ToreB

Hi guys.
I most likely have a defective CCU in my '90 964 C4. Exchanging it into a replacement unit fix the problem and everything works OK.
But, a replacement CCU unit is costly, therefore I try a post here before doing just that.
The issue is as follows:
Setting the temperature control to anything higher than minimum on the CCU starts the A/C condenser blower in the front fender intermittently on max and off. The fan blows max in one second, and off the next.
Also, the two heater air regulator servos in the trunk goes to minimum heat and "pulses" a little back and forth in the same manner.
If i pull out the A/C condenser fan relay in the trunk everything goes to normal behaviour, exept for no A/C of course. Both heat servos then opens to normal positions, and heating regulation works OK. Relay is OK, same behaviour with a new relay.
All other functions in the CCU, upper and lower air flap controls as well as blower fan speed is OK.
I've checked and resoldered most solderings inside the CCU, no change.

I've been wondering about these things:
Maybe there's a feedback signal from the A/C condenser blower/relay that fools my CCU into doing this to me?
The normal A/C relay current may cause an internal fault in the CCU?
Maybe a faulty grounding of the CCU?

Maybe someone have seen the same behaviour and have some tips?
I've already been reading a lot of posts here on the forum as well as other recources as http://www.porsche964.co.uk/technical/electrics.htm
Thanks,
Tore
Oslo, Norway

AlfredF

Dear Tore,

I have exactly the same problem as you describe.
Did you found out what the cause was to this problem?

I'll hop to hear you soon,

Thanks,

Alfred
the Netherlands

ToreB

Hi there Alfred. I found the cause, a defective driver IC inside the CCU. I was able to identify the component due to very slightly brownish/burned circuit board below the Siemens IC.(there are at least four there)
I was very lucky to have a spare CCU circuit board available, changed the IC, and Bob's your uncle..
The component in question is pictured and mentioned here:
http://www.porsche964.co.uk/technical/climate.htm

Best regards,
Tore, Oslo, Norway

AlfredF

Thanks tore,

I'll check this tommorow.

Greetings,

Alfred, the Netherlands

mojo747

I'm going to be pulling mine soon. Do the cruise and wiper stalks get in the way?

AlfredF

After reading another topic on this subject I found a solution from Rollingjack to bypass the rear blower relay
See: https://rennlist.com/forums/964-foru...***-box-2.html

I remove the cover and suddenly I saw that the plug from the rear blower was disconnected

After connecting the plug my problem with the CCU was over

LouZ

I was just getting ready to write this exact response! I had a similar problem last year, and sure enough, I found that the plug to the rear blower was disconnected! Popped it back on, and all is well!

LouZ

No, You need the "radio removal tool" (bent coat hanger) to release the unit from the dash, and it will slide out without much trouble.

mojo747

My unit has fan control but not temp control. It is heat all the time. It bleeds through the vents on the highway and cooks you. It blows hot from the center and right vents, right floor and defrosters, but cold on left side vent and floor. ??? The shop said I need a new CCU. Any thoughts?

ToreB

I'll say your shop is wrong and you have a defective heater flap servo on the right side. It's placed in the footwell behind the side wall carpet cover. Remove the cover and look. If the actuator arm is stuck when you regulate the heat you have identified the problem. You can compare this with the servo in the other footwell. I'll bet that is moving normally...
Post the result here mate.
Regards,
Tore
Oslo, Norway

mojo747

I'll check it, but the left side is cold all the time. I can change where the air flows, ie. floor, vents, def. Just not the temp.

AlfredF

100% sure that it's one of your servo's.

Look here: http://www.porsche964.co.uk/technical/servos.htm

Succes, Alfred

mojo747

Thanks. I just printed off that article. I'll be spending this rainy Sat. under the boot. Wish me luck. I'm thinking both servos for mixing are out. Driver's is stuck on cold, passenger on hot. Time to start wrenching
I'll update.

Nine11

Mine was blowing hot all the time in all directions. Didn't matter how you set the airflow or the temp, it was my CCU. I got a new 993 unit.

mojo747

Got the passenger servo out. Took it apart and it had some scorching on the little springy fingers and a melted spot on the board where the scorching occurred. Put juice to the servo motor and it is good but the board appears to be bad. It will only rotate to the scorched place and then stop. I haven't taken out the driver's side servo yet. Here are some awful phone pics.