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Losing computer connection on start up

1K views 10 replies 6 participants last post by  imps 
#1 ·
So I haven't posted here in a long time and if anyone here is part if ACMR, you'll probably see my post about this there if you haven't already.
My issue is that every time I start my car, all the electronics shut down. All the lights, guages, electronics, everything. The car starts right up and runs and drives fine, but everything is reset like the battery was disconnected. Steering feel, oil life, MPG, etc., are zeroed out and my drivers side window doesn't go up and down automatically or go up when I close the door. The stereo is not reset however. Even the power outlets lose power while the engine is cranking and come back on once the engine is running. Everything is fine once the car is running and is fine when it's off but when I turn the key, it all shuts off and resets.
The battery and alternator are good and battery holds the correct voltage. Terminals are tight and clean. I've checked every ground point I could find and it all looks good. No blown fuses either. Computer connections are tight as well.
I'm stuck here. What could be causing the ecm to lose connection when the starter engages? Car is a 2012 5.0 if that helps any.
 
#2 ·
How are you sure the battery is good? How was this tested?
 
#3 ·
don't most electrical items intentionally shut down when cranking due to amount of amperage draw required to crank?

am I mis-understanding your issue?

edit so just the resets of feel/oil life are problems...+ 1 on I would try battery/connections although it sounds like you know what you are looking at.

Mud
 
#4 ·
The symptoms point to a weak charging system, that is dropping below the minimum voltage required to power the computer, when the starter motor is engaged.
If you are positive that the battery/alternator are both in good shape, then the next thing to look at would be the Constant Control Relay Module.
 
#8 · (Edited)
The symptoms point to a weak charging system, that is dropping below the minimum voltage required to power the computer, when the starter motor is engaged.
I'm not convinced, I'll agree with voltage drop but not weak charging system......it ain't never going to charge enough amperage to overcome cranking draw.....How does it crank nice and fast or meh? still thinking battery/connections because Occums Razor


plz note NOT a mechanic but I fix machinery
 
#5 ·
So the battery is maintaining the right voltage both when the car is off and running. Something like 12.3 volts, if I remember correctly, when off and 14.7 when it's running and it doesn't fluctuate or drop at all. Like I said, everything works perfectly fine when the car is off and when it's running minus the fact it's all been reset after starting.
Where do I find that constant control module? There's gotta be something that's killing power to the rest of the car for that few seconds.
 
#9 ·
I suppose that if one were to hook up a set of jumper cables to a running vehicle with a healthy charging system, before starting the car, then the charging system could be ruled out as the cause if the computer continues to reset itself during engine startup.
A proper load test is the best form of diagnosis though. As it will also detect a faulty starter motor that is drawing excessive amperage.
 
#7 ·
Sorry. I believe that it is actually referred to as a Body Control Module (BCM) on your car. It is the computer that controls lights, windows, guages, etc.
I have no idea where it is located on your car, or if it has anything to do with the problem that you are experiencing. Just throwing it out there as something to consider during your diagnosis.
 
#11 ·
Does this system use a PCM Power Relay? That relay turns ON when key is turned ON, and stays ON while cranking. If they used a multiple contact relay, which may be possible since all functions controlled by the PCM are powered-up by that relay, it could have a bad set of contacts. I'd try plugging in a new one. Just thinkin' out loud. imp
 
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