Here's an odd one.

Working on a 4cyl TBI setup. Randomly (quite often), it has startup idle problems that I believe are related to the IAC being out of position / in the wrong position at startup.

For example, sometimes it might start with the IAC shut down too far and it has problems idling to the point that it will idle well below desired idle RPM from the ECM, yet the IAC is *not* retracted nor commanded to be retracted (verified by monitoring ALDL stream). no idle speed error is set even after a minute or two of idle speed error in excess of 400rpm (commanded 1000, observed 500).

In other cases, the idle rpm will be much too high (around 2000rpm), which immediately sets an idle speed error code, yet again no IAC movement is observved nor commanded.

In either case, simply restarting solves the problem.

Now, onto the interesting point:

this car also has an alarm system with a valet mode. The valet system is enabled when you shut the car off, by: turning the key to the OFF position, then to the run position, then back off. This causes a single alarm chirp and valet mode is enabled. It looks for a brief pulse of power to enable valet mode.

The darn thing chirps nearly every time you shut the car off. It seems to be sensing an extraneous pulse after you turn the car off, even though none was commanded from the ignition switch. (the ignition switch has been replaced with a brand new one).

So, the question is: if there was a brief moment of power to the ECM during it's shutdown procedure (where it's moving the IAC and preparing for the next startup), can this botch the shutdown procedure and cause unpredictable results? How sensitive is the ECM to stray signals on the ignition wire?

Does anyone know what might cause such a pulse in the cars electrical system? Car has been this way for 70,000+ miles, through various alternators and batteries. It's been suggested to me that a diode be placed in line with the alternator to prevent the alternator from "spiking" the electrical system but i'm short on details.

any input appreciated.

 

Is there any audio amplifiers or something that may be storing some "holdup" power when the car is turned off?
Possibly when it finally gives up there is a voltage generated by the collapsing field.
Just a hunch.

 

thanks for the suggestion, hadn't thought of it. I tried unhooking the 'stuff' but it didn't seem to have any effect.

Sometimes the alarm will chirp the instant (and i mean THE INSTANT) you flip the key to "off", sometimes it will take up to a second or so (engine is still windmilling down - a low compression smogger wheezer that idles at 1000rpm and it really will windmill for a bit). Kind of also supports the alt theory because it's still spinning whereas the ign switch etc. are already off.

I suppose I could try unplugging the alternator for a few short startup/shutdown cycles to determine if it is the alt.

will post again when I have more info.