[deledink]

I know that a lot of carb'd cars have a device that increases the throttle when the AC is switched on, but what about on a 2.5 TBI? Is there some type of circuit that ups the idle speed a bit when you hit the AC? On mine, the rpms seem to drop down enough at idle to slow down the blower motor and drop the ammeter gauge, but if you give it a little gas, the gauge goes up and the fan speed goes back to normal.

Also, vacuum reading with AC on goes from 18 down to about 15, but that might just be due to the rpm drop. I think the idle dropped down to about 600 rpms

 

[Bob Lincoln]

The AIS motor (Air Idle Speed) on the front of the throttle body is driven by the computer to speed up the idle just milliseconds before the A/C compressor engages, to prevent bogging down and stalling. If the idle is normal under all other conditions, it's possible that the mechanical load of the compressor is increasing as it wears out, and drags down the idle due to increased friction. So the AIS may not be able to overcome the drag.

Normal idle is about 650-700 RPM in drive for these cars, with automatic transmission. With manual transmission, it's about 1000 RPM.

 

[deledink]

Thanks again Bob. The compressor is new, so I'm thinking it should be ok. Haven't rebuilt the throttle body yet, but just wanted to get an idea of how the TBI setup compensates for idle drop when the AC is switched on, as I didn't see anything obvious to my eyes (I'm used to working on carb'd engines and this is my first time working on a fuel injected engine, so I'm still learning). Wonder if the alternator not producing enough voltage at that particular rpm could cause this?

 

[John Wood]

Thanks again Bob. The compressor is new, so I'm thinking it should be ok. Haven't rebuilt the throttle body yet, but just wanted to get an idea of how the TBI setup compensates for idle drop when the AC is switched on, as I didn't see anything obvious to my eyes (I'm used to working on carb'd engines and this is my first time working on a fuel injected engine, so I'm still learning). Wonder if the alternator not producing enough voltage at that particular rpm could cause this?

It shouldn't be the alternator at low RPM's since the battery, assuming it is good, would maintain the voltage.

I had a similar problem with my Dodge a few years back and cleaning the AIS motor of all carbon (including the seat) and re-sealing the TB to the intake manifold helped. Also, using aerosol carb cleaner, I cleaned the TB throat and the throttle plate of all dried up build up (a little carbon and maybe some gum from dried gasoline).

 

[Bob Lincoln]

Thanks again Bob. The compressor is new, so I'm thinking it should be ok. Haven't rebuilt the throttle body yet, but just wanted to get an idea of how the TBI setup compensates for idle drop when the AC is switched on, as I didn't see anything obvious to my eyes (I'm used to working on carb'd engines and this is my first time working on a fuel injected engine, so I'm still learning). Wonder if the alternator not producing enough voltage at that particular rpm could cause this?

The AIS motor moves a plunger in and out of an air passage that goes under the throttle plate, so it changes the mixture slightly to change idle speed, by allowing more or less air to bypass the throttle plate. That passage will be visible on the underside of the throttle body and through the AIS mounting hole. It's often clogged with carbon and gas deposits, and as John said, a good spray cleaner such as Gumout or CRC will remove them.

 

[deledink]

Pretty sure the battery is good. Haven't had any reason not to think so.

Thanks again for the good information!

 

[deledink]

Had the battery tested and while it was good enough to start the car, it had a shorted cell, and would only charge up to 10.4V. Replacing the battery fixed the problem and now the car idles normally with the AC on and blower speed and gauge reading remain constant

 

[Bob Lincoln]

Excellent.