I believe I need a new alternator. My car quit on me while in a drive thru (McD’s). The car has the battery light on and it will die. I checked the voltage at the battery while it was running and it was 11.5 VDC and dropping. A new tensioner did not fix it (that’s another story).
I see I can get a 90 amp or 120 amp alternator and I don’t know which to get or if it doesn’t matter.
Can someone tell me if the output matters?
mspart
The voltage should be over 12 when measured. If you have a lot of acessories, and a rear window defroster that’s used alot, I would say go for the big one (120 amps). All police cars and taxis have these larger units.
Your battery also needs to be tested first. Even a weak alternator should be able to charge up a good battery. Don’t think of replacing the alternator until it is clearly indicated that you need one.
We put a Die Hard in the car last October so we are not thinking that is it. If the alternator were good, I would think I would get 14 volts at the batter cables. But i get about 11.3 volts with the car running. I’m not real knowledgeable about this but it seems that this would indicate the alternator is bad.
Do you agree?
mspart
At idle, it doesn’t take many things to be turned on before the output of the alternator is exceeded. Then, the battery starts to supply the power (amps) demand, and begins to discharge. At idle (700 rpm), the output of the alternator is only (about) 30 amps. As engine rpm is raised to 2,000 rpm, the alternator output rises to 120 amps. There is nothing that can do done to change this output curve; so, you have to turn some electric motors off, and the headlights off, to lessen the amp load; or, you can raise the engine rpm toward 2,000 rpm.
The alternator will charge the battery at 13+ volts when there is less electrical load on the alternator that its output capacity at that engine rpm. Raise the engine rpm high enough to keep the output demand on the alternator lower than it can output, during testing. It would help to do an amp draw, during testing, to see how much load the alternator and battery are carrying, and to compensate expected test values.
When the alternator from my 93 Caprice tested bad at Advance Auto it was only putting out 11 volts or so.
The replacement alternator had the same rating (105 amps) as the original. Before replacing the alternator make sure the battery is in good shape.
Ed B.
Yes, if the diehard battery still tests out OK, then you would have a problem with the charging circuit, which consists of alternator, cables, voltage regulator, etc. A good shop can locate the exact problem very quickly.
Last year our Nissan would not start, only turned over slowly. We thought, battery, perhaps alternator, but it turned out to be the starter motor. A good reason to check it all out. Alternators are expensive, and if it is somerwhere else in the charging circuit, you might be wasting money.
Yes, fair enough. The car died at the McD’s drive through, we got it jumped and that lasted all of 5 seconds. My son and I pushed to a neighboring parking lot, borrowed some keys from a McD worker we know and hooked the batteries up for quite some time with both cars running. We called my son’s friend and he came and went home with us in case we needed another jump. While riding home the clock digits disappeared, the car started bucking and then it wouldn’t go anymore. The AC was off and I tried to get the blower going but it wouldn’t go (before it finally died).
Believe me, I don’t want to spend $150 on an alternator but I don’t see much way out. The battery appears to hold a charge from this experience, I just don’t think it is getting charged.
I appreciate all the things you guys have said. If I’m off base, let me know and I’ll do some more checking on the car.
Thanks,
mspart
Ok, thanks for the information. I think I’ll get the alternator, test the car and probably install it. If anyone has tips for how to test I’m all ears.
mspart
When you checked the voltage, WHERE did you check it? If you put your voltmeter test lead on the battery post, that may not be measuring the actual output of the alternator. To check the alternator output (all electric stuff turned off, and engine at idle) place the positive test probe on the alternator output post where the big wire is usually held to the alternator post with a nut.
If there is a difference in the voltage measured, that varies with changing where on the alternator-to-battery wire the measurement is made, something, usually corrosion or a loose connection, is causing the difference.
Disconnect the battery cables, use a round wire brush (or file, or sandpaper) and remove the corrosion from inside the loops of the cable clamps. If it has side mounted battery posts, use the wire brush. Shine the battery posts with sandpaper. Smear petrolatum jelly on the cable terminals and reconnect to the battery. The pet. jelly will prevent more corrosion.
[Anecdote: Recently, an acquaintance had this cleaning done (by Yours Truly), and it cured a stall/no-start problem that a new alternator (by some “mechanic”) had failed to cure. The alternator wasn’t the problem. The corrosion was the problem.]
The charging circuit for the van involves more than the alternator. The BCM and PCM modules are also involved so the trouble may not be with the alternator itself. Check to see if the voltage on the smaller wires tied to the alternator are close to the battery voltage while the engine is running. If they are then I would guess the trouble is with the alternator.
Thanks everyone for the great troubleshooting ideas. I tested the voltage at the battery cables. To get to the wires at the alternator is impossible. I know because I just replaced it. You are supposed to take out the window wiper module but I didn’t. I think the alternator r/r would have been easier but I wouldn’t have finished any quicker.
The good news is that the charging circuit is now ok. Fortunately. Alternators are not cheap. Cougar is right that the computer regulates the voltage. I didn’t know that until later but that could have been the problem.
You know, the air bag light had been on for quite awhile. It and that battery light are now extinguished. It took my son and I about 3.5 hours to do the job. I had to remove the tensioner to get the alternator bracket out, the book doesn’t say anything about that. We got it back together alot faster than taking it apart and we rested in the hot tub.
Thanks again for all your help. I very much appreciate it.
mspart
Glad you got it fixed and thanks for the update.
I’m glad you found what was the most likely cause of the (too) low charging voltage (11 1/2 volts) from the alternator.
When checking the output voltage of the alternator, one wants to check voltage (ideally) on the alternator output post. If one can’t check there, one should check the output voltage, on the wire from the alternator to the battery, as close to the alternator as one can get. What if a connection, especially where the wire is attached to the alternator output post, were poor, and (thus) dropping the voltage? One would, then, measure a low output voltage and assume that the alternator output were low, when, actually, it wasn’t. That would be a wasted alternator change.
The question is, what to do when one can’t access the alternator output post, and, thus, can’t directly measure the voltage? When one has the alternator removed, and the wires are still attached, one could re-connect the battery, and check the voltage on the alternator output post. Compare that voltage to the voltage on the battery post. If the voltage is lower on the alternator post than the battery post, there is a resistance which is dropping the voltage between the battery positive and the output post of the alternator. The alternator may not, after all, be faulty, in this circumstance.