My Car has been doing this for some time....

Sometimes when I activate the Aircon , switing it to 'Normal' the engine will stumble momentarily, if I do it when the revs are low, for example
whilst sat at lights it can eveb cause the engine to stall. As I have to do this frequently to prevent over heating it was getting to be a pain
and as there is no refrigerant gas in the system anyway I thought that I might just as well disconnect the compressor to prevent the problem
as I had assumed that it was due to the AC pulley placing a load on the engine.

I removed the AC pulley belt and disconected the wiring from the compressor.

To my surprise that has had no effect, the car STILL stumbles when I engage to 'Normal' switch.

Can anyone here explain what is happening ?

the A/C running should signal the CPU to up the idle
so TPS aka IAC plug on the side of the TB

I have some luck pulling of the connector and giving it a bath in CRC or WD-40 on both sides
if that fails try carb cleaner inside the TB

then look for bad wires or connectors up the chain a/c to CPU back to the IAC

next you get to buy bits like a IAC

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Question wonder and be wierd
are you kind?

[This message has been edited by ray b (edited 08-17-2011).]

you turn on the A/C to trigger the el fan to cool the car ?

fan should trigger if the car gets real hot [225+F] by it's self

maybe a short in the fan wires is causing a voltage drop
as you unhooked the A/C bit

also could be bad ground wires
or a weak battery or alt connections

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Question wonder and be wierd
are you kind?

quote Originally posted by ray b: also could be bad ground wires or a weak battery or alt connections

That's probably it.

87 and older Fieros had the old style SI alternator, those didn't put out much to begin with and then you're adding a huge electrical draw with the AC, even with the AC dissabled, that still allot of juice. You're going to need to check out the electrical system, after all, it's 25 years old, start by having the system load tested, battery stores or auto parts store can do that. Look at cleaning and or replacing the grounds and battery cables. Check and clean the main alternator feed up to the main electrical terminal then up to the battery. The main feed is often overlooked, it is important to check it all out.

When my alternator went bad I upgraded to the 88-Fiero CS alternator which is a plug n play replacement, that made a huge difference, car ran much better, didn't have idle drops like before. If you already have a CS lalernator or your car is an 88, definitely look at all your electrical connections as everything else is a moot point until that is up to par.

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jetman
Silver 86 SE 2M6 4-speed, with "check wallet light"
Now fortified with 8 essential slices of bacon goodness

An electrical problem certainly makes sense, but during the rebuild we replaced all the wires, added good grounds, a reman alternator which I think its the old style because it was advertised as for an 86 model was installed, and it has a good quality new battery (relocated to front).

Still, activating the a/c switch does cause a significant voltage drop as the lights will dim momentarily at the same time as the engine stalls.

What is most puzzling is that as well as the momentary stumble, the car definately looses some power with the a/c switched on the throttle is noticably less responsive and acceleration and power are reduced. It actually feel like the car gains several hundred pounds when the fan is running ! All this and remember the air compressor is not connected (no belt).

Could an electric drain in the fan system be the problem, could this cause the drop in performance ?

one or two wires to the relocated battery ?
single wire can cause bad ground voltage drops

buy a cheap VOM and check for voltage drop when the fan is running
jumper wire the fan and see if the drop is still there

Likely bad ground.
Google/youtube Search voltage drop
See https://www.fiero.nl/forum/Forum2/HTML/117863.html

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

I have added one long wire between the positive terminal of the battery and the starter motor.
The negative battery terminal is grounded in the front compartment near the windscreen washer fluid tank.

All the grounds, wires and conections are good, there is only a 0.1v drop between the battery and the C500 connector.

I will check the fan operation in the morning to see exactly what the current drain is when its engaged.

bingo

add a second huge ground battery to motor

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Question wonder and be wierd
are you kind?

quote bingo add a second huge ground battery to motor

I might give it a try, but to be honest it already has one huge ground, the space frame.
And there are three very good grounds from the space frame to the engine so i'm not sure how much it would help, but still
its easy enough to install so I suppose it wouldnt hurt.