Engine ECU ground question

91GTstroked

Active Member
Jun 14, 2007
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Hey guys I have a question regarding the ecu ground. The previous owner relocated the battery to the trunk. My car has a wire tuck, so most wiring is in drivers side fender.

It appears that they cut the ground wire that went to the negative battery terminal.

So, can I pull the remaining wire out of the existing terminal and add a new 10ga wire and run it back to the battery? Or should I just use a new ring terminal and tie the two together?

I attached a picture, might be hard to see.

Also, what problems could I have by leaving it as is?

Thank you.
 

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hftrud

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Feb 22, 2020
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Bremerotn
Hey guys I have a question regarding the ecu ground. The previous owner relocated the battery to the trunk. My car has a wire tuck, so most wiring is in drivers side fender.

It appears that they cut the ground wire that went to the negative battery terminal.

So, can I pull the remaining wire out of the existing terminal and add a new 10ga wire and run it back to the battery? Or should I just use a new ring terminal and tie the two together?

I attached a picture, might be hard to see.

Also, what problems could I have by leaving it as is?

Thank you.
Yes, grounding the ECU is important. Shouldn't have to go direct to the battery. Any close by known ground is good. I created a ground strip that I can connect any ground to that is in question in the engine compartment.
 

91GTstroked

Active Member
Jun 14, 2007
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Yes, grounding the ECU is important. Shouldn't have to go direct to the battery. Any close by known ground is good. I created a ground strip that I can connect any ground to that is in question in the engine compartment.

Okay, I believe it's still grounded. Just not using the factory wire from ecu plug to negative battery terminal.
 

hftrud

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Feb 22, 2020
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Bremerotn
You still do not have to go to the battery to ground it. Run the ECU harness grounds to a good ground source near by. The battery is supposed to be grounded to the engine, frame and body. That means just about any clean metal surface is a potential ground. You can check this by putting a volt meter "+" lead to any hot 12 volt source in the car and put the "-" lead to where you want to ground. Should read battery voltage. If it drops more than 0.2 vdc. find another ground point. Also make sure full battery voltage is coming through the source used for the "+" lead. A hot fuse is a good battery source.
 
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NXcoupe

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Dec 3, 2009
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Dayton, OH
If the ecu was a relay or a light bulb, yes, any good clean ground would work. Problem is, it's not, so it needs to be connected directly to the battery negative terminal to keep the signals clean. Solder the 10g wire after cutting off the ring terminal. I will not tune a car on my dyno that does not have the computer grounded this way for a reason. Too much time spent chasing my tail has taught me.
 

91GTstroked

Active Member
Jun 14, 2007
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If the ecu was a relay or a light bulb, yes, any good clean ground would work. Problem is, it's not, so it needs to be connected directly to the battery negative terminal to keep the signals clean. Solder the 10g wire after cutting off the ring terminal. I will not tune a car on my dyno that does not have the computer grounded this way for a reason. Too much time spent chasing my tail has taught me.

Thanks for the advice. So in the picture I included you can see they cut one wire and the other is connected to that pigtail connector. Do I solder those two together, then run back to the battery?

Sorry I'm a little confused and want to do it right.
 

7991LXnSHO

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Sep 1, 2010
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Avoiding the proximity of an ignition box reminds me of home stereo ground loop noise, especially with that thing that spins vinyl discs. It’s surprising we do not have more electronic interference with computers, radios and amps on cars, even with good wires and resistor plugs. The MSD coil that came with my car physically and electronically sang. The replacement one must be isolated better, but there’s a lot of EMF surrounding any of these coils that are not shielded by an oil filled, steel container. And the alternator cases are too well vented to act as a faraday cage.

Cut off the ring terminal and solder a 10g to it. Run that back to neg battery terminal and do not put it anywhere near where an MSD box is grounded.