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THIS DISCUSSION / SOURCE-DERIVED OVERVIEW
Discussion summary
YOUR RESEARCHYet another AC Thread
Leading reported evidence
Check '02 wiring diagram and saw
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Read evidence and context
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I now checked the '02 wiring diagram and saw that the "AC demand" pin on the DME is not used anymore.
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Suggestion
There's a signal wire going from the climate control module to the DME to tell the DME the demand level of the a/c compressor (PWM signal) so the DME can adjust the idle speed.
Leading evidence: 'Check '02 wiring diagram and saw' (no explicit attributable resolution; this is a reported action or suggestion). Across 1 matching discussions, The most frequently discussed action is 'Adjust idle speed' (1 discussions). For that action, 0 discussions report a resolution and 0 report an unsuccessful attempt. No action has an explicit, attributable resolution report under the current rules. These are forum reports and suggestions, not a confirmed diagnosis.
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Performed · outcome unconfirmed · Original poster
I checked idle speed prior to disconnecting the battery:
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Performed · outcome unconfirmed · Original poster
So I cleaned the MAF and went through the adaptation again.
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Suggestion · Original poster
I checked idle speed prior to disconnecting the battery:
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Adjust idle speed
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Suggestion
There's a signal wire going from the climate control module to the DME to tell the DME the demand level of the a/c compressor (PWM signal) so the DME can adjust the idle speed.
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Suggestion
If that does not correct the issue disconnect he battery for 10 min then reconnect then relearn the idle strategy ie turn key on without starting engine wait untill you hear a series of "clicks" from the engine compartment , that teaches throttle setting, then start engine let idle for 3 min A/C off then 3 min A/C on.
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Outcome without a verified action link
The first time I turned on the AC, RPM jumped to 750 for a few seconds, then back to normal idle (700).
My AC system has the following symptoms (note 2002 C4S):
1. Blows cold air - no problems here.
2. Idle with AC off - ~700
3. Idle with AC on - ~700
Isn't the system supposed to increase the idle when the AC system turns on?
That does sound weird. The idle should be bumped up to about 800-900rpm. However, I'm not sure if it makes a difference when the outside is really cool so not much load on the a/c.
Do you actually see your compressor is spinning in the engine compartment?
Yes. The clutch engages, the compressor turns. It isn't making any strange noises, but the idle is slightly rougher when it is engaged, probably because the idle isn't increasing.
That's the strange part because the idle is controlled by the DME (by changing the e-gas throttle plate angle) and there's nothing you can adjust. Our a/c compressor has variable output, which also means variable load taken from the engine. The higher the ambient temp, the higher the load. The lower the temp, the lower the load (so less drag on the engine). You observed it during a hot ambient temp or cool ambient temp?
I think you could try to scan the climate control module and the DME with Durametric and see if anything pops up. There's a signal wire going from the climate control module to the DME to tell the DME the demand level of the a/c compressor (PWM signal) so the DME can adjust the idle speed. That's the only thing I can think of but it needs some low level diagnostic.
It's just my speculation so don't get your hope too high
However, a technician should be able to back probe the DME (behind the rear seat parcel shelf) to read the PWM signal and see if the signal looks normal.
Actually, no. I now checked the '02 wiring diagram and saw that the "AC demand" pin on the DME is not used anymore. I think the DME and climate control unit just communicate through the CAN bus for DME7.8. The "AC demand" was used on older DMEs (5.2).
I also saw that the DME actually powers the compressor relay so if the compressor is engaged, the DME knows about it. Sorry I don't have further theory about your issue now.
I still recommend scanning the car for error codes, which may give us some clues.
You know what, I just tested my '02. The idle speed barely changed with a/c ON. My a/c has been working fine so I think that's normal behavior. I mentioned 800-900rpm but that was based on wrong recollection.
Do the simple things first. Clean the throttle body around the throttle plate thoroughly. Sounds like it is all working or the idle speed would actually drop when the A/C is turned on. The throttle plate may be actually advancing due to signal from the DME for A/C load but not much increase in actual air flow due to dirt build up. If that does not correct the issue disconnect he battery for 10 min then reconnect then relearn the idle strategy ie turn key on without starting engine wait untill you hear a series of "clicks" from the engine compartment , that teaches throttle setting, then start engine let idle for 3 min A/C off then 3 min A/C on. Should fix the issue, if not more complex diagnosis will be necessary.
Thottle body is clean:
It was also mounted sideways until I cleaned it and mounted it properly.
I will try the re-calibration noted above, and check for codes.
If that doesn't work, my Indy shop has told me to bring it in and they will have a look at the compressor to make sure it isn't on the way out.
So there are no codes in the Climate Control system. There are no codes in the whole car.
I checked idle speed prior to disconnecting the battery:
Cold Idle - 1000
AC off - 700
AC on - 700*
*Note the first time I turn on the AC, it jumps to 750 for about 3 seconds then settles back to 700. Subsequent pushes of the AC button do not result in a changed RPM.
I disconnected the battery for 10 minutes. Then reconnected, turned the key to run. There were clicks from the engine compartment. I started the car, let it run for three minutes then turned on the AC for three minutes. The first time I turned on the AC, RPM jumped to 750 for a few seconds, then back to normal idle (700).
Again, the compressor is working, the air is cold, and it isn't making any noises. Maybe I am being too picky.
Same on my 2003 there is very little change in idle speed with the AC on vs off. Maybe the 996.1 cars are different in that way.
AC works great in the car.
The funny thing is the car ran just fine with the throttle body on sideways. I turned it to the right orientation, I just didn't take a picture.
I was surprised that Porsche didn't offset one of the bolts so that it couldn't be put on that way.
It makes me wonder how many other critical engine components could be mounted 90 degrees to their factory positions and the car still run.
So I cleaned the MAF and went through the adaptation again. It is a little better...is it great? Having not driven many 996s, I can't say if it is good enough.
I note that when I turn on the AC there is a little dip in the voltage, just momentarily. I should have my battery checked...maybe.
