I have an '86 XJ-SC with charging problems.
For about 12 years, after I start the engine, the alternator would only come on line when I turned the aircon on.
This worke fine for some 12 years.
Recently I've added extra earthing leads.
Now it won't. Symptoms are, car voltage gauge shows about 14v. BUT I've put a cheap volt meter in the cigarette lighter and it only shows about 12.4v (which I presume is battery voltage) and it slowly drops.
Do I need another alternator or is it a wiring problem?
TIA.

 

Maybe both.

My gut tells me you have some crusty connections in the system....which is probably why adding some ground wires brought some normalcy to the system.

There are various ways to approach this but I'd begin by cleaning connections at the alternator and cleaning the two "+" junction posts on the firewall.

There have been some cases where the heavy alternator output wire corrodes under the insulation and develops a lot of resistance. You check the resistance with an ohm meter. Or test voltage directly at the alternator and compare it to voltage at the "+" junction posts

If no joy it wouldn't be too hateful to remove the alternator and have it tested

Others will chime it. I'm just two sips into my first cuppa coffee

Cheers
DD

 

Further to Doug's points, there are two huge earth clusters just to the right of the battery in the boot. Pull the side trim and the carpet and you will see it on top of the chassis rail by the battery tray RHS. This is often very corroded.

 

Hey Greg. Thanks. I think you are onto something.
I put a jumper lead from the -ve battery terminal to another part of the chassis and it appears to now be working well.
Tomorrow, I'll locate and clean those terminals.
Thank you.

 

I got through 4 second hand bosch 115amp alternator in a very short time so having seen reputable specialist suppliers offering 120amp brand new with warranty that are all perfect fit for the Lucas/bosch bracketry for just over a ton I purchased one and it's been perfect.

 

Rex
Where from please?

 

manchester auto electrics via ebay range rover 120amp

 

Latest UPDATE.
I checked all the earth connections in the boot. All good. No corrosion.
So, I thought. I connected one jumper lead to alternator housing and the other end to chassis.
So far all good, although I haven't done a road test on it yet. But I have started the car quite a few times and all good.
Fingers crossed.

 

Good find and not uncommon.

Look at and around the LH engine mount and the associated brackets. The OE engine earth strap was a 2 strap thing, engine to cradle, then cradle to chassis, using open braid cables. They fail, mechanics dont refit them as they were, and what you have is the result.

Get a GOOD solid Battery cable, eye to eye, and run it grom a sump bolt to the chassis direct. All mone, 6 & 12 were retro done that very early in their life with me.

THEN, run a 4mm Black cable from the alternator casing to the chassis, as you have the jumper lead now.

THEN, run a 4mm Red cable from the alternator Pos Stud, over to the chassis, and up and around the Booster, to the Pos battery post tucked on the firewall just near the trans dipstick.

That renews, and includes, the aging OE wiring, and sharpens up all the electrickery of the beast.

About a 4 beer task.

 

A new alternator has been fitted.
So far so good.
Thanks to all.