My recently-purchased 1987 XJX V12 shows an interesting and consistent charging behavior.

When I start it, the voltmeter hovers at or just above the lower red line. If I drive it around at city speeds (40 mph or so), it stays about there. However, if I put my foot into it when getting on the interstate, the red warning light comes on, the meter goes to dead center, the red warning light gradually goes out, and the meter stays at dead center for the remainder of that trip, regardless of how fast or slow I drive.

Is this common behavior?

I'm happy when I see it go to the center of the meter, but I wonder if the charging system is struggling at slower speeds.

Thanks,

John

 

Have you checked the tension of the alternator drive belt? It could be slipping at speed although that usually generates a squealing noise.
Steve

 

might be a dodgy connection on the warning light circuit (most likely on on near the wire on the back of the alternator). The warning light acts as the exciter to the alternator and starts the charging at low revs.

 

It also is highly possible the alternator is failed. Have you measured the output voltage? The gauge in these cars is not anything to use for diagnosis.

 

I've not been under the car yet simply because it's so low to the ground (much lower than anything I've ever had before) that I'll need to get ramps to elevate it enough. As soon as that happens I'll try out these suggestions. Higher priority at the moment are the windshield wipers (two speeds...off and painfully slow) and instrument lights (off no matter what). Since it's not a "need to drive everyday" car those two are not a big deal (yet).

Thanks so much for the suggestions. I'd rather track this stuff down myself than pay someone else (who may not know what they are doing either!).

John

 

You can check the charging voltage at the battery. Do not need to crawl under the car for that. Do it at idle and at high rpms. Ideally when the warning lights is doing it's thing. Also turn off all loads and then on all loads. Record all the voltages.

As for your wipers it is a common problem. Huge voltage drop to the motor. Much of it is in the switch itself. The rest in the vehicle harness connectors. Also the motors get water inside them and this causes havoc too. There is a plastic shield over the motor but usually the original one is disintegrated.

 

The warning light is over voltage. This could be caused by 2 things dodgy connections in the dash or a bad regulator on the alternator.

As the battery reaches full charge the regulator will reduce the voltage the alternator supply's to the battery to prevent overcharging. I would get this looked at as it is possible for a battery to explode if it overchaged.

 

Just got back from a voltage test run;

Before start: 12.33
30 sec after start: 11.85
1 min after start: 11.90
Voltmeter on panel is sitting at the top of the (low end) red.

Heat on high, headlights on, radio on, flashers on, rear windscreen heat on: 11.25

Turned all the above off, engine at idle: 11.75

Sitting in park, ran engine up. At 2500 RPM battery warning light came on, voltage jumped to 13.7 volts. I held the rpm at 2500, warning light gradually faded off, voltage stayed at 13.7. Panel voltmeter jumped to center and stayed there.

Brought engine back to idle (800 rpm): 13.7

Turned all the stuff on as mentioned above: 12.0

Turned off all but engine: 13.6

Shut car down while I went to store. Came back, started it up, voltage was around 11.8. Brought it up to 2500 RPM, same deal. Voltage came up to 13.7, warning light came on and faded off, and voltmeter in instrument panel centered and stayed there.

John

 

That's exactly the behaviour I would expect with a faulty exciter/warning light circuit.
Check the external wiring (inc bulb wiring), then look at either getting the alternator checked or changed.

 

Sounds like a bad diode in the alternator. There are many options when it comes to a replacement alternator. Your original is a Lucas unit. I had no luck in getting a good rebuilt or finding good quality parts to rebuild mine. Only Chinese junk that local rebuilders could get. After 20 years my original alternator was dead and then I went through 2 rebuilt units and some parts to rebuild my original. The rebuilt units have a lifetime warranty but that is not a selling point to me. If a unit only lasts 3 months and just dies what good is the warranty. i am still stuck and I get to do the same job over again.

I finally went with the John's cars bracket and put a GM in the original alternator's place. I was able to go with a 94 amp unit. I think it is 94 amp. It was 5 years ago and I have had no trouble. Plus parts are readily available should I had problems on a trip.

There are others who have put a GM CS130 type where the air pump used to be. I think some have used the Bosch from later model XJSs and modified a pulley to use with a V-belt.

 

My response in black below

Before start: 12.33
This is too low but might be just your meter reading low. We need to find a base line.

Do you have a battery charger? if so put it on your battery and fully charge it. Then re-measure the voltage of the fully charged battery it should be 12.6volts

At 12.4volts open terminal(disconnected), a car battery has only 50% charge.

30 sec after start: 11.85
1 min after start: 11.90
Voltmeter on panel is sitting at the top of the (low end) red.

Heat on high, headlights on, radio on, flashers on, rear windscreen heat on: 11.25

Turned all the above off, engine at idle: 11.75

Sitting in park, ran engine up. At 2500 RPM battery warning light came on, voltage jumped to 13.7 volts. I held the rpm at 2500, warning light gradually faded off, voltage stayed at 13.7. Panel voltmeter jumped to center and stayed there.
I think you might have 2 problems here - one the panel meter is not working correctly and 2 your alternator sounds like it might be over charging

Brought engine back to idle (800 rpm): 13.7
This should not bring on the warning light, your meter might be reading low. 14.4V is normal charging voltage

 

Let the battery charge overnight, and charger indicates it's done. Voltage is 12.98 volts with one lead disconnected from car, and stays the same (at least in the short run) with the lead connected.

Are the alternator leads only reachable from under the car?

Thanks!

John

 

Does the warning light come on when you connect the battery and start with the battery voltage at 12.9V ?

Alternator is on the RHS down very low. The best place to measure voltage is at the battery. There are spinning things near the alt and with meter leads too easy to stuff caught.

 

It's going to the local shop tomorrow for a look at the electrical system. They have a factory-trained Jaguar mechanic. He's going to find the problem and give me an estimate before he does any work...which is good since if the alternator needs to be replaced this could get expensive quickly.

I'll report what he finds.

Thanks for the help!

John

 

In 'normal' alternators this behaviour is due to the exciter brushes being worn out.
You can usually find spares on EBay for <$10 or adapt near (oversize) ones by reducing on a flat file.
I've fixed many alternators by doing this.

 

Well, it's getting a "new" alternator. I've really no way to get it up in the air to try and pull it off, and with winter coming on I don't want to risk the thing running out of electrons and me getting stuck somewhere. Anything I can reach from the top I'll take a swing at, but getting underneath it seems out of the question.

The good news is that I've got the bonnet off for a new liner, so that will make it easier to put some homemade cold air inlets behind the grill. I'll start a new thread when I have something to show on that topic.

Thanks for all the help!

John

 

JCR

May I suggest that you get a 115 amp alternator fitted? The standard 75 amp one is not powerful enough for the car's needs. My car ate them until I did this and now lots of volts and amps and no further problems. Also, very important:

Ask your garage to clean up the connections on the charge cable from the alt to the starter and the one from the starter to the firewall. Clean up the firewall post this cable connectes to. Clean up the terminals/renew the earth strap from the LHS engine bottom to the front subframe and thence to the B bank bottom chassis rail. Put an extra earth strap on from the alt support bracket direct to the chassis.

All these things can cause the alt to not charge properly, and however good your garage guy is, he may not know about all of them;

ps: Alternator maintenace points copyright G Francis.

 

Thanks as always for the suggestions. I got the higher output Bosch alternator put on (as well as shocks, ball joints and a steering rack.

I've got the air pump off now, so it seems like a good time to add an extra earth strap to the alternator.

Thanks,

John