Quick back story: Yesterday, my starter seemed slow in the morning. I drove to work (50 miles), daytime, didn't use any accessories because I was worried something was wrong.

When I left, it started fine/fast, even stopped a couple times at places and started it again. Drove back in the dark, 50 miles, headlights and radio on.

This morning when I went to start it - battery dead, wouldn't turn fast enough to start.

Charged it -- battery now reading 12.5V.

Idling at first, it was reading something around 13.5, but as it continued to idle, got up to around 16v.

Here's the weird thing - when I rev the engine, the voltage drops, back to around 13.5 or 14v.

16v seems way too high, like it could do some damage.

So somehow it is reading too high at idle, but not charging the battery properly enough with headlights and radio running at highway speeds.

Any ideas? Voltage regulator?

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Batterys can fail quickly. Take it to a battery shop and have it load tested. If it is in poor shape it won't take much of a charge and the voltage will vary somewhat as you have described. It is cheap to check and at least eliminate one potential issue. Cheers Dan

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It's always something simple
91 300D 603.960 (from japan) 194K
92 Toyota Diesel Landcruiser HDJ81-t 116K
02 Golf TDI new head courtesy of PO
87 300D
97 BMW 525 TDS Wagon 5spd
bunch of Onan and other diesel generators

With engine running at idle, disconnect the positive side at battery and measure Volts DC and AC. Edit: From the positive cable to the ground cable with the battery out of the mix.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)

Thanks so much for the responses so far.

Here's some more numbers:

Battery prior to starting: just under 13V

Alternator only (w + bat terminal disconnected) -
- at idle: between 13.5 and 14v DC, between .75v and 1V AC.
- revving: up to about 15v DC, up to 1.25V AC.

Battery + alternator:
- at idle, at first, about 13.5 or something around there
- when revving AND idling after revving - up to +16.5 and holding there steady or even climbing (I took the cable off the terminal again when I saw it climbing that high).

The voltage would charge high like that, and then when I disconnected the battery, it would drop again. Connect it again and it would start normal and climb and hold high again, repeat...

Hmmm... maybe it is the battery acting wacky? The alternator doesn't seem too off on it's own. The voltage only climbs when it is battery + alternator in combo.

Battery was new in spring, but has been through a dead alternator/lots of jumping/charging in July. Alternator was just rebuilt in July.

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I second the load test on your battery. I think that might be your problem.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

The voltage regulator has failed. That voltage is too high and has most likely boiled the battery until it is low on acid. That voltage will kill the battery, bulbs, glowplugs, etc.

Sounds like the regulator is not regulating...

A battery, which is a series of (6) chemical cells at a nominal 2.1V per cell cannot, and will not 'ask' for higher voltage. The regulator increases voltage output of the alternator to drive current into the battery to charge the cells.

Alright, I just learned a bunch of stuff.

1. Took the battery where I bought it and they tested it and said it is fine.

2. I (stupidly?) drove with my + battery terminal disconnected (for fear of the voltage that was reaching it from the alternator frying it) and fried my headlights and turn signals within the first block of driving. I don't know if that would normally be the case, but well... $55 in lamps later I guess now I know not to do that.

3. Voltage with battery is now reading 13.25-13.5v with no rising voltage. WTF? It drops a little, like .25v when revving, but otherwise was holding steady. With car off, battery reads about 13v.

4. I disconnected the battery again and my windsheild wipers started moving on their own.

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Stop running the car with the battery disconnected. You can do a lot of damage. If the car was a lot newer you could already have been into the thousands of dollars to sort out the mess.

Figure out a more normal way to troubleshoot. Obviously the windshield wipers cannot run with the battery disconnected and the engine not running.

I don't know a more normal way to troubleshoot. That is why I am here asking questions.

The windshield wipers ran on their own only with the car running. Sorry if that was unclear.

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Sorry did not mean to sound severe. I just wanted you to stop damaging things. There are contacts that open up when the wipers are in the park position to stop them. Somehow those contacts may be welded together now. Especially if the wiper switch is turned off and the wipers will still go with a battery installed and hooked up.

Pull the wind shield wiper fuse to get them stopped so that you can concentrate on your basic problem first. Wiper issue later once the voltage issue is solved.

Okay just lets back up a little. When you read the sixteen volts was it at the actual battery posts or was it on the battery clamps? If on the battery clamps you may have dirty battery ternminals but I really cannot see the car starting with them that bad. Other than that the regulator in the alternator is suspect or the ground from the body to the engine perhaps. Sounds like the regulator may be a little erratic and perhaps quite intermittent as well.

X2

Never disconnect the battery or alt with the motor running in a car with an electronic regulator !!!

You have killed the reg & who knows what else.
Electronic regs need a battery in place to stabilize their output. It takes only a micro fraction of a second to fry the power transistor in an electronic reg. There are those with a background of working on old cars with generators and electromechanical regs, their testing procedures can not be used on cars with electronic regs.

If you have measured 16v dc with the motor running & the battery installed, the reg is toast.

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

as LB40 just said:

"Dis-Connecting the Either the Positive or Negative Cables from the Battery
when the Engine is Operating KILLS the Voltage Regulator."

V.R. Run Amuck will KILL other Electrical Appliances.

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'84 300SD sold
124.128

Thanks,

I will see if I can get a new regulator locally tomorrow. There is a re-builder not very far from where I live.

When I measured the voltage, I had the probes from my multimeter wedged by the bolts of the clamps, but I think I also read the same while directly touching the battery posts with the clamps connected.

I am glad I only fried what I did. I guess one has to use more caution when taking advice.

If I measure resistance from the engine to body, what would be a reading that would tell me it is suspect/poor connection? Probably anything over an ohm? Or even less?

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My suggestion was to only *test* the alternator output with the battery disconnected. If the battery was bad, it would effect the voltage readings of the test.

Absolutely DO NOT drive or run the car with the battery disconnected as shorts could and probably will occur.

I have performed this same test for many years - over 30 at least with no ill effects to any car I've ever owned. However, I would not rule out the possibility that the battery might provide a load for the charging system in the event of an overvoltage condition.

Now, if the Voltage Regulator is defective which, after testing, sounds like it is, the battery would have little to do with any over voltage condition. I agree damage could occur to electrical systems due to a faulty regulator.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)