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Old 03-13-2013, 02:20 PM   #1
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Default battery disconnect bad for the ecm?

I have a cut off switch on my negative battery terminal and had been pulling it off everytime I got out of the car. I have read that disconnecting the power to the ECM loses the LTFT and resets it. Is this something I should not be doing?
I am about to start rewiring, so it wouldnt be a big deal to change it, but I like the idea of having a complete system kill switch and its right behind my seat so I can just reach back and grab it out.

Should I just run a kill switch in the ignition and not the main power? Does it matter?


thanks in advance for the input.
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Old 03-13-2013, 03:03 PM   #2
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1. The disconnect should be on the pos cable.
2. There should be a fused circuit to keep the ECM hot, even w/ the disconnect open.[Wired to the batt side of the disconnect. The alternator should also be on the batt side]
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Old 03-13-2013, 03:47 PM   #3
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Quote:
Originally Posted by Old Geezer View Post
1. The disconnect should be on the pos cable.
2. There should be a fused circuit to keep the ECM hot, even w/ the disconnect open.[Wired to the batt side of the disconnect. The alternator should also be on the batt side]
I appreciate the response
To further my question, I am not concerned about where the cutoff should be. It was installed this way when I got the car and it still kills the power as should. Havnt had a problem in 4 years.
I would like to know if there are any issues with cutting the power to the computer? I am about to start tuning for FI and dont want to mess with the fuel settings in the computer if it affects them by doing so.
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Old 03-13-2013, 08:06 PM   #4
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prob would be a good idea to leave power to the ecm. You could do this a few ways. Seperate small dry cell battery wire to a relay that keeps only the ecm powered when the disconnect is made would prob be how i would go about it. Why do you have the disconnect?
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Old 03-13-2013, 08:58 PM   #5
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It isn't hurting anything, but it will just take a while to relearn the trims and spark stuff, and the transmission adapts, every time you put power back to it.
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Old 03-13-2013, 09:13 PM   #6
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If you have pending obd2 codes, the PCM will not store them. Also I'd you are running in speed density mode it may take a ignition cycle to start the car after complete power removal. This depends on how the MAF fail is set in the PCM.
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Old 03-14-2013, 03:26 PM   #7
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Changing the maf test to run to .01 will fix that issue.
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Old 03-14-2013, 04:05 PM   #8
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There is nothing wrong with disconnecting the negative as apposed to the positive. Most battery istallation instructions say disconnect the negative cable first. I'm sure some engineer can say why but not me.
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Old 03-14-2013, 07:36 PM   #9
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Quote:
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Most battery istallation instructions say disconnect the negative cable first. I'm sure some engineer can say why but not me.
This is because if your wrench hits the chassis while you are disconnecting the positive, you will create a short. Take the negative loose first, and it won't matter if you hit the chassis.
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Old 03-14-2013, 07:40 PM   #10
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Quote:
Originally Posted by wait4me View Post
It isn't hurting anything, but it will just take a while to relearn the trims and spark stuff, and the transmission adapts, every time you put power back to it.
So does keeping power going to the pcm going to avoid this?
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Old 03-14-2013, 08:24 PM   #11
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This is because if your wrench hits the chassis while you are disconnecting the positive, you will create a short. Take the negative loose first, and it won't matter if you hit the chassis.
Never thought of that, however when I was in the Coast Guard we where taught to disconnect the negative terminal first when swapping batteries. These batteries where not located in an area where you could accidentily ground the positive terminal. No telling why they did it that way though.
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Old 03-14-2013, 08:45 PM   #12
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keeping pcm powered will avoid the "relearn"
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Old 03-15-2013, 01:34 AM   #13
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Quote:
Originally Posted by ss performance View Post
Never thought of that, however when I was in the Coast Guard we where taught to disconnect the negative terminal first when swapping batteries. These batteries where not located in an area where you could accidentily ground the positive terminal. No telling why they did it that way though.
I would say it is just good practice to always do it the same way so than when there is a chance, you will be doing it the right way.
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Old 03-15-2013, 01:53 AM   #14
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Quote:
Originally Posted by newman2ng View Post
prob would be a good idea to leave power to the ecm. You could do this a few ways. Seperate small dry cell battery wire to a relay that keeps only the ecm powered when the disconnect is made would prob be how i would go about it. Why do you have the disconnect?
Would you mind explaining this in more detail? I need to do this and I don't quiet understand what you are saying to do.

Also, is this still NHRA legal to keep the pcm powered?
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Old 03-15-2013, 01:55 AM   #15
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OP, do you have the switch for NHRA rule purposes?
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Old 03-15-2013, 06:52 AM   #16
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Quote:
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OP, do you have the switch for NHRA rule purposes?
thinking when I posted.
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Old 07-16-2013, 01:41 PM   #17
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If you leave the ECM B+ terminal powered, but kill power to the ECM IG1 terminal, injectors, coils, etc, what will the ECM do? Will it throw any codes?

I'm thinking about having the alternator output and E38 ECM "B+" (terminal 20 X1) direct to the battery (with fuses) and switching all other power with the kill switch.
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Old 07-16-2013, 01:41 PM
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