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I've read conflicting info with regards to grounding the ECM/PCM itself. I'm talking about the metal case itself. Both of my Jeeps have 7747's and they are mounted against the FW and are definitely getting ground. They seem to run fin though. Getting ready to move to a 7427 and starting to read up on 'em.
So? Ground? Yes or no?
NEVER ground the case. While some have got away with it, it affects RFI sheilding and can cause failures in the ECM/PCM.
IIRC, all of the PCMs i've torn apart are already using the case as a ground via the bar that all of the output transistors are connected to for heat dissipation.
not 100% on that though.
You are correct Robert, the ECM case is grounded through the ECM internals, but this is not the same ground potential as the chassis ground since the ECM gets it's ground reference from the engine block, which has it's own potential and (electrical) noises in it. Isolating the ECM case form the chassis ground eliminates ground loops and shields the ECM from other noises, including RFI radiated through the spark plug wires, through cancellation.
Don't ever for an instant believe that engine ground = chassis ground, or even that chassis ground is the same through the entire chassis, there are different currents and therefore electrical noise at different parts of the chassis, that can really mess with the ECM if the ECM case was grounded to the chassis.
The only ground wire I would say could be acceptable to attach to the ECM case externally could be one that is attached to only a ground wire feeding the ECM. I would be hesitant to do this as well, since it could act like an antenna for high frequencies and possibly introduce other noise that could cause issues with the ECM and therefore how the engine runs.
I think the easiest argument to this is GM never grounded the ECM/PCM case! So why would we?
that isn't always a valid argument though.... GM missed out on doing a lot of things that should have been done.
since the ECM's case is already grounded internally, and those grounds run to the engine block ground points, i think it would be safe to say that you could run more grounds to those exact ground points. or rewire the existing grounds with fatter wires.
Why would you even bother?Quote:
Originally Posted by RobertISaar
The OEM set-up IS the way it should be done, I'm not talking from a manufacturing stand point, but from a noise rejection stand point.
i wouldn't bother, since i've never had problems with the way it is setup as-is, but i've seen people add grounds as i guess some type of troubleshooting(or band-aiding) and have it work well enough to leave alone.
The location I have mine, and the way I have to mount it, makes it extreemly hard to mount it without it getting grounded. I have had my old 7747 mounted this way, and my current 7427. I have not had any issues in the couple years I have had it mounted like this. I remember reading this once before so I unbolted it from the firewall and let it hang down and set on the carpet (no ground on the case at all) and it made no change what so ever to how it ran. So I just bolted it back into place.
Quote:
Originally Posted by JeepsAndGuns
on my FSJ's, I have it bolted to the FW using the factory heater box bolts. Those bolts are spaced perfectly for this.
Ive worked in a Salvage yard for 15 years and pulled a many Computer over my time here. I've seen them mounted in metal brackets and plastic both. Some that come to mind are the 95-01 Dodge trucks that bolt directly to a metal bracket on firewall, and earlier Ford cars and trucks that had metal jackets the computer slid into. However best I recall the GM's are usually isolated by plastic in some form or fashion.
Exactly, GM has decided or did back then to isolate the ECM from chassis ground as a form of sheilding.
I'm confident that if you were to open the ECMs from other manufacturers that are monted to metal will usually have the extrnal chassis isolated from the internal ECM ground through a resistor to isolate noise. At least that's the theory.