Hi Voltage Reading

[Krlyman]

1993 RMW: Took it in because voltmeter is sitting steadily at 17 volts. Mechanic replaced alternator and voltage regulator and it still shows 17 volts steady. The garage folks say it must be the meter. Is this true or is it something that I'm missing? They bench tested the new alternator and it is functioning fine.

Thanks folks,
Ken

 

[HotZ28]

Has anyone put a digital volt meter on the battery while the engine is running? Of course, voltage will be higher when you first start the engine, but should never go to 17v. Normal would be 13.8v -14.5v. Have the voltage checked at the battery, that will tell you if the voltage is reading correct on the IP.

 

[buickwagon]

When you turn the ignition to "on", without starting the engine, what is the voltmeter reading? If it's still up around 14 or 15 volts, then the voltmeter is likely at fault. If it's down around 12, then maybe you have a problem. The next sign of true overcharging is low battery electrolyte, as the overcharge "boils" the liquid away*. Have you been going through a lot of water in the battery lately?

Now, I'm wondering if the garage that tested the new alternator is the same garage that replaced it? Didn't the "mechanic" verify that it was actually over-charging with the very simple double-check that HotZ28 mentions above? Fro that matter, didn't he verify that the new alternator cured the complaint? If they can't do something that simple, how could they possibly bench test the alternator? Did they bench test the old alternator? Are they covering up the fact that they mis-diagnosed the problem in the first place?

These cars use the CS alternator, with 4 terminals and one post. The post of course is connected by the heavy cable direct to the battery. The P terminal (tachometer output) is not used (the tach is driven from the ignition circuit). Neither is the S terminal, which is for external voltage sensing and control. The F terminal supplies switched power to the L terminal, which powers the charge light on the dash. The voltmeter is not directly connected to the alternator at all, it's signal comes from the same circuit that powers the tach and speedometer.

When the S terminal is not connected, the alternator uses internal voltage sensing to regulate the output, so if the alternator is truly overcharging the battery, then the problem is within the alternator and the new one is defective. If the alternator is not overcharging the battery, then the voltmeter is defective and the "mechanic" misdiagnosed the problem.





*technically, it's not boiling, even though it can get quite warm. It's releasing hydrogen and oxygen gasses by electrolysis, but it looks like it's boiling.

 

[Krlyman]

Thank you both for quick replies.

With engine off, key off battery reads at 12.25 volts.
With engine off, key on the voltmeter on dash reads at +/- 13 volts
With engine on voltmeter on dash reads almost to red area or into red area (volts? as no number affixed), battery volts is at 14.2 v.

The car runs as though nothing wrong. But this is concerning to me. As to the work the garage did, I can't answer. I can only state what I was told and as we all know -- people will tell you what is needed to justify the replacement of an apparently good alternator with another apparently good alternator of a lesser quality. Maybe I should do the PowerMaster route.

Regarding the batter water -- I just received this wagon last week and have been going through things. The battery is an Interstate and it took a fair amount of water to bring it up to normal but it wasn't dry. My gut feeling is that it is the battery and it won't take a charge. The voltage at the battery is much lower than what the voltmeter shows on the dash.

Ken

 

[buickwagon]

How long had the battery rested before measuring the key-off voltage at the battery? If it was several hours/overnight, then that's not too bad. 12.5 to 12.8 would be better. I would load test the battery and compare the corrected CCA to the battery's rating to see how much life is left in it. Since the car is new to you, nothing can be read into the low electrolyte level -- that could just be lack of maintenance by the PO.

Charging at 14.2 volts is right in the target range. No problems there. If the dash gauge is showing something silly, then the gauge is at fault. As I pointed out earlier, the dash gauge takes it's feed from the battery, through a fuse and the ignition switch, not the alternator and there is no way the voltage climbed between the battery and the dash.

I don't know if the previous alternator was good or not. It may have been, since we don't know if the mechanic measured the voltage at the battery or not. However, voltage alone is not the only measure of an alternator. Maybe it had a failing bearing, or an open or shorted diode in the internal rectifier. A bad diode will prevent the alternator from putting out full power under load, but will still show reasonable voltage under low load. If the mechanic was really on the ball, he could have looked at the output voltage with an oscilloscope. The effects of a bad diode are visible in the waveform. Or he could have connected a carbon pile to check for voltage drop under heavy load. Let's just say that's what must have happened and therefore you didn't waste your money.

 

[Krlyman]

The battery had been resting for approx one hour prior to measurements. I suppose a two hour drive to Sears Automotive for a DieHard may be in the future. About the only battery I have faith in.

Regarding the meter however, if it reads the wrong voltage, does that mean the trash bin for it? Is the gauge adjustable somehow? How difficult would it be to place a multimeter across it to measure the actual voltage at the meter? I've got the service manual for the wagon but its not terribly useful.

RIP, Tom from Click and Clack passed away today.

 

[buickwagon]

Well, if you look at that not-so-terribly-useful FSM, you might notice that the volt gauge is electronically driven by a logic module in the dash, so a volt-meter on the gauge itself is probably not terribly useful 🙂. You could meter the voltage going into the cluster itself, but trust me, you won't find 17 volts at the dash if there's only 14 at the battery. I'm not aware of any way to adjust the volt meter, but I could be wrong. There may be some discrete components mounted that have failed -- I had that happen on my 01 Regal cluster (although that affected the digital display, not the value). Maybe the pointer can be repositioned? I've never had cause to investigate the voltmeter on a Roady. Repair may mean a replacement cluster.

1 hour is not much resting time for a battery. Overnight is more traditional. So odds are that your true battery resting voltage is somewhat lower. I'd definitely have the battery load tested because it may be getting tired. I don't know how cold it gets where you are, but around here you definitely don't want a weak battery in the middle of winter!

 

[Krlyman]

Well, if you look at that not-so-terribly-useful FSM, you might notice that the volt gauge is electronically driven by a logic module in the dash, so a volt-meter on the gauge itself is probably not terribly useful 🙂. You could meter the voltage going into the cluster itself, but trust me, you won't find 17 volts at the dash if there's only 14 at the battery. I'm not aware of any way to adjust the volt meter, but I could be wrong. There may be some discrete components mounted that have failed -- I had that happen on my 01 Regal cluster (although that affected the digital display, not the value). Maybe the pointer can be repositioned? I've never had cause to investigate the voltmeter on a Roady. Repair may mean a replacement cluster.

1 hour is not much resting time for a battery. Overnight is more traditional. So odds are that your true battery resting voltage is somewhat lower. I'd definitely have the battery load tested because it may be getting tired. I don't know how cold it gets where you are, but around here you definitely don't want a weak battery in the middle of winter!

Things seem to be running fine and I looked in the Not-so-terribly useful FSM and can only figure that the voltmeter is attached to a 'black box'. Less than useful. I've got the parts manual coming in the mail and perhaps there I can see if the meter is available by itself or should I just get used to it being so high all the time. I'd hate to spend >100$ on a battery to find that it wasn't the problem of the mysterious meter.

Thanks to everyone who has responded - this is a great forum.

Ken

 

[Krlyman]

Update on the voltage problem. I have replaced the rebuilt alternator with a brand new Bosch alternator (includes the regulator). I've replaced the batter with a high quality Optima. I've replaced the ECM with a rebuilt one. Still, the voltage reads high. Perhaps this is my purpose in life, to figure out what is wrong? LOL

Any other thoughts about this out there? I figure the little black box (logic module) might be bad. Is this individually replaceable? I'll have to check the voltage at the battery with the engine running tomorrow.

Thanks again folks.

 

[Krlyman]

I checked the battery voltage after starting the engine. It was still idling fast but voltage was at 14.75 volts. Right where I think it should be right after a cold start. The meter however, is still reading about 17 volts.

 

[HotZ28]

Obviously, the meter is at fault.

 

[Krlyman]

I finally figured it out. It WAS the meter. I replaced the instrument cluster and the new (salvage) is reading 14v. Right where it should be. Getting the dash out gave me a chance to replace the broken radio too. These AC Delco radios aren't cheap it seems. And getting the dash out. Man what a chore. When the manual says to drop the steering column a bit. They really mean it.

Problem solved.

Thanks folks,
Ken

 

[HotZ28]

I finally figured it out. It WAS the meter. I replaced the instrument cluster and the new (salvage) is reading 14v. Right where it should be. Getting the dash out gave me a chance to replace the broken radio too. These AC Delco radios aren't cheap it seems. And getting the dash out. Man what a chore. When the manual says to drop the steering column a bit. They really mean it.

Problem solved.

Thanks folks,
Ken

Brilliant deduction of the obvious! Congratulations on the repairs. I hope you have many more miles of trouble free driving without worrying about excessive voltage on the gauge.

 

[ryenie]

Fuse #13 is the fuse that powers the voltage gauge, i would suspect an issue in the printed circuit board for the cluster. Do the tach and speedometer read accurately, they seem to be share the same ground from the solid state ignition. The ground for the circuit is grounded to the rear cylinder head on the drivers side near the exhaust manifold, i would check it for corrosion and tightness.

 

[Krlyman]

Brilliant deduction of the obvious! Congratulations on the repairs. I hope you have many more miles of trouble free driving without worrying about excessive voltage on the gauge.

I figured it out some time ago but had a chance to buy a nice instrument cluster for 50$. Just thought I'd bring this to a close.

The new tach and speedo are working fine as well.

If anyone wants an instrument cluster with a faulty voltmeter, just send me a message. I'll sell it cheap.