How to r and r transmission to chassis ground

[Snitzler]

Total time about 4 hours, most of it spent cleaning. The 2 bolts are 13 mm in diameter. Removing the ground strap from the transmission was relatively easy, it took some experimenting with extensions and sockets to get the right length so that I could get the ratchet in there. You have to go in from the front reaching back and there is not a lot of room. The transmission mount in in the way. Brake cleaner and a brass brush cleaned up the filth. I should have worn gloves, black hands. Cleaned the ground strap ends with a brass brush and sprayed the ends with Deoxit. Installation of the strap to the trans was by needle nose pliers and feel, you can’t get your fingers in there. All the hardware was brass brushed and sprayed with Deoxit as well. R and R of the strap to the chassis was much easier. Again, clean and brass brush. Soaked everything with Deoxit and torqued the nut. I was amazed at the poor condition of the old ground strap and the contact surfaces were totally corroded. The contact surfaces on the transmission and chassis were filthy as well. I wonder what kind of current wasn’t going through the ground strap and going through the other grounds? I think I am going to go after all of the other grounds now.

Old transmission to ground strap.


Old ground strap to chassis


New ground strap installed to transmission


Another view showing old strap


Tools


Old strap

[Discseven]

Snitz... thanks for taking the time to photodocument---nice pics. Did some reading on DeoxIT. Interesting chem. Thinking about testing as fan metal preservative.

[pete3799]

1/4 inch drive extensions and swivel work well.

[Gretz]

Thanks for the pics and info. I completed this today in about 2.5 hours. As you said, most of the time was cleaning. Also, I agree that 1/4" drive extensions worked well. I didn't need a swivel but see how it could help some folks. Lastly, on my 82 SC, I could see/touch/access the rearward bolt from the rear but still needed to remove it from the front obviously.

[rokemester]

If you have need to replace the mounting washers for the transmission or need to R&R the bracket much easier to get at the ground strap.

[Flat6pac]

Rome, on a G50 the removal of the trans cross bar there is little to support the trans because lowering the trans facilitates the changing of the starter. Also, if you depend on the swivel in the back seat to support the trans that’s a couple hundred dollars.
Agreed, the removal of the cross bar on the SC trans, after supporting the trans, would have been much faster.
Bruce

[rokemester]

Yes, should have mentioned that I made sure that the transmission was supported with sturdy jack stand and heavy metal brace.

[Jdub]

This is a great post and reminds us all how old ground straps become increasingly resistant to carrying loads over time. A solid alternator > trans > chassis connection gives you real peace of mind and is an essential component to working back on alternator and charging issues.

I was stunned at the corrosion found under the skin of my old alt > trans wire when I last replaced my alternator. And I am obsessed with bad grounding having worked on British iron for so many years.

[Clumsy me]

Reviving a dead thread hoping someone can help me out. Recently picked up a 76 912E. When I got it the grounding strap was not attached on the transmission side of the car. it was just hanging off. I got a new strap but due to the location of the stud its hard to tell what the thread size of the nut is.

Hoping someone can help or point me in the right direction.

[917_Langheck]

13 mm nut is usually fitted to an M8 bolt or stud. You will need to measure the thread pitch. If you don't have a pitch tool puzzle in a 1.0 or a 1.25 mm pitch bolt to the stud to see which one fits, and that will be your nut size. Maybe try fitting the nut on the chassis side on the transmission side to see if it's the same. It's likely to be M8 1.0 given the other hardware used in that area.

[ErrorMargin]

Originally Posted by Clumsy me

I got a new strap but due to the location of the stud it’s hard to tell what the thread size of the nut is.

I happened to have easy access and measured for you on my 84 911. The transmission ground strap uses M8-1.25 nuts on both the body and transmission.

[917_Langheck]

M8 1.25, check! ✔️

Thanks for corroborating the correct spec.

[Ian Comerford]

Originally Posted by Discseven

Snitz... thanks for taking the time to photodocument---nice pics. Did some reading on DeoxIT. Interesting chem. Thinking about testing as fan metal preservative.

Try ACF 50 if you can get it, it works really well on my fan

[Clumsy me]

Originally Posted by ErrorMargin

I happened to have easy access and measured for you on my 84 911. The transmission ground strap uses M8-1.25 nuts on both the body and transmission.

I really appreciate it! Really hoping this finally gets rid of my brake warning light on on the dash!


Guys just wanted to follow up with this. That is the correct size and it did fix my brake warning light. Thank you for the help.... now on to the next round of issues.