andrew911

Well, I have seen a plethora of HVAC questions lately- must just be coincidence that a few of us are trying to get our HVAC straightened out. Anyway, this weekend I was looking over my system, and and still confused. Here is what happens- when I turn my temperature setting to the red dot, the heat should go on. What happens is I hear a fan going on in the rear, AND the A/C fan under the left front of the car turn on for a few seconds, then both cycle off. This happens continuously, until I turn the temp switch back to the blue dot...and no heat!

This weekend, I removed the cover of the servos, and found the two servo motors for feet and windshield control to be working fine. What was happening is the two servos that operate the heat valves were moving a little, then stopping and returning to the fully closed position in time with the cycling of the front and rear fan motors. My mechanic said maybe the insulation from the A/C line that runs by one of the motors was catching the motor and sending it back, so I gently removed the insulation (I ripped off the whole 10 inch or so piece of it- I'll fix it later and be sure there is clearance for the servo arm). Anyway, this did nothing to help the problem.

With all that said, here is my key question. I have recently replaced my HVAC control unit with a used unit (I believe from a 993, but "supposedly" the model number that would work with the 964), and it controls the fan and A/C fine. Is it possible that the HVAC control unit is defective, or is it the servo motors which also often go bad? I was thinking the servo motors, but both of them cycle back in forth at the same time (even though I don't think they're connected) AND the A/C fan up front cycling on and off at the same time is a little odd. What should I do- I'm as confused as my poor car... When I dis-connected the lever from the servo and pulled it out, heat was coming out, so I know the heat itself works if I could just get the computer or servo straightened out

Randall G.

Hey Andrew,

What's the suffix on your "993" CCU? If it's -01, it's for a 95-98 993--I have no idea if it will function properly on a 964. If the suffix is -00, it's the updated 964 CCU. The part number should be 993-659-047-00.

When you say A/C fan, do you mean the condenser fan in the left-front fender? If it's cycling when you take the temperature control **** to red-dot, you definitely either have a CCU problem or a wiring problem of some sort. Of course, that's assuming you don't have the AC on. Even then, the fan should run continuously--it shouldn't cycle.

Adrian

There is also another thing. Model year. If you have a 1989, 1990 or 1991 built prior to July 1990 (USA only) these 993 units will not work. A wiring modification has to be carried out. If you have a RHD conversion of same models listed above this gets even harder.
Ciao,
Adrian
964C4

andrew911

I'm sorry- I should have specified, I have a 1993 LHD C2. I was told the unit had the "00" suffix, however being as it was a used unit, the sticker magically disappeared. I will call the people I bought it from to ask. I thought if the unit was "01", it wouldn't work at all- mine controlls the A/C, fan motors, and windshield/feet adjustment fine- its only with the heat that I have the issue. It does seem odd though- the A/C condenser fan is cycling, even though the A/C button is not pressed...

andrew911

By the way, I also have my original 964 unit...would it be worth a try to have someone "rebuild" it, since I have it out of the car anyway, and then just swap it in? Is that expensive? It sounds more like the HVAC control unit than the servo motors I must admit...

Randall G.

Hey Andrew,

Did the A/C condenser fan cycle with your old CCU? Can you install it long enough to test? What caused you to replace your old CCU?

Hate to say it, but a fan (A/C condenser, in this case) running on its own volition sounds like a control (CCU) problem. However, I would try not to spring for a new unit until you have an opportunity to try a donor CCU long enough to see if it fixes your problem.

With respect to rebuilt CCUs, Ottos of Venice does this--advertises in Excellence Magazine. I don't know how much they charge.

Best of luck, and let us know what you find.

andrew911

Well, the original HVAC unit that was in the car was constantly running the small fan motor by the fan switch- the tiny fan that runs for a few minutes after the car is turned off would not stop running on my original unit, and this was causing the battery to drain in a day or two. That issue would have to be fixed on my original unit in order for me to use that one again. I don't recall any problems with the A/C cycling at that time...

If I remember right, the heat was working when I bought the car in November too- I drove the car home & drove it for a weekend, then the car was stored until the spring so I never had occasion to try the heat since replacing the HVAC contol unit until the other day when I noticed this.

By the way, I'm hoping the HVAC control unit is what people are also referring to as the "CCU"...please tell me it is and that this HVAC system isn't even more needlessly complecated than it already is!

Adrian

Dear Andrew,
They are the same. CCU. HVAC, aircon and heating control unit. I would suspect you have a late model 993 unit and this is th eproblem. The only way to prove this is to install the correct unit or have your existing unit repaired. Maybe you could borrow one from a fellow rennlister who is nearby or install your unit in their 964.
What I also could suggest is that you remove the power to the temp sensor fan on your old unit and reinstall it. This will not cause you any major problems. Then check if the problem with the heating system still exists. This should be easy for you to do.
Ciao,
Adrian
964C4

andrew911

OK- I'll have to see if I can remove the temp sensor power from my old unit, or have my old unit "refreshed/rebuilt". I would try someone else's, but it is kind of a pain to get these units out and in- took me 45 minutes (and I think I got lucky in finally getting the rear connection to take), and I'd have to do that with 2 cars plus go borrow the removal tool from my mechanic again. Well, I'll talk to some people tomorrow to get a price on a rebuild.

I guess its possible that the A/C and the other functions would work properly and only the heat function is messed up when using the improper 993 "01" unit, but I would have thought that all functions would fail with the wrong HVAC control. Its tough to say...