Hey folks,
Is there a pic of the major ground points on a V12? Last night while I was driving my 88 XJ12, the lights dimmed and engine cut out while pulling away from a stop sign. It was close to home so I managed to get it home but it did the same routine a few times. It sure felt like a loss of electric power, so it'd time for a grounds cleaning exercise.

 

TheWarlock,

The wiring diagram in your Parts & Service Manual has good diagrammatic showing all the power, earth and loom connection points.

Good luck

Paul

 

If you dont have the manual I would suggest go digging. Thats what I did the first weekend I had the car.
I would start with ground points beside battery. There are many. Then move to the engine bay. A small wire brush or sandpaper will work. Be sure it inspect where the wire enters connector. Clean the screw and ground point on car body. Replace any bad connections.
It takes some time but saves it down the road while troubleshooting other issues.

 

Mine has done that before on several occasions when my old voltage regulator went out.

 

A tip for gounds.

Clean the metal with sand paper and when you have screwed the grounds back on use seam sealer over them to prevent water and rust.

 

I still have some leftover POR15...I'm assuming that would work just as well as seam sealer?

 

Any wire that is black is a major ground. Any wire that has black as a secondary color is a keyed ground. Good Luck!

 

Thanks guys, I do have the service manual and while it has wiring diagrams, I have not found a diagram that shows physical locations on the car. For example, I've seen ground straps under the engine, and that would not show on a wiring diagram.

The tips on black wires and seam sealant is great, thanks.

The alternator is a new 1 month old unit, so I really hope it's not the rectifier inside it.

 

A list just to help, but lights dimming seems pretty drastic so starting at the back:

earths:

• RHS of boot on tunnel top under the buttress. Several here
• Battery terminal connections
• Loads around the dashboard and under the console, though these are unlikely to be your problem
• there is a really important earth from the engine to the lower chassis rail on the LHS (USA driver's side). It goes first to the subframe and then bolts to the lower chassis rail. It is a VERY good idea to get a second earth strap and connect it from the alternator bracket fixing bolts directly to the chassis.
• There are earths each side of the radiator top panel connected by the panel fixing bolts. This is a three layer earth connection (wire/bolt/panel/body) so ensure you clean each layer to shiny metal.
• If you pull out the headlights, there are earths in the nacelle area too.

Positives:

• There are two posts on the firewall, one each side about level with the wiper grill. Good idea to disconnect the battery and clean these.
• Also check the positive thick wire from the alternator to the starter to the firewall post is in good shape and the connectors clean.

Greg

 

Thanks for the detail, Greg. very helpful as always. On another thread someone mentioned the famous Grant rewire technique. Any links or info on this?

I've been trying to reproduce the problem before touching anything, so that I know it's fixed. That's been futile so far, it starts and runs like a champ around the block.

So now I'm scared to venture outside the block and the neighbors are going to call the cops on a black XJ12 that is repeatedly staking out the neighborhood in alternating high speed and slow speed runs

 

I just dab on some Vaseline on ground points. But, I live in a semi arid climate.


It seems to work.


Dates back to my off road days and fording streams in old pickup trucks.


Carl

 

The alternator positive feed to the battery is connected to the LHS firewall post via the starter motor. For some reason the alternator feed to the starter consists of two thinner wires instead of one thick one. Grant suggests replacing the two wires with one thick one and connecting it directly to the firewall post, rather than piggybacking it via the starter motor.

What I did, as a halfway house, was to change the alternator-to-starter wire to one thick one. Also fitted a 115 amp alternator and also fitted (as Grant recommended) an extra earth strap from the alternator bracket direct to the frame. Before, my car just ate alternators and charged Ok but not really strongly. Afterwards, it has been strong at 14.4 amps all the time.

If you do go direct to the firewall, you must disconnect the alt to starter wire. Be very careful with routing and securing the new wire, so you do not get any danger of chafeing and therefore short circuits on the way!

Greg