Car: 1998 Magna Altera LS 3.0l V6 Auto
This just started happening today. When the car is in drive and I switch the aircon on, the revs drop quite substantially and usually result in the car stalling. With the aircon off, the engine remains running.

I did a test before where with the car stationary, I shift to neutral, then to drive and turn the A/C on. 9 times out of 10 it stalls. Any ideas what this could be? I'm thinking its alternator related but it could possibly be the ISC.

Battery is less than a few months old.

Silvia has a very similar problem at the moment mate, its the alternator for me...pretty sure it COULD be that for you also

Does your ISC buzz when "ON" but not turned over? If so it's ok. PCV valve is the biggy on that age car. Cleans up nicely with a nail, cotton bud and TB cleaner.

Ill give that a go tomorrow, sounds like a plan.

Ill check the ISC tomorrow aswell - so it should buzz when IGN is on but ENG is off?

Ill give that a go tomorrow, sounds like a plan.

Ill check the ISC tomorrow aswell - so it should buzz when IGN is on but ENG is off?
Think there is a test you can do with a multimeter in the gregorys manual to check of the ISC is ok.

Checking the voltage of the battery when the engine is running would give you an indication of whether its the alternator or not I'd imagine also.

Why is the mechanic asking for advice? lol

If it turns out your needing an ISC let me know, i have a few new ones sitting around here.

Checking the voltage of the battery when the engine is running would give you an indication of whether its the alternator or not I'd imagine also.

Why is the mechanic asking for advice? lol

I have a general idea of what it is but I was wondering if it was anything specific, or something that has come up before for someone else.

Fault diagnosis is done best by a specific and targeted approach where you gather information from the problems you have and apply it to the solutions that are known. Far better, cheaper and less time consuming than a shotgun approach or just plain throwing parts at it. :)

I'm going to get my foreman to help me give it a look over. I reckon its also possible that the idle stepper motor is gone and as a result does not up the idle upon engagement of the A/C.

Seems highly likely.

But I'll give that a go and let you know how it goes. Thanks Magna00 for the offer, I'll keep ya posted.

Ill give that a go tomorrow, sounds like a plan.

Ill check the ISC tomorrow aswell - so it should buzz when IGN is on but ENG is off?

When you flick the ignition from OFF/ACC to RUN (without going to START) it should buzz for a bit.

Nope no buzzing, i think its screwed - it stalled without the A/C on this time aswell today.

I had a go at playing around with the idle screw, didn't seem to change the idle speed up much but the revs did appear to drop quite substantially on reverse adjustment. I have also checked the PCV and it was clean.

Even with adjustment, dropping into drive and turning the air con on from neutral with air con off the revs would drop to nearly 400rpm then hunt up and down like it was going to stall, and on occasion did. To me it sounds like the isc on its way out more than anything else, but I'll give it a check over tomorrow with a multimeter. The other strange thing is that the isc does seem to be working at least basically because with the isc unplugged the car idler at 2500rpm but once plugged back in drops to within the 700 +/ -100 rpm acceptable range.

I was thinking of diagnosing following this page; http://www.nardek.com.au/testisc510-10.htm

Car is clicking over? Starter motor

Is their any warning lights on? Alternator

Do the headlights and everything else power on? Above

Make sure the idle is set correctly - ISC

Also make sure all connections are secured, this could also happen if you have disconnected the battery recently

Mum had the same problem in her TJ wagon years ago. turned out to be a fault with the gearbox comptuer. Was replaced and never happened again