NP993

I think my CCU is acting up. The fan will only go on high-speed with the temp **** turned all the way to cold. As soon as you turn the temp **** to anything other than full cold, the fan goes down to about a quarter speed. Adjusting the fan speed **** does nothing. In other words, there's no way to get heated defrost with an effective fan speed. Anyone else had trouble like this?

quasr

Not sure about the symptom but my fan speed control quit working all together. I sent it to this place: www.autoecu.com and should get it back by mid-week. They rebuild the unit and replace parts that are either bad or need upgrading to current tolerances (their words). They charge $249.00 for the service and do the work in 2 days. I found a couple of places in CA that do the same service but at a cost of $500 or so. Will keep you posted on the results. They say it should be plug and play on return.

Lorenfb

"The fan will only go on high-speed with the temp **** turned all the way to cold. As soon as you turn the temp **** to anything other than full cold, the fan goes down to about a quarter speed."

That's a normal condition, i.e. When the rear blower (or it's circuitry) fails,
then the front blower only operates normally when the temp **** is all the
way counterclockwise (cold).

There're many CCU modes of operations that most don't understand and assume
that the CCU is bad. This is also the case for many Porsche shops. The 964/993
CCUs are very reliable and rarely fail. Because of this, many waste time and
money assuming the CCU is bad and have it unnecessarily "rebuilt" by some
who also don't fully understand the CCU's functionality.

NP993

Thanks Loren -- in my case, I have an RS heater bypass tube in the back, and the factory blower motor is removed. Is there a way of fooling the CCU into thinking that the rear blower motor is working just fine?

Lorenfb

"Is there a way of fooling the CCU into thinking that the rear blower motor is working just fine?"

Yes, the CCU needs to "see" a feedback voltage (+12) from the blower relay.
This can by accomplished by jumping G35 (#15) to pins G1 & G19 on the back
of the larger connector (35 pins).

Wilder

I bought my car with the RS bypass already installed and a working CCU. A few months ago the CCU started failing with the exact same symptoms as Noah's CCU.

I followed your instructions and it didn't quite work for me. After jumping G35 to G1 and G19, it worked but when I removed the key from the ignition, the ignition lights and door chime came on and stayed on. Through trial and error I made it work by jumping G35 to G1 only. It worked for about half an hour then reverted back to the original symptom. It just seems to work arbitrarily. Could my problem be in the circuit board in the CCU or am I missing something? What confuses me is that about a month ago we took out my CCU and tested it on another car and it was working fine.

I've sold my car and he wanted to take delivery right away but I agreed to fix the CCU at my time and expense. Any help would be appreciated.

Lorenfb

"After jumping G35 to G1 and G19"

Jumping like this should not be done, i.e. It's always best to fix the problem properly
and not Mickey Mouse the wiring to solve a problem.

Pin 35 is the 12 volt supply to the CCU and should not be connected abitrarily to other pins.
Pin 19/1 are the blower motor supply voltage feedbacks.

Most likely the problem is in the engine compartment and not the CCU.
It's related to the rear blower and/or its supply voltage.

NP993

Okay, I'm more confused than ever. First you said:

Now you say:

Wilder

x2 on being confused.

I would think that replacing the heater fan with a bypass is a modification for any 993 but an RS so I don't know that there is such a thing as a "proper fix" aside from fooling the system into thinking the heater fan is still in place.

I tried "fixing" the problem by installing resistors in the wiring of the heater fan located in engine compartment as instructed in the link below but it didn't work for me.

http://www.porsche964.co.uk/technical/noheater.htm

I then did a search and came upon this thread with your instructions, which I followed to a tee.

Lorenfb

"Jumping like this should not be done."

That's correct! And only used for non-factory installs or for troubleshooting.
Less not compare different cars and installs as part of other's problems
and diagnostics.

Time to fix the problem right!!!!

Wilder

Is there anyone who can be so kind as to help with this problem? I'll start at the begining...

The car is a 98 C4S with an RS heater fan byass tube. The CCU worked well despite the RS bypass tube for some time until one day it arbitrarily decided to stop working. My symptoms are exactly those posted by Noah at the top of this thread. As per my post above, I tried the fix at www.porsche964.co.uk as well as the fix originally suggested by Loren. Loren's instructions worked with limited success.

Does anyone have any clue as to how I might be able to troubleshoot this problem?

Lorenfb

"Does anyone have any clue as to how I might be able to troubleshoot this problem?"

First, explicitly define the CCU problem and avoid generalities, e.g. "CCU doesn't work.":
1. What exactly doesn't function?
2. What does function?
3. Have all the fuses (front & rear) been checked?
4. Have all the connectors been checked and cleaned, e.g. blower amplifier?
5. Have all the servo actuators been monitored for their functionality?
6. Does the CCU get the "X" power from the ignition switch?
7. What mods have been made to the wiring?
8. Have the voltages (#30 & #15) been checked with a voltmeter?
9. Has the CCU been FULLY tested in another vehicle?
10. Have all the sensors been tested and connections checked?

Remember, using the "shotgun/throw-a-dart" troubleshooting approach rarely if ever yields
results especially with complex ECUs such as the 993 CCU.

Wilder

Loren,

Thanks for the reply. Please read below.

So, to answer your questions:

1. What exactly doesn't function? See above for detailed description
2. What does function? See above for detailed description
3. Have all the fuses (front & rear) been checked? Yes, all working
4. Have all the connectors been checked and cleaned, e.g. blower amplifier? No they have not but see #9
5. Have all the servo actuators been monitored for their functionality? Sorry, this is beyond the scope of my knowledge. Can you expand on this and explain how to check for this?
6. Does the CCU get the "X" power from the ignition switch? I'm not sure I understand but I can say that the CCU has the original wiring. Can you expand on this and explain how to check for this?
7. What mods have been made to the wiring? The ones you suggested above
8. Have the voltages (#30 & #15) been checked with a voltmeter? No.
9. Has the CCU been FULLY tested in another vehicle? Yes, it works fine.
10. Have all the sensors been tested and connections checked? No.

rob964.c4

I have a 964 so don't know if this wil help

I'm not sure but I forgot to put back the plug witch connects to the fire board and mine ccu didnt worked anymore only at full speed and hot air.
You can see the plug on the foto.


robert

mr_bock

It seem that this thread started out well and then withered....

The BIG Question is How To Fool The CCU when the rear blower is no longer installed?????????

I don't get the 'Fix It Right' remark as The Rear Blower Is Removed and Will Not Be Reinstalled!!!!

Any real fix for this situation Please!!!! No fault lights, No fault codes!!!!!

Many Thanks to the person who can answer this.