928GT8108

During turn on the A/C, the rpm will drop quickly when press the clutch for shift gear. Sometimes will make the engine off due the low rpm. Any location should i check or fix it? Thanks.

FredR

Check that the throttle position switch idle contact is working.

Mrmerlin

has an intake refresh ever been done on your car?
how many miles are on it

928GT8108

Where is the throttle position switch? May i have the part number?

Thanks

928GT8108

The intake refresh at about 2 years ago. The mileage after refresh is about 2000km only.

FredR

On a left hand drive vehicle it sits in the driver side of the Vee to the side of the intake to cylinders 7/8.

A primary check is to place your ear close to this position and listen as the throttle is opened slightly to see if you can hear the switch "click". it is easy to test- look for make/break on the appropriate terminals of the LH computer plug- I do not have ccess at the moment to my manuals to advise the terminal numbers- perhaps someone will chime in on that one.

If the switch is not working the computer will not know the motor is at the idle position and thus will not send actuations signals to the ISV [Idle stabiliser valve- also located under the inlet manifold]. It is also possible that the ISV has failed but less likely.

If the ISV is not working or responding the shuttle inside remains stationary and passes a fixed amount of air to allow an idle of sorts to take place when the motor is warm and the a/c is not used. If the a/c is switched on the compressor places more load on the motor and the rpm's thus drop and can stall the motor if the throttle is not opened.

worf928

Don't worry about the part number yet.

You can test the switch if you have a multi-meter.

Directions are my inspection guide (linked in my signature.)

GeorgeM

Does this only happen with the a/c on when you push the clutch in? If you have your foot off the clutch, does it drop?

For the experts.... Where does the signal come from to adjust idle when a/c is turned on?

Lately, when I turn a/c on, I get no fluctuation at all in the idle. The tach used to flicker just a bit and RPM would rise slightly with a/c on. No more.

Further, my idle has been a little low lately. It also hasn't been settling so well after an RPM drop, i.e. when pushing the clutch in from, say, 1500 RPM or higher. The tach will drop to ~500 RPM, then come up slightly and settle, instead of settling at 750 directly.

The TPS checks good. Intake refresh 14k miles and 4 years ago.

FredR

The control signal for the ISV is generated in the LH computer or so I understand.

Ref your low rev issue see post 6 last sentence.

jcorenman

The AC signal goes to the LH at pin 14, which raises idle when the AC clutch is engaged (from 775 to 875 IIRC, for a GT). This connects to CE panel W24 (0.75 Green/Red) which is the voltage to engage the compressor clutch.

So if the AC works then that clutch circuit is OK.

If the RPM doesn't increase then the problem likely lies within the LH. (That may be the case with the OP's situation also, but check the idle switch first).
I think that AC signal goes through the ceramic "hybrid" circuit, which is suspect if the LH hasn't been rebuilt-- it is a circuit technology which does not age gracefully and will eventually fail.

Louie Ott handles rebuilt LH's for John Speake (JDS Porsche in the UK), he can provide better info on this.
Louie's website is here: https://www.performance928.com/products.html
Contact info here: https://www.performance928.com/contact.html

GeorgeM

Hmm. ISV isn't that old. Got one of Louie's LHs back in 2009.

All this appeared to start after I installed a new throttle cable... the one that goes around the wheel to the throttle body.

Sorry, don't mean to hijack your thread 90GT.

FredR

George,

The whole point of these threads is to share knowledge, experiences of a similar kind, suggestions etc to help resolve whatever the problem may be so something similar/relevant is not what I would call a hijack which [as I understand] is when a subject is introduced that takes the thread in another direction altogether.

Thus if the idle is not working correctly it could be one of several root causes such as when I refitted the inlet manifold and did not push the ISV connector fully home- easy done but quite possibly relevant. Even a new ISV can fail prematurely unlikely as it may seem.

worf928

How did you determine that the Idle/WOT switch was good?

GeorgeM

Bosch Hammer. I've only had it for a few days, so I have a lot to learn.