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Engine slows when AC is switched on

earthworm on Fri May 30, 2008 12:46 PM User is offline

Year: '96
Make: Saab
Model: 900S
Engine Size: 2.3
Refrigerant Type: R134a
Ambient Temp: 70 F
Country of Origin: United States

Please help settle a discussion.
I hold that a signal is emitted from the AC compressor system to the ECU causing a slight increase in RPMs to compensate for the compressor drag..

But I have no idea how this works.

Some on Saab Central wonder why their engine slows so much when the AC is engaged, others, including me, barely notice when the AC is on.
Please advise.

'96 900S
'97 Honda, air is inop

bohica2xo on Fri May 30, 2008 1:33 PM User is offline

On SAAB models with Motronic or Trionic ECM's that use both TPS & IAC units, the ECM does indeed get a signal calling for a compressor start.

The drop in idle speed is not acceptable, and is caused by one of several things. The ECM is running out of authority to correct the low idle speed, probably from one or more of the following:

1) Mis-adjusted throttle plate setting, reducing IAC authority.

2) Dirty throttle body, restricting airflow at idle setting.

3) Dirty or damaged IAC unit, limiting range of compensation.

4) air leaks in the IAC circuit.

Inspecting the throttle body should be step one. A buildup of just .010 inches of crud at the edge of the throttle plate can affect idle. Clean the throttle body before making any adjustments. If the throttle body is filthy, the IAC is as well & should be cleaned.

After everything is clean and you have checked for any IAC or TPS codes, you can adjust the throttle plate for maximum IAC authority. Factory emissions specs want the IAC range centered, so it can compensate for a split hose or other big leak. This reduces it's range of compensation for loads like the A/C. I have found that setting the throttle plates to allow more IAC authority improves cold operation & keeps the idle in range with the A/C on.

Setting the IAC for maximum authority should be done on a warm day, with the engine at operating temperature. Using a sensitive tachometer, let the engine idle with the A/C off. Carefully increase the throttle plate opening about 1/4 turn of the screw and check the tachometer. The idle RPM should settle back to the ECM spec after adjustment. keep adjusting until the IAC / ECM can no longer reduce the idle speed to spec. Then back off 1/4 turn and check the idle speed. Blip the throttle a couple of times, and make sure it returns to proper idle speed. After you have set the throttle plate check the TPS for proper adjustment, moving the throttle plate setting affects the TPS adjustment.

I have owned several 900's, and just recently sold my 900SPG. I have been through this many times. The same procedure works on most MAP / TPS systems as well, like most of the OBDI ford stuff...


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

earthworm on Fri May 30, 2008 1:46 PM User is offline

Thank you, sir, for a quick and informative reply.
Generally, I consider AC to be AC, regardless of the make of vehicle.., I have no qualms in referring people to this site - it is that good.

'96 900S
'97 Honda, air is inop

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