Thom

Hi all,

I searched the archives but nothing truly meaningful came up.

When I switch the air con on, the motor vibrates for something like a second, during which the revs decrease. Those vibrations are exactly like those typical of a worn out clutch disc (clutch judder when engaging first gear).
It happens ONLY on idle and periodically, that is each time the air con compressor is engaged - when the "click" somewhere behind the dashboard can be heard.

While (sort of) normal that the revs decrease because of increased effort put on the motor, why is there such vibrations when just the air con is on ?
I'm thinking of worn out motor mounts but I have read here of vibrations related to the alternator bearing.
The TB and BB are ok.

Any idea on the subject truly appreciated.
Many thanks in advance.

Chris_924s

If the AC bearings are going you can check movement by wiggling the pulley with the belt off. My AC compressor never made noize, the clutch just seized throwing the belt. My Alternator squealed like a SOB when the bearings were going/ overloaded by belt tension.

retension the AC/ALT belt to spec then run. then loosen the belt up a bit and see if the vibration goes away.

Motor mount failure you will feel in the steering wheel (Ask me how I know this..)

HTH-

Dave in Chicago

My '88 did the same thing, as does my '92 (both cars have perfectly operational A/C). I have considered it to be pretty normal in these cars.

hosrom_951

My NA and Turbo does that along with another guy here who rebuilt his 968, so dropping of the revs and slight vibrations should be normal.

Try adjusting the idle first http://www.clarks-garage.com/shop-manual/fuel-08.htm

That is IF the car is operating normally

Peckster

The load is greatest while the AC is starting up. Maybe your compressor is on its way out, but it doesn't sound like a major problem to me, especially if it goes away.

Revs drop slightly for a sec on mine, but it's barely noticable.

Thom

Thanks for the replies, guys.

There happens to be "some" vibrations in the steering wheel along with those AC-related vibrations but they have been there ever since I bought the car 3 years/40,000 miles ago. Both don't seem to have got any worse with the miles being piled on.
Moreover, I drove yersterday a '91 951 with everything new under the engine hood, and there are absolutely none of those vibrations when the air con is being switched on.

I'll have a look at the AC bearing before replacing the engine mounts.

Thanks again for the replies.

Roger Collins

I had a similar problem on my 85.5 NA. The way is works for this model is: when the compressor kicks in, it sends a signal through the A/C relay to the DME to compensate (raise) the idle. The DME then sends a signal to the idle control valve to raise the idle. Sometimes the idle control valve is gummed up and doesn't work correctly, or it has a vacuum leak. In my case, the pins on the terminals of the DME unit which is supposed to send the signal were dirty. I pulled the unit, sprayed with cleaner, plugged it back in, and solved the problem. Hope this helps.

Thom

The motor mounts were replaced yesterday.

The exhaust side MM was completely shot :





When switching the air con on, the vibrations are now 1/4 as intense as they were before, which feels a good improvement.
The intensity of gearshift vibrations has significantly reduced too.