Is it the battery? No... Is it the alternator? No... Then what the heck is it?!

I’m a novice at best, so pardon the lack of technical jargon and what have you. The story is long, but if you’re looking for a head scratcher I probably got it.

I’ve got a 1998 Pontiac Bonneville SE with close to 200,000 miles on it (bought it at 130K). 6 Cylinder 3.8 Liter. No major problems with it before except a dead fuel pump, and my engine “vapor locked” once. Both of these problems happened 3+ years ago.

I recently had an issue with my car in that it seemed it started hard (it would turn over more than it usually did and it would shake when the car started). I don’t know how long this went on and I kind of just took it to mean that the car was getting old as I didn’t see any other characteristics manifest themselves. However, about 3 weeks ago my car wouldn’t start. I got a jump and upon further driving noticed that my batter gauge would fluctuate wildly. It’d read 13 or 14 volts after I started it up (and after the jump it read about this) yet when I was driving it, it would dip to as low as 5 or so.

I called my father and he figured the battery was the problem. We took out the old battery, which did seem to be old and somewhat corroded, replaced it and GREAT! It seemed to start up smoother, the gauge didn’t flux at all and everything seemed hunky dory.

Side note: The battery that’s in the car now is a “blemish” (meaning aesthetic imperfections only). I was told that THIS IS NOT A REFURB (but I am now taking that with a grain of salt). I fear mentioning this as I don’t want to insert bias into my question, but it should be known. I’ve known people with these batteries and they’ve not had problems.

I drove the car for a couple weeks and then the battery gauge thing started happening again, where it would fluctuate, sometimes wildly and dangerously low. I took it to a local parts store and got both the battery and alternator tested. They said the battery was OK and that the alternator was dead, it was only putting out 19 amps when it should be putting out around 95.

I bought a brand new, though off-brand, alternator, figuring I don’t need two non-new parts to question if it didn’t work, and installed it with the help of my ever-so-gracious father. Immediately after we installed it the car started up like a dream and the needle was firmly planted (an eensy bit of shake) near the 14. It ran smoother and sounded better, too.

While driving it home that night the battery gauge started doing its thing again though less consistently. It’d mostly stay around 14 (just under) but it would dip down a bit. Typically not as much as before but there was one instance when it did go waaaay down. However, that wild of a fluctuation has not happened since that night, which was last night.

Side note: That night, I was able to link the needle flux to something odd. It dipped noticeably twice when I was at a stop light and turned the wheel hard left. It only did it twice, despite subsequent turning of the wheel and I haven’t been able to recreate this situation, so perhaps it was just a fluke.

I drove the car around for probably about 30+ mins that night figuring the battery needed a good charging after all that. I also took it back to the parts store and both the battery and alternator checked out OK.

Today (the day after I installed the new alternator) I started the car up and it seemed fine. The needle was again near the 14 and it looked to move only very little. I chalked up the previous night’s needle fun to the fact that the battery was probably near-dead and the alternator was most likely very busy trying to charge it back up.

I took it out at lunchtime today to run an errand and it seemed OK. Very little flux again. I stopped at home to get something and when I came back out, maybe 10 minutes later, it started up real hard again. I watched the needle move as I drove it and it did seem to move, perhaps slightly more than before (perhaps not), but it never went much below the 14.

Another thing I noticed, though this could be completely unrelated, is that I seem to be running slightly hotter than I did before. This could be due to the fact that I have a working alternator as, I assume, a working alternator will produce more heat than a busted one. I also noticed that the heat didn’t necessarily correspond to the activity of the car, meaning that if I revved up, the needle sometimes went down (sometimes up). The temperature gauge now seems to be moving more than usual, but only after I replaced the alternator. Like I said though, this could be unrelated.

I’m going to let it go for a couple days as is and see if it starts to balance itself out as I think it could still be an issue of the alternator trying to supply most of the power it produces to the battery. However, I doubt this will solve anything.

So, I am MORE THAN OPEN to any suggestions anyone has regarding this. The only other thing I can think of is a short or other electrical problem SOMEWHERE.

A hearty thank you and good day to all who read this far.

I may be dead wrong, but from your discrition of the problem it seems to be headed for the starter. That is the only thing between the alternator and the battery. Poor contact there can give a whole lot of porblems and it is hard to see. Have that checked.

Besides removing/cleaning and replacing the battery connection at the starter, ensure ALL the ground cable and wire connections are clean and wrench tight too.

As well, ensure a solid ground from the engine block to the firewall or chassis.

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I don’t think its the starter. Maybe I missed something in that post, but I don’t think there was ever a slow or no crank problem.

I’d guess fuel delivery myself. Check the pump, the filter, and the injectors.

I’m just about to leave work; I’m checking the starter on the way home. Would a fuel problem really cause the battery meter to fluctuate, especially if replacing the battery and alternator seem to semi-solve the problem or solve the problem for a short time? The hard starting was “solved” twice for a brief period of time, first after I replaced the battery and second after I replaced the alternator. Do any of the parts you suggest require power drain, as it seems that it’s almost exclusively an electrical problem.

well if the fuel pump has a wiring problem it could.

Otherwise, don’t dismiss the idea that you have two separate problems - - such as a bad ground making the meter fluctuate (btw in-dash meters are always inaccurate as hell - -go get a multimeter and get a real measurement :wink: ) AND a problem with fuel delivery.

That night, I was able to link the needle flux to something odd. It dipped noticeably twice when I was at a stop light and turned the wheel hard left.

Might be worthwhile to check the accessory belt and tensioner as well. Sounds like it might be slipping under load.

although i am not a mechanic, (nor typist) i think y ou are confusing normal gauge readings.

typically the meter will read 14 volts while it is charging the battery. when the battery is charged back up the volts drop down. if your alternator is reading +14 all the time, there IS something wrong.

that said, the other issue of hard starting is another issue. fuel filter?

someone else mentioned starter. if you have a starter that is without solenoid, i am assuming that it takes alot of current to get going. if it does have a solenoid, then maybe its going.

how to check, ???

If the temp gauge is hotter than it was for most of the time you’ve owned the car, that could indicate a problem, but it is separate from the electrical problem, assuming the current voltage reading is the same as it always was.

If the voltage gauge is slightly higher than it used to be, and that could happen if the voltage regulator is built into the alternator, and the temp is only slightly higher, that is simply that the temp gauge is measuring a percentage of max voltage. If the voltage is higher, then the gauge needle moves up. Say you used to have 13.8 volts before, the low end of normal from the alternator. If normal temperature sent 50% of that voltage to the gauge and 7 volts move the needle to the halfway mark, the needle would be slightly below half way, 6.9 volts. Now the new alternator puts out 14.2 volts, the half voltage at normal temp is 7.1 so now the temp gauge will be slightly above half way. If your change in temp is greater than this, than there may be a problem.

As for the hard to start. If the engine turns over slower than it used to, the starter is probably going out. If it turns over as fast as it normally does but takes more turns to catch, the the problem is in either the ignition or fuel system. If it turns faster than it used to, there may be a compression problem.

Changes in load or engine rpm can temporarily change the system voltage. Low rpm and heavy load (fan on high) can really drop it.

Any battery, new, blem or recon can fail early. They have warrantees to cover that. Most batteries will last 5 to 6 years but I have seen brand new, highly priced, good reputation, batteries go out in only a few months. Not very often, but it does happen.

I have discovered a correlation with the fluctuation of the battery meter. I think this is a new discovery, though I can’t guarantee that it DIDN’T do this before:

When I move the car from (e.g.) park to drive, the voltmeter meter dips down, pretty significantly, too. This does not happen if I go from drive to park. It does not always dip down the same amount, but it usually is fairly significant. It also does not do it going from (e.g.) drive to 2nd.

Any ideas with this new information?!

Also, the starter passed diagnostics (it takes more turns to catch, it doesn’t turn over slower, which someone said indicates a fuel problem). For giggles I got the battery and alternator tested again, too, and they passed.


When I engage my door locks, the needle dips. Only if going from one state to another (lock to unlock, not if I try to lock while already locked).

This leads me to believe it’s a short, perhaps a bare wire touching metal on the inside of the door (or elsewhere).

Any takers on this theory?! :slight_smile:

I very much doubt there is a short problem causing the fluctuations you are seeing on the voltmeter. A shorting problem should blow a fuse.

It is normal for load changes on the power buss to cause small changes in voltage levels (say, up to around a volt). You need to be concerned when the voltage goes less than 12 volts and greater than 15 volts. Voltage readings in between those limits aren’t much cause to look for trouble.

If the hard starting issue you are talking about is the starter motor turning slowly there are a couple of places to check for trouble. The starter solenoid may have bad contacts and that causes a voltage drop across them. This causes the motor to get less than the needed power to run it. There could also be a bad wire connection in the main cable to the solenoid. If the voltage on the main battery cable, at the solenoid, is around 10 volts while cranking the engine, then the cable is ok and the solenoid may need to be changed out or repaired.

Honestly, at this point I think your are over-reacting to the normal minor needle fluctuations. When you go from park to drive the engine slows down a bit and the alternator slows down, so it may not produce as much voltage at idle in gear as it does at idle in neutral or park. You may notice this at night with headlights turned on the needle drops a little when you are at a stop. Those are normal sorts of changes. As long as it holds steady while you are driving along at 20MPH or more, I’d say you don’t have a problem. I suspect your alternator was the original problem, but now you are just so gun shy of every little thing that you are worried about normal gauge movements.

The engine temperature will also fluctuate as the thermostat opens and closes, and as the radiator fans cycle on and off. That too is normal. If it’s consistently near the read zone on the gauge you should have it checked out, but small changes as you drive are normal.

Let me try to explain the alternator to you. An alternator is a three phase AC generator. Within the the alternator are three coils, 120? apart in the case. There is a coil in the center that spins. The coil in the center provides the magnetic field that cuts across the three stationary coils inducing current flow in them. The amount of current induced in them is dependent on the strength and speed of the spinning magnetic field.

As the field spins faster, it generates more current. The amount of current can also be controlled by controlling the voltage applied to the spinning field, there by controlling the current flowing through that coil and controlling the strength of its magnetic field. To keep the current out constant as RPMs go up, you would reduce the voltage to the spinning coil. That is the job of the voltage regulator.

Now if you know ohms law, you know that voltage is equal to the current times the resistance. The resistance is from the load. The load is the things you turn on like lights, fans, door lock solenoids etc. As the load varies, that is you turn something on or off, that changes the resistance. Lets say you turn something on, resistance goes down. The alternator is putting out a fixed amount of current based on the previous load, and now that current is facing less resistance, so the voltage goes down. When the voltage drops, everything else gets less voltage and therefore, less current. The voltage regulator senses the drop in voltage and supplies more voltage to the spinning coil, that makes more current flow in it, increasing the strength of the magnetic field, increasing the current output, and voila, more current through the new resistance, voltage in the circuit increases.

The voltage regulator is trying to maintain a constant voltage in the system by controlling the strength of the magnetic field in the alternator. A sudden change in the load can cause the voltage to fluctuate while the regulator is trying to stabilize it. The max voltage that can be applied to the spinning coil is battery voltage or system voltage. Under heavy load (lots of stuff turned on) and low RPM, the voltage can dip to the point that the alternator can’t keep up. This can drain the battery. Idling in gear with the headlights and AC fan on high and the megawatt stereo on full blast will kill the battery.

Also inside the alternator is rectifier bridge. Its job is to convert the 3 phase AC to DC for the cars electrical system. It consists of 6 or 8 diodes. If one or more of these diodes burns out, the maximum capacity of the alternator is also reduced. Short circuits in the cars electrical system, or in an aging battery and draw more current than these diodes are rated for and can cause them to burn out, but only at higher RPM. At lower RPMs, the coils are not capable of making enough current to damage them. Sustained high speed driving with a bad battery can ruin an alternator FYI.

I hope this helps you a little. Some fluctuations are normal, a drop when idling in gear is normal, drops when the door lock solenoid engages is normal. The drop because you turn the wheel sharply to the left while stopped isn’t, but it could be due to the sudden demand being put on the power steering pump that it momentarily causes the belt to slip, the same belt that drives the alternator, thus causing the alternator to momentarily slow down, dropping the voltage for a moment.

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Thanks to everyone who has responded so far. FYI> I tried a new-new battery and I still have the problem.

I’m figuring that I have an electrical problem somewhere, a bad ground or frayed wires or something. I think that my needle fluctuations are abnormal because (1)it did not used to do this, (2) it will fluctuate even in idle without intervention (3) I had a recent problem where I was in idle and the needle dipped (fairly harshly) suddenly and all my lights went dim.

My last car I was starting to run into this problem as well, and I think it was due to the fact that I had an unprotected entrance for into my electrical system for water/moisture/etc in the form of a busted headlight case. It got so bad that the lights would flicker as I was going over bumps and things.

At this point my solution is to ignore it and hope it goes away.

JMHO, but if the car were mine I would go over all of the battery cable ends (including up under the insulation near the cable end) and I would especially take a look at the fusible links.

I think this car uses a link between the alternator and battery. Battery charging is provided by the alt. through this link and due to the car’s age and mileage it’s possible the link end could be corroded or scaled over; especially so since a lot of current passes through this connector.

A schematic is not available to me at the moment but my guess is the link will take off from the positive battery cable terminal on the starter. At least the older ones did.
A crummy connection here can also cause the battery to fail to charge at times along with causing voltmeter fluctuations.

Hope that helps anyway.

Thank you to EVERYONE that replied. You’ve give me a great deal of information to go off of. My first order of business will be checking the connections. My second order of business will be to see if I can see a spark or anything when the engine is running. My third order of business will be to go insane. :slight_smile:

Thanks again.


There is no problem that can’t be discarded. A Bonneville with over 100,000 is nothing to keep. 200,000 is really not worth keeping. More really bad problems are usually on the way.

I know I am almost 10 years late to this conversation. My car was acting the same way today. I have an explanation. The battery is fully charged so the system drops the voltage to 12 volts. It is a way to protect the system. When my car voltage was dropped to 12 volts when I was driving, I just turn on my headlights and the voltage went back up to 14.23 volts. Since the headlights was using up energy, the system started charging again and the voltage went back up to 14.23 volts. I will observe my car the next few weeks and see if my theory is correct.