So, shortly after getting the front subframe in car and driving it a bit in June, the charging system was a bit erratics. Sometimes 13.something volts, sometimes 11.something.

So I had my fundy shop install a new Bosch. I was occupied with a new alternator installs in my 928, TVR and Ford Ranger..

So after I return here in October, Im back to the same behavior. We put the car up on the lift, and found something strange. Dependin on how we flexed the chassis, the voltage would jump to 13.something.

Obviously ground straps under the engine. All others in the car were removed and cleaned in June.

I found two issues. First, when the replacement subframe was painted, the point at which the drivers side strap was attached to the frame, the paint had not been scraped away. Then, near the motor mount on the passengers side, a ground strap was hanging.

I have always been a fan of installing a big fat (00) size ground strap from the alternator to the ground point on the frame. My boat has a 175@ alternator, and I have a 00 cable running direct to a negative bus bar. Unfortunately, cars don't generally have buss bars.

Are there more ground straps to be found? Does the wiring diagram show them? Should there be a ground strap on the alternator?

I have a feeling that it wasn't the alternator at fault

 

Good sleuthing !

I don't know if there are more factory installed ground straps.

Never hurts to add extras.....which might be easier than searching for existing ones.

The alternator should ground via its mounting bracket....although it might be loose/dirty/corroded. You could add an extra ground if desired.

I added an extra heavy ground from the engine to the body. I used one of the engine brackets (air injection pump?) as a contact point.

Cheers
DD

 

That hanging strap is a "thing of interest". Coupled with the movement and volt out put!!! Bingo!!

My lump got into anew hissy mode. No crank. Just clunk as the solenoid hit but failed to do anything else!!

An added ground from the one on the wing wall to a handy bolt on the alternator and cranks happily and better yet fires up, fast..

I did create a ground bus for a series of relays needed for the engine swap. Expanded the exisating one on the cowl. Hid the VIN. so ??? Relay rack and ground bus now are located there... a

 

Upgraded wiring with bussbars

I know its not a Jag, but at least it’s British!

TVR wiring with BlueSeas busbars, auto reset circuit breakers and waterproof relays. The bussbars make connections for the B+ and ignition circuits so much easier. 10ga wire to the headlights, horns, and dual fans. Tinned braided copper connection for the grounds everywhere.

Excess wiring gets cleaned up after a few test runs this week.

For the astute, those are the door cards for the 928. Upgrading the speakers.

 

A quick update on the alternator problem.

I ran a 2 ga cable from the alternator case to a mount point on the passenger inner wing. Voltage jumped to 14.17. Dipped to 14.14 with lights, wipers and rear defroster.

I was thinking how ridiculous it is to ground the engine to the subframe. The subframe is pretty effectively isolated from the chassis (ground) by the rubber engine mounts



Problem solved!

 

Well done.

Once upon a time there was a 2nd strap to earth the subframe to the chassis. Common problem that one is missing after a vist to the vet.

 

I would think I would learn my lesson. I have 10 sports cars and 3 sailboats. All except the 928 and the Jag have dedicated ground cables to the alternators.

So why am I chasing my tail on the Jag trying to find out why my new alternator isn't putting out when their is no ground connection to the alternator?

On the Jag, the difference was incredible. Not only was I getting 14 volts at idle, but putting additional loads like lights, blowe, defroster and wipers there was virtually no drop in voltage at the battery

 

Just an update on the grounding of the alternator via a dedicated strap, and not through the alternator mounts, to the block, to the subframe, to the chassis.

There have been lots of discussions on the Rennslist 928 forum regarding corrosion on the aluminum heads. I pulled my 928 engine down last fall for regasketing and cleaning.

On aluminum block/head cars, there appears to be electrolysis of the heads and the steel fire rings on the head gaskets. Coming from the marine electrical environment, I am well aware of galvanic corrosion, and mitigation methods.

One theory put forth about the auto head corrosion is that by passing the heavy currents from the alternator through the block, you are creating an ideal situation for galvanic corrosion ( commonly referred to as electrolysis).

By providing aN appropriately sized ground strap from the alternator to the chassis, you can hope to minimize some of that current, thereby forestalling the head and gasket damage. Of course the cable needs to be sized appropriate to the current the alternator produces.

But if you think about it, most engine mounts are rubber. And the subframe mounts are rubber vibration isolators. The factory tried to ensure grounding of some sort by a ground strap between the block and the frame. That is appropriste for the sensors that ground to the block. But it doesnt mean it is optimal for the current produced by the alternator.

I have done this now on my XJS, my 928, my TVR, and several of my Benz’s. I have seen a noticeable increase in voltage stability, especially at low speeds.

For marine engines, it is standard practice to provide a separate ground strap for the alternator.