I understood that the 3-gauge cluster was nothing more than two good looking idiot lights and a clock. However today my voltmeter actually moved off center and began reading about 11v. When I turned off the AC (it's 104 F today) and radio, it went up to about 12v.

So is my battery of undetermined age dying or do I have an alternator problem? I had the battery checked a few weeks ago while tracing down another issue and it was pulling 12.67v at rest.

No AC or a dead battery with triple digit temps is no bueno. Suggestions please.

 

if the voltage drops with the motor running it really can only be the alternator I would have thought.

 

The voltmeter is much-maligned, but it's not so bad as dash instruments go. It will give you resolution of a few 10ths of a volt.

Let the battery rest overnight and then do a voltage measurement before starting the car. We'd like to see at least 12.4 volts or so.

Good luck.

[edit] But re-reading the thread ... battery or no battery, the behavior sure sounds like charging system trouble. Let us know about the battery voltage, first off.

 

I made the mistake of loaning out my voltage meter, but will try to pick it up tomorrow. Good to know about the gauge. And while I am inclined to agree, I hope Seakiwi is wrong about the alternator. Let's see what happens with the battery.

 

OK, how about a quick-and-dirty test using dash voltmeter. All non-essential electrics off ...

Ignition on, but starter not engaged, voltage = ?
While cranking, voltage = ?
After car starts, say at 1500 RPM, voltage = ?

 

Ignition on position 2 = 11.5v
Cranking = 11.2v
At 1500 rpm = 12.2v
Idle speed = 11.9v

Readings are approximate, but reflect needle movement relative to full marks. These were taken this morning after car sat all night. All possible electrics shut off during readings. Normally the needle sits at the 13v position (12 o'clock). I'll try to get by Autozone today to see if they can test the alternator. I think they have that service.

 

Mine does the same thing if my AC is on along with the headlights. If I turn one of the 2 off, I'm fine. The moment both lights and AC are on, my meter goes below 12. Hovers around the 11 mark.

 

The factory voltmeter is wired to pick up voltage that is fed on the branch circuit power at the instrument cluster, instead of direct at the battery where it should be sensing.

This causes it to read the result of voltage drops in the branch circuit wiring and connectors. It then displays false readings due to varying loads....this is not a good electrical design.

 

Hello, I have been going thru 2 months of this exact same thing scenery .
When A/C , lights, radio and then come to stop sign and the engine fans come on I drop to 11.2 for the initial fan startup and then go up to 11.5 and I would at times get ABS and no trac codes.

Ok after much ado trying to diagnose, there are MANY factors to check.
Alternator, wire harnesses, grounding points there are many.
I had a new alt. Put in 8/11 they gave me a new alt. About a month ago under warranty.
Drove home and within 1 mile the same thing happened.
So they did more diagnose, now it came out that the alternator wire harness was bad(a known issue for Jag) between the alt. And the engine mgt fuse box and starter. They found a 1.2 volt drop there and they had to prove it to and after Market warranty adjuster. The warranty co. paid the repair bill plus the adjuster(there is a long story behind all this, but we'll leave that for another day)
The new wire harness from jag is 170.00 or so the labor 600.00 they ended up making the harness because Jag's was 6 gauge and they made my new one from 2 gauge wire.
All other points of harness and grounding front to back checked out well, my car has absolutely no corrosion anywhere.

Ok now I get back up to 13.6 or so at 1500 rpm's good. Story not over yet, remember your voltage readings will be completely different from a cold car to a warmed hot car especially in summer heat 100+ degree weather. My voltage is definitely better with new harness, but still getting the drop under load with fans on still down to 11.5volts. Had someone else do more diagnosing everything front to back all positive and ground contacts there are many, found nothing, then took to another repair shop and put on big machine and did electrical system test under load. It turns out the new alternator is bad again after 1 month! Go figure!
Now I have to go back to original shop who installed the second alternator already under warranty and get them to warranty the new alternator.
What a mess, now big attitudes and testosterone come flying out of nowhere about me going other places to get checked. WOW!!!

Anyway the moral of this story is there is not just one place to check for voltage drops, yes it can be alternator but there are MANY MANY other factors involved with the charging and electrical system of these cars.

Good Luck and,

Happy Motoring...

 

This is my daily driver, and I do not have a backup car right now. So my question to the group is: am I going to be stranded somewhere if I continue to drive this a few days until I can get it diagnosed and repaired? (assuming it is a bad alternator or the wiring harness)

 

Hello, that would be a question that only the future will know.

It depends on what the problem really is, but my feeling is you should be ok for a time, based on my experiences that seem to mimic yours.

The only thing is when voltage gets to low the car will start throwing codes, like I said the ABS and ASC and traction systems are very sensitive the low voltage but easily remedied by turning the ignition off and restarting the car and driving, that should clear the code until the next low voltage.

The point is, until you get a definitive diagnosis you do not know what is actually causing your problem. The problem could be as serious as a bad alternator and it actually breaks down or it could be as simple as a loose ground wire that needs to be tightened or lite corrosion that needs to be cleaned off, the trick is finding the problem that takes time and knowing where to look and what to look for.
Hope this helps.

Happy Motoring...

 

The battery should read 12.56 v. at rest. With the engine running at or above 1500 rpm, it should read 14.4v, the rate that the alternator charges. If you are reading much less than this, you need to get the system checked. No battery test is good without a load test, requiring a special meter that puts a load on the batery wile testing it. Without a load test, you don't really have a true picture. If you buy a load tester, be sure to get one for 1000 amp systems.

 

From your measurements, it seems the battery is -- for whatever reason -- probably not being recharged by the alternator to any useful extent.

So I'm going to go pessimist on this one and say I think it's likely to strand you before too long, absent some intervention. If you have an external charger, probably best to have it hooked up whenever possible, until you can get the car to the shop.

Good luck.

 

So I stopped at 2 different Autozones tonight. Both had broken/missing testers. Arrgh! Weird part is, just before I reached the house the gauge went back to a normal reading. I kept AC & radio off while I drove around some more to charge the battery just in case. Everything fine until I pulled into the garage. Just before shutoff, it dropped down to just before the 12v marker. Running the rpm at 1500 would take it back up again.

I'll check cold start again in the morning and try another parts house for the alternator tester. Running without AC when it is 106 F is really, really bad and I don't recommend it.

 

106! Downright biblical.

Do you have your voltmeter back yet? Getting a few numbers right at the battery would remove some uncertainty coming from relying on the dash gauge.

 

Voltmeters are mighty cheap these days. Even the $3 one at Harbor Freight is probably better than the one on the dash.

I just went through all this on my 99 XK8 last month and it was the alternator. Got mine at Autozone.

 

The Autozone in-car tester says I need a new alternator. "Not doing it's job", was the technical lingo I think he used. The two replacement options they had were an OEM reman'd unit or a new Duralast (house brand). Duralast was ~$30 cheaper and Duralast has a limited lifetime warranty.

Neither were in stock, so I have some time to research and consider. I have seen on the forum where plenty of folks have had good luck with the Autozone version. Any thoughts guys?

And no, I haven't even bothered checking the dealer. Probably another part made out of unobtainium that will cost more than my daughters college education.

 

Forgot to add, I checked my battery with hand tester and it was OK. Autozone guy also checked and confirmed 12.57v.

 

When i start my 1995 XJ, the voltmeter sits at about 11V. A few hundred yards down the road, it hits 11.5-12V then, about a half mile later 13.5-14V on the meter.

Is this normal for this car or do i need to start looking at the regulator?

Allyn

 

Allynmey, just be glad it starts and runs......sounds like you're okay if the voltage comes back up in a short time. ? battery a little weak, but why worry. OP...agree put a trickle charge on overnight.....should get you through the day until the new alternator comes in (unless you drive mucho miles/day).