

Indycam

Is it april 1st already ?

"Is is possible that both CCUs have the exact same fault? "
Is possible but very improbable .

Cross wired ? That all I can come up with .

Oilslick964

Well, I thought about that but wouldn't it just work in reverse? This one, if power is to it, regardless of where the servos arm is, when you turn the key on, the arm moves to full cold. If the ccu is on heat, the servos arm moves to full cold. If the ccu is on full cold, the servos arm moves to full cold. After that, the ccu has no effect on the servos arm on that side. It just stays on full cold untill you run hot power to it.

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JasonAndreas

Have you checked the resistance level of the left mixing chamber temperature sensor?

Oilslick964

You have surpassed my ability to comprehend...

No, just kidding. I understand what you are saying but I have absolutely no idea how to go about doing that. Do you know how? Is it something a normal, average guy like me can do?

KirkF

If it was me I wouldnt try and diagnose this system without a bosch hammer. With it you can check for ccu faults, open and close all the servos, start and stop all the fan motors and see the inputs to the ccu.

Just my .02

Kirk

Colin 90 C2

Paul,
Did you check out
http://www.porsche964.co.uk/technical/servos.htm

Seems Rory had the same problem as you. I did not read the entire article but it may help.

Colin

darth

Is it possible to swap the wires in the connector between the working servo and the one that isn't working? If the driver side servo starts to work then one can conclude you have 2 CCU's broken in the same (i.e. what are the odds) way or at least it may lead you to the problem.

Just a suggestion
Bill
90C2

Oilslick964

Colin,
yes, I took out the servos and they are operating properly.

Bill,
I did that too and the left (Non-working) servos works perfectly when it is hooked up to the right side and the right (working) servos does not work when connected to the left side. Do you think it could be the same problem on both CCUs?
If so...Colin you wanna buy a CCU back? ha ha!

darth

Paul

Like I said what are the odds - I guess the only thing I could suggest is another CCU or intall the ones you have in a friends car and see how they react. Or if you're elctronically inclined, reverse engineer the CCU board. There is one side of the circuit which works and you can use for comparison. Even doing a visual inspection for burnt components would be a first step and if that doesn't reveal anything a good multimeter with a diode checker is all you really need to check the resistors, diodes and transistors on the board but you may have to unsolder at least one lead of a component.

Sorry I couldn't be more help
Bill

JasonAndreas

If you have a multimeter or ohmmeter you measure the resistance between pins #18 and #23 on the back of the CCU. You're looking for;

0C/32F = 30.6 - 34.7 kOhms
25C/77F = 9.5 - 10.5 kOhms
50C/122F = 3.4 - 3.8 kOhms

The CCU uses the difference between the inside temperature sensor at the back of the CCU with the mixing chamber temp sensors and compares that with the temp selection **** on the front of the CCU to figure out how much the mixing chamber servos should be opened.