Hi everyone. I have a late S3 V12 XJ, 1992 MY. Lately, I have had an issue with the battery voltage gauge (battery condition indicator) showing 12V after engine start up (top of red arc on gauge). It sits that low for the first 5 minutes or so of driving and then it suddenly goes to the 13V mark and stays there. This would be the normal indication. I changed the battery for a new one and it made no difference. I verified with a volt meter connected to the battery that the voltage is only 12V with engine running for the first 5 minutes.

The ignition warning light doesn't work. I'll have to change the bulb. The other warning lights work fine. I suspect that the alternator is not working initially and the ignition warning light should be on.

Is this a sign of a failing alternator? Or is it something else?

Any help would be appreciated.

 

Replace the bulb first and see what happens. The alternator "excite" circuit may need the bulb resistance to kick start, so to speak, the charging process.

Cheers
DD

 

Interesting. Thanks Doug. I'll do that and report back.

Steve

 

Doug, I swapped the bulb with the hazard warning bulb that I know works, and the ignition warning light still doesn't work. I guess I'll have to figure out why this light doesn't work. Do you think it might be an issue with the ignition switch?

 

I am with Doug.

The fact the "new" globe is still dead, that circuit needs to be traced and rectified. I doubt the switch, as the engine runs, so the Ign side is doing what it should.

The alternator needs that circyuit to " excite" the innards. YES, it will self excite, usually at about 1500rpm engine revs, but not always.

Being a 90 year, look and see if it is fitted with a Load Dump Module, a box looking thing on the back of the alternator casing. If so, disconnect it, they are generally more strife than originally designed.

While there, probe the small Ign wire, with the Ign ON engine OFF, it should read battery volts, if not, track it back until the fault is found, and the globe will possible come alive due to that.

 

Check the wiring at the back of the alternator. The "excite" wire to the dashboard warning light is small gauge, brown/black. Make sure it isn't broken. Clean/tighten the connection.

Cheers
DD

 

Thanks Grant and Doug. I'll have a look at the wiring for this circuit. I'm glad I didn't go down the path of a failed alternator and/or ignition switch!

 

Hi Doug and Grant. I finally had a bit of time to look at my car this morning, and you guys are absolutely right. Once I removed the air cleaner and realized what I thought was the alternator was actually the air pump, I found the alternator and could see the loose black/brown wire that is causing the problem. Now the next step, reconnecting it. It is just a plastic spade connection that has come off. I can see where it attaches to the back of the alternator. The hard part is reaching it. I can't seem to reach it from above. I will jack the car up and see if it is easier from below. Thanks again, and I will let you know how I get on.

 

AH, the simplicity of a V12, priceless.

From below is normal, unless you have a keen as mustard child that loves crawling into places, I did.