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Old 12-12-2018, 07:37 PM   #1 (permalink)
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Annoying Saturn ECU Behaviour

I haven't posted in a while, but the contest announcement has inspired me to do so. :-)

For those who've followed my 2001 Saturn Mod thread, you know that I've installed kill and restart buttons. The restart button is for convenient restarts at lights (at speed, I use the clutch partial-release, press, release technique another poster suggested for smooth restarts with minimal wear).

Anyway, to get to the point, one problem I've had ever since installing the kill switch is that after using it for a day or so, the ECU starts to get the idea that the car is prone to stalling. What it does to compensate is keep the idle high, sometimes very high. I can be driving along at say 1800 rpm, and when I release the gas and push in the clutch to shift up or down, the engine revs up to 2500 or so. This works out reasonably well when downshifting, but it's annoying when upshifting.

I spent a long time scratching my head, reading various Saturn forums, replacing my TPS (although the readings I'm getting from Torque suggest all sensors are working properly), all to no avail. As a software guy, I finally realized the problem is a bug, or rather an unintended consequence of making the engine operate in a way the ECU designers didn't account for.

If I stop using the kill switch for a day or so, the problem goes away, as the ECU relearns that the engine is fine.

Has anyone else seen issues like this? Found any solutions?

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Old 12-13-2018, 11:24 AM   #2 (permalink)
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First question: does it throw any DTC codes?
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Old 12-13-2018, 11:34 AM   #3 (permalink)
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I don't think so. It certainly hasn't turned on the SEL (and I did have a different problem in the summer, P0410 caused by a clogged pipe, that did turn on the SEL; there were no other codes besides the one describing the problem). I'll scan it again when I get the chance.
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Old 12-13-2018, 12:17 PM   #4 (permalink)
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Well that’s certainly no good, but I can imagine that being a result of making a kill switch... perhaps alternate between the kill switch and cycling the key?
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Old 12-13-2018, 01:44 PM   #5 (permalink)
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The problem with cycling the key is that in the day time, the day time running lights turn off when I do that, so oncoming drivers think I'm flashing my lights at them. Cycling the key also disables the airbags for about 10 seconds, and there's the risk of turning the key too far and locking the steering wheel. Those are the reasons I installed the kill switch in the first place.
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Old 12-13-2018, 02:41 PM   #6 (permalink)
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Turning the key to the accessory position will kill the engine without letting you lock the steering wheel. Not sure about itís effect on lights or airbags. Kill switch is definitely safer though.
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Old 12-13-2018, 02:56 PM   #7 (permalink)
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Originally Posted by Daschicken View Post
Turning the key to the accessory position will kill the engine without letting you lock the steering wheel.
But as I said, there's a danger of overshooting the accessory position and ending up in the off position. Been there; done that.

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Not sure about itís effect on lights or airbags.
Turns them both off on my car.
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Old 12-13-2018, 09:37 PM   #8 (permalink)
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Just a weird thought. Could the IAC be partially open upon shutdown? This could be causing the high idle. ....
Try removing it, extending it fully then reseating it, then leave it unplugged.
Just a thought.
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Old 12-14-2018, 08:31 AM   #9 (permalink)
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Does the ECU control the lights? How did you wire your kill switch? Assuming the lights are just on with the key on (through relay and whatnot) and you just kill power to the ECU it should turn the car off while leaving the lights on and an electrical circuit will keep you from overshooting the acc. position. It doesn't solve the airbag problem though.
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Old 12-14-2018, 08:05 PM   #10 (permalink)
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I'm wondering if there's a push-button way to simulate momentarily moving the key to a lower position that will kill the engine.

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