So I haven't poked my head under the hood yet (it is raining) but the last 3 times I have driven my XJS, it has read low voltage for half of the drive and then normal later on.

It actually started to be a problem today ( I wasn't sure if it was a finicky gauge) when I could tell my voltage was actually low, as my gauges started bouncing around. It read between 9 and 13 and dropped a little lower, then it suddenly popped right up to the middle where it should be.

Maybe a connection issue? I recently pulled the battery for charging.

 

Could well be? Had same problem, discovered the lead on the alternator was lose, tightened the nut and all back to normal.

 

Sounds to me like bad grounding on the instrument cluster. Check the grounding.

Pull the ground terminals off both ends on the body side clean the metal, make sure its bare metal and use a star washer between the terminal and body. On the cluster side ensure the terminals are clean. If you have a DVM resistance should be less than 0.5ohms, Don't forget to Zero the meter with probes shorted before measuring.

 

I've run my car with a failing belt and no belt a few times. This is exactly what the gauges will do when the car is running just off the battery and it starts getting low. I can also feel it affecting the ignition system.

I just thought coming back on and working normally was odd, didn't know if it could be part of my charging system or just a bad connection.

 

On some low voltage issues in the past, I suspected my alternator.


I made a patch cord for my HF sourced VOM. Two plugs, one red, one black for the meter from my "wire stash". A cigar lighter plug from another "saved part" of a defunct gadget.


So, I could read volts in real time under various loads and RPM's.


Alternator exonerated, battery convicted. New one resolved all.


CAVEAT:


Clean, tight and lubed terminus of three big cables essential
A. Battery + at each end.
B. Battery - at each end.
C. Transmission to chassis - strap at each end.


Carl

 

A failing alternator most likely won't give you the symptom you had where the gauge recovered, unless the regulator is intermittent. A dodgy connection will. Could even be a bad connection on the alternator or battery. In an automotive charging system the Alternator generates 3 phase AC voltage which is rectified then regulated the battery acts like a BIG capacitor bank smoothing the voltage spikes.

I've been fixing electronics for a living for 35years and more than 60% of the failures are bad connections or dry joints. I would look for bad (dirty or loose) connections.

 

Well, battery connections were fine. Checked the battery voltage and it definitely isn't seeing any charging coming in from the alternator.

Last time this happened, my voltage regulator went out, so I wouldn't be surprised if a similarly aged lucas alternator did the same thing.

 

It just sucks because it is so buried that R+R is the same amount of work to even inspect it.

Guess I can test it while it is out at least.

I pulled it about 15 minutes ago and the connector and spade terminal are totally green. At least cleaning doesn't come with a core charge.

 

I've had some over the years where 'the green' had crawled well into the length of the output wire. Just something to check for. A slightly bulbous appearance to the insulation would be a clue....but checking resistance would be the best bet. Unfortunately the other end of the wire is very difficult to access

Cheers
DD

 

Yeah, I will definitely be doing some reading after it is hooked back up. It is funny that the connector makes me want to change to a GM alternator more than anything else. I know some kits are sold to bolt in with no wiring, but something a bit more waterproof would be nice.


I can't really see, but it looks like the ground cable inside a metal protective tube to the starter, is that right?

 

UPDATE:

Bad voltage regulator.

I took this alt off my other engine when the regulator in my previous alt went out. It appears that is the most likely thing to break on these old lucas alts.

New regulator was $15 at XK's

 

High school chemistry!!! Copper oxide!!! I've seen it invade battery cables, end to end. Insidious as not always obvious.


Imperfect connections giving rise to heat by increased resistance plus water accelerates the process of two elements forming a compound.
Here, definitely an unwanted one.


Fix or retardant? Star washer + a dab of Vaseline or even hair spay aka clear lacquer....


Carl