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Starts but stalls immidiately with unaltered AMM
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Posted by m-keith (more from m-keith) on Tue, 10 Aug 2004 22:19:24 Share Post by Email
In Reply to: Re: 87 900S- bogs off idle but with 2 strange twists, JohnK, Tue, 10 Aug 2004 15:21:20
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Thanks for the input. CTS tested good, & all 3 of my AMM's have the same part #. Not sure about the O2 sensor, but being that this problem is present when starting the engine from dead cold, I would presume the O2 sensor to not be a factor. Had a chance to do some more testing this evening, & here's where I'm at.

Cleaned the throttle body & AIC resulting in no change, but they sure needed cleaning anyway.

The following tested good: CTS, AIC, TPS

The AMM failed the burn-off test, & I did not get 4 volts at terminals 1 & 4 after shut-down. This being the case, the Bentley manual instructs to check for voltage at pin 2 & ground at pin 4 of the AMM connector. From here, assuming there is a fault at pin 2 and/or 4, the manual states that there is a wiring problem between the AMM & the control unit. In my case, pins 2 & 4 tested good, & the manual provides no direction if this is the case.

Also, the resistance between terminals 3 & 6 was a whopping 1028 ohms. I believe this is due to the fact that its mixture screw has been tampered with.

Anyway, I have 2 spare AMM's, neither of which appear to have been tampered with. One reads about 630 ohms between 3 & 6, & the other reads 382. The car will not run with either of them. It starts but stalls immidiately. I'm going to use the one reading 382 because that is within spec. Also, it is the one that came off of my parts car, & according to the person who sold me the parts car, it ran well until it had a wiring harness fire. Therfore, I consider this AMM a known good unit. Even though it seems like I'm taking a step back, I now plan to trouble shoot this as a "starts but stalls immidiately" type of problem.

That being the case, I'm thinking I can eliminate the following from the driveability troubleshooting table being that the car will run on the tampered AMM: ignition; system relay; fuel pump relay. The following have already tested good: AIC; TPS; CTS. I don't think it's a vacuum leak because I get a steady 18" of vacuum at idle, & most of the vacuum lines & fitting appear to have been recently replaced. If it is a vacuum leak, it'd have to be both very inconspicuous & very large. That leaves me with an elecrical connection or ground problem, a problem with one of the input signals to the control unit, or a faulty control unit.

Have I overlooked anything and/or am I off track in my approach?

Thanks again,

Michael Keith
Houston, TX

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