Ground strap condition a problem?

[ausgezeichnet]

In checking all the DME system ground locations, I checked the Engine-body-Transmission ground strap on the right side transmission mount. It was completely covered in black greasy grime and the bolt attached to the frame is a little rusted. Could this cause a bad ground?

[ZOA NOM]

Absolutely. Grounds are almost always neglected, and are the source of all the current flow in the car. Clean it up and replace the strap.

[911pcars]

It doesn't sound like the best of grounds. Not sure about the electrical resistance created by grease around a conductor. I'd say it's nil, like the oil-covered interior of an engine block. However, it's substantial if the grease is located between the ground strap connector and ground. Same with rust. The connection should be clean and tight.

Sherwood

[350HP930]

The slime on the strap is not important as long as its not corroded.

The condition of where the strap is connected at the ends is critical. It never hurts to inspect and clean those areas.

[ausgezeichnet]

As with a lot of other things on a 20 year old car. It probally will not hurt if it is replaced. Thanks.

[geof33]

I'd replace that strap and the one on the battery as well. Can't have a "too good" ground... But you can easily have a "sorta bad" ground....

[azasadny]

When in doubt, swap it out!

[88911coupe]

Is there a way to TEST a ground strap? I have the engine out of my '88 and the transmission ground strap was replaced a few years ago and looks fine. I have no problem replacing it but can it be tested? I don't want to just assume that a "new" one is necessarily better since it may be a lesser quality or poorly made? I assume test for resistance but isn't that going to vary based on how much current is being used? I hate electrical things...it seems like voodoo to me.

[targa80]

Originally Posted by 88911coupe

Is there a way to TEST a ground strap? I have the engine out of my '88 and the transmission ground strap was replaced a few years ago and looks fine. I have no problem replacing it but can it be tested? I don't want to just assume that a "new" one is necessarily better since it may be a lesser quality or poorly made? I assume test for resistance but isn't that going to vary based on how much current is being used? I hate electrical things...it seems like voodoo to me.

You could remove one side of ground strap then use an ohm meter to check the resistance between the loose end of strap to bare metal surface near where strap is connected. Next reverse process, reconnect loose strap end and disconnect other end of strap and test for resistance. The resistance should read zero ohms or a very low fraction of an ohm. A high resistance indicates a bad strap or corrosion/poor contact between strap and metal frame/ motor/ trans connection.

[dannobee]

Voltage drop test when the engine and trans go back in the car would be best. With the engine out, the best that you could hope for is to clean up each end and the attaching points and verify nearly zero ohms resistance with your digital volt ohm meter.

[spuggy]

You can never have too good a ground...

Every sensor on the motor - as well as ground for the alternator, IIRC - needs a ground path, and that ground strap is the only electrical connection from the motor to the body of the car.

Fiberglass rust removal pens work well for getting nice shiny threads & body contact area with the absolute minimal removal of actual material.

A liberal application of dielectric (silicon) grease will probably help protect the strap from oxidization - otherwise, they can turn into crumbly dust pretty quickly - like only a few years...

[wazzz]

You can't measure the resistance of such a thick ground strap with an ohmmeter. What you will get is the DMM probe resistance. Near-zero resistances can only be measured with a milliohmmeter.
If your strap is clean, not brittle and properly tightened at both ends, it should be good. You said you changed it a few years ago.

[AlBackus36]

Non-conductive slime on it can certainly affect the performance of the braided strap. It's an exposed braid that relies on contact between the strands to provide the overall gauge.

If you swap it out, you can get covered/sheathed 2/0 cable with heat shrunk ends to replace the braided type. A battery shop or similar should be able to build it for you if you can't. Clean both contacts on the trans and body, and install the new strap with dialetric grease or something to keep future moisture out. Never worry about it again.

[PeteKz]

Buck, if it's clean on both ends and clean where it contacts the body and trans, and securely connected, then you're fine.

If you really want to test it, here's how:
With the engine/trans in the car and the starter connected, crank the engine. If it cranks strongly, your ground strap is good. The current to the starter returns through that ground strap. Around 80 amps when cranking cold. DO NOT try to crank the engine with the ground strap disconnected. There are several other wires through the harnesses between the engine and the body and if you try to crank the engine without the ground strap, the return loop of current will go through those small wires and fry them.

[GH85Carrera]

Originally Posted by azasadny

When in doubt, swap it out!

Far any ground as cheap as that one, 100% agree. Get a new strap, and clean the connection points. One unarguable fact, you simply can't have too good of a ground. More is always better.

[pmax]

Originally Posted by azasadny

When in doubt, swap it out!

The connection points are more likely to be the cause of poor grounding.

Besides, copper is easily renewable.

[Old H2S]

The factory ground points on the G body are copper plates silver brazed onto the steel chassis and are an indicator of how important Porsche took the looping problem. They shine up pretty..

[NickW21]

Rekindling the ground connections discussion... I'm looking to sort out potential ground issues as my EFI setup is producing some odd electrical outputs at the ECU. One thing I found odd on my car (1979 SC) is the the ground strap coming from behind the fuel filter is connected to the electrical mounting plate where the CDI box, fuse, relays are. This has in turn made the entire plate grounded. Was this intentional? Are there any issues that this could cause? Seems odd that they would want this to be grounded, but perhaps there is a reason for it. Any ideas?

Best,
Nick

[E Sully]

It was intentional and it prevents issues. To reduce vibration the plate uses rubber silentbloc mounts. The plate and components need a chassis ground and it eliminates static charge issues. A ground strap is needed (911 612 008 00 Ground strap plate on your SC).
If you are having issues with your aftermarket EFI system, you will need to review the installation documentation.

[sugarwood]

I remember trying to replace that strap and giving up.
I think one end of the strap was blocked from view and hidden.
I didn't feel confident about reinstalling it blind