Dodgy ground connection at firewall - frying tonight?

Been having some intermittent starting issues - 8 times out of 10 it starts immediately, hot or cold, on the first turn like it’s a brand new car, but 1 time in 10 it will offer a single grunt but won’t turn over and another 1 time in 10 it’s stone cold dead.

My handy dandy “GOOLOO 1500A Peak SuperSafe Car Jump Starter with USB Quick Charge 3.0” (perhaps coincidentally, and after some wiggling/kicking/hitting with hammer) fired it up instantly each time so I blamed the battery and replaced it. Low and behold on the way home with the new battery installed I had the same dead-on-restart issue and this time the jump made no difference, but after wiggling everything it fired up from the battery perfectly.

I think I’ve traced it to a dodgy ground connection where it’s bolted to the firewall. It’s tight but wiggly, if that makes any sense, and the old white (now yellow) insulator part of it has started to fall apart. Wiggling it about fixes the problem, whether coincidentally or not.

First questions: is this the right replacement part?!/English/parts/ebcfb3a3-a9ca-4672-916d-2d17d2275688

Second question, born of ignorance and possessing significantly fewer brain cells than I was born with, is if the ground connection is lost while driving am I doing any damage? A quick search on the Internets manifests opinions ranging from “no, not at all” to “you’ll fry the alternator” through “you’ll fry the car” to “you’ll fry yourself”.

Of course due to my wiggling everything there’s a lot of potential inconsistency here, and it might not even be that. I’m 99% sure the battery indicator gauge has stayed in the 13+ amp range at all times, although there is a possibility it could have dropped off moments before I pulled to a stop just prior to having the issue upon attempted restart. (The battery charge indicator would drop to 0 if the ground connection were lost, wouldn’t it…?) Or it could possibly be that the car does a specific shake as the engine shuts down that sometimes breaks the connection at the firewall.

This may be a red herring but perhaps worth mentioning that the issue usually occurs when hot/warm – the car typically starts perfectly in the garage but becomes recalcitrant once I stop, usually either for gas or somewhere where I’m surrounded by admiring - and soon to laughing - members of The Great Unwashed. This isn’t 100% the case, but it does seem to be a trend.

Usual story – 100% reliable in the driveway, less so out of it and reduces exponentially with 1) increased distance, 2) proximity to the cloak of darkness and 3) rain.

If it’s OK for me to drive with a dodgy ground (?), and assuming I can’t resist doing so in the next 3 days before the new part arrives even though I risk further embarrassment, what else should I check if it happens again? So far when it’s happened I’ve usually had people standing around laughing at me so I’ve taken the hi-tech approach of wiggling everything in sight before trying again as quickly as possible, but of course a loose connection sometimes reconnects with a wiggle and sometimes it doesn’t, so next time I want to check things a little more systematically.


On my 69 S2 everything goes to just one connection… looks like that lower bolt in your photo:

I do have that terminal post you link to but that is for the ‘hot’ connections:


Not sure what purpose is served by the (insulated?) post in your photo.

Your ground connection should be direct to the metal of the car, the plastic piece that you show may belong to the (+) battery connection that is near the Voltage regulator (VR). The piece you show on the SNG site is for the positive (+) battery connection.
Disconnect the ground, remove the plastic piece, and clean the ground area and reconnect the ground wires and you should be good to go.
Maybe while you are at it clean the (+) connections also.

Regards, Joel.

Yes, what Joel said. The plastic piece shouldn’t be there. I’d take a round wire brush like comes with a rifle cleaning kit available separately for a couple of bucks, and clean up the female threads in the bulkhead. Then wire brush the terminals, clean up the bulkhead like in Geo’s picture and run a fresh bolt in there with a star washer.

To be clear, the insulated post is what the ground strap is attached to - that’s the thicker of the two black cables. The smaller black cable also attached to it disappears into the firewall under the regulator, no idea where it goes.

Worth mentioning that on the inside of the firewall the post connects to a kill switch, the yellow wires coming out of the regulator going through the side of the firewall to also connect to the kill switch.

Of note the internal end of the post that feeds the kill switch also has an insulation layer.

Eeesh, this little corner of the car is the worst part, looks awful. The paint corrosion is from a leaking brake master a few months back. Honestly the rest of the car isn’t this bad…!

OK, so this is odd. The red kill switch cable is connected to the ground terminal on the firewall. How is that right…?

btw - the kill switch and terminal post along with its plastic insulator has been on the car since I bought it - sorry, since I became custodian - never had an issue until a few days ago.

Depends: is that a ground kill, or a hot kill?

Sorry Paul, been using your name in vain with all this talk of wiggling. All I can tell you is that the car is dead when it turn it 90 degrees.

Hmm, on unscrewing the firewall post the hole it goes through is 1/2 wide, with the plastic insulation on either side apparently to anchor it. Considering the ground needs to be to the car itself I can’t see how it worked at all, but it certainly did.

Dare I connect the battery ground to the bolt below it instead…? Not sure what else is grounded there as the wires join the main loom, but something is.

Although that then leaves the kill switch unconnected to the battery ground as it has been, but if simply reground that by simply putting a washer outside the firewall then that’s OK, right…? Battery is grounded to the bottom bolt, kill switch grounded to it’s existing upper bolt minus the battery ground.

I was unaware you had a kill switch. My reply is based on ‘normal wiring’, I know nothing about a kill switch except people do put them on cars. My suggestion is to put the wiring back to ‘normal’ and then wire in a kill switch if you still think it is necessary. If you are not handy with wires and current have someone that is do it, you don’t want to end up with a pile of ashes!

Regards, Joel.

Another theory, since the upper ground bolt has plastic insulation on either side of the firewall, is that maybe this isn’t the ground location at all, that the insulation serves a purpose to take the ground through the hole and actually ground somewhere along the kill switch wiring.

Does seem very odd that there is this insulation on both sides where theoretically there shouldn’t be any.

Whether it’s a ground or hot you’d be better off with a continuous cable running from the battery terminal to the kill switch. Just make sure you have a good rubber grommet in place at the hole in the firewall. Ir you might be able to fit the cable in the grommet that already holds the wires that go from the engine compartment to the driver’s foot well. The theory being the fewer connections you have in any wire run the fewer failure points you’ll have.

The bolt below the hole is the correct place to attach the battery ground cable. Of course, that would eliminate the kill switch.

That is exactly what is going on.

Yes, the simple, more predictable solution would be to restore the ground to the stock location (battery cable to the lower bolt along with everything else) but you will need to work out what other non-original wiring is in place & what it is for.

Specifically the 2 yellow wires and the 2 black wires that are running back to that grommet (and will be appearing in the engine bay near the voltage regulator). These wires:

That yellow pair and black pair are going to appear thru the grommet in the center of this pic…

…knowing where they go next or what has been added there may be a clue.

I assume those are the wires that kill the alternator.

The black wires run standalone from a random connector right next to the kill switch out through the firewall grommet where they split - one is pegged to our favorite top grounding peg and the other to the +ve connection above the regulator - so should be OK to just disconnect.

The yellow wires pass from the kill switch out through the grommet where one connects to the “F” tab on the regulator and the other feeds up behind the very firmly fixed bracket the regulator is attached to - cant get to it without taking the fan off that’s above it, but it seems - seems - to route back around as part of the wiring loom that connects to the +ve terminal again.


In looking at the kill switch it almost looks like it (and therefore the battery) is grounding right where the switch is mounted, but it’s difficult to tell without taking it partially apart and impossible to get a camera in there.

So…if i now try to just put it back as it was for the moment, with the battery ground passing through the bulkhead to the kill switch without grounding, what are the ramifications if the insulators fall apart allowing it to actually ground at the firewall as well…?

Update - so far so good. But wondering now if the collapsing insulation was the cause of the problem in the first place - intermittently grounding at the firewall before the kill switch?

So it looks like the yellow wires disconnect the Field terminal on the voltage regulator when the kill switch is disengaged. If you remove the kill switch you would also want to connect the F wire directly to its terminal on the voltage regulator.

From the pictures and your description it would seem that the black wires are providing paths to ground and a fused (7.5a) power to or from something that is no longer there…

So that would seem to be unrelated to the kill switch. Just a guess looking at that connector, but someone may have been using it to connect a battery tender.

If you want to keep the kill switch you may need to start by checking and assuring all connections are clean and tight including however a ground is achieved right at the switch.

If you choose to eliminate the kill switch you (move ground to lower bolt and connect the voltage regulator as original) you will eliminate several connections, likely including whichever one is giving you intermittent continuity.

Rob – total aside and not at all related to the point of your thread (and I apologize) BUT – please tell me the manufacturer and color name (or paint code) of the gorgeous red on your Jag.
Inquiring minds need to know!
(My guess is PPG Viper Red)