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ongoing idle drop problem
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Posted by edriggers [Email] (more from edriggers) on Sat, 30 Jun 2001 17:27:09 Share Post by Email
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I have a problem that may be interesting, maybe trivial.

After warmup on my '89 900S (80K miles, manual trans.),
the cooling fan on the driver's side kicks on--I assume
this is normal. Simultaneously with the fan coming on,
the idle drops from a normal 850 to 100, and then oscillates
between 100-900 for the remainder of the drive whenever the
car is idling.

I got out the Bentley manual, read a lot of postings here, talked
to a saab-ophile friend, and did the following:

1. Checked output from O2 sensor. It was bad, so I replaced it,
but problem still there. After replacement, re-ran LH diagnostics
through check engine light: code 12111 (bad mix while driving)
was gone, but 12112 (bad mix idling) still there.

2. Checked for vacuum leaks. Found a few cracked hoses, and replaced.
Still have the problem.

3. Checked the AIC valve for "buzzing," according to Bentley. No
buzzing, so I cleaned out the valve and the throttle body. Still
no buzzing, so went through rest of checks, and all looks right.
Also ran the LH 2.4 component test, and the AIC actuator circuit
was opening and closing the valve properly.

Running the component test brought up the interesting part: the
"automatic transmission drive position" signal code came up, and
reminded me that this car originally had an AT! A shop in California
swapped in a manual (I think from the same year). According to the
Bentley book, there is a switch with connection to the LH control
unit which ups the idle when the engine is in drive. I wonder if
this signal over-rides other idle control signals for the LH unit?
The LH unit seems to think that it still has an AT to deal with
(unless the component test flashes the AT-drive signal for all cars,
even manual).

Should I: 1. Just replace the AIC valve because it is not "buzzing,"
even though it seems to respond to LH signal?
2. Pursue this AT signal to the control unit? I've
considered removing the connection at position 30 in the
LH connector just to see what happens--any thoughts?

Thanks much.
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