KC John

My car wouldn't start the other day, so I put a charger on the battery and brought it up to 12 volts. I had inadvertently left my security solenoid on in the car and killed the battery.

I thought all was well until this morning when it wouldn't start again after driving it for about a half hour to 45 minutes. I jumped the car by pushing it and then put a meter on the alt (pos) and the neg side of battery. It read 10 volts, but was still hooked to the battery and it also was at 10 volts.

Later after another drive, I did it again and it said 6 volts and that is what the battery was at also. I took the alt to O'Reilly's to have it tested and the guy said he was having problems with the machine, but he'd try it and if it didn't work bring the car by so he could test it on the car. It tested out OK on the machine inside the store. Just for kicks, I bought a new alternator. Put it on my car and got the same results, so I returned the alt. and drove the car to them.

The guy came out with a tester that was for testing the alt. on the car, and once hooked up, it read "PLEASE USE ON 12 OR 24 VOLTS SYSTEMS." He said he had never seen that message before, but I didn't have confidence that he really knew what he was doing. He called someone else and he was stumped too and he was the assistant manager.

I'm charging the battery again so it will start on it's own, but I need to figure out what I might be doing wrong if anything. If I remember correctly, it should be 14+ at the hot side of the alt. What am I missing?

buns

Well it seems to me that the problem is probably the voltage regulator. Here is a simple test you can perform. It will bypass the voltage regulator.

KC John

I did what you said, and unless I had a loose wire the voltage droped as I raised the idle. Everyone is in bed and I didn't want to run the car too long.

KC John

With battery at 100% I turned on the lights and the needle never moved off 12 volts even with raising the idle. What does that mean, bad alt or bad regulator?

5thvet

Check the battery itself with the engine running at about 1500 RPM. Test with voltmeter and you should be getting about 13.5 if the alternator is charging the battery.

toddalin

I'm thinking bad cells or maybe a cracked piece of metal within the battery. Each cell is worth about 2 volts. If you had some bad cells, you could reduce the battery voltage capacity which could be why the machine told you to use it on a 12 or 24 volt system.

Also, when this happens, the battery typically won't hold a charge under load.

I use several of those 12 volt sealed lead/acid gel packs which are not identical to a car battery, but similar in concept and workings. I even charge the larger ones with my car's trickle charger. Sometimes if I discharge one too fast (e.g., accidential short), I will loose a couple cells and the battery will then only go up to 8 or 10 volts when charged, and will not hold any charge under load.

Try another battery.

KC John

I started the car and pulled the positive battery cable and the car died immediately. It has to be the alternator. The battery is only needed to start the car, I know that much. What I'm still not sure of is wether or not the alternator or the voltage regulator is what's gone south.


For what it's worth, I used jumpers and another battery with mine disconnected, and got the same results.

buns

The test that I posted and that you apparently performed, bypasses the regulator and you still have not enough voltage from the alternator. This tells us the alternator is probably the cause but you said you replaced the alternator and the new one didn't work either.

So what are the chances the new alternator was also kaput? Slim but still very possible. Or you have some issues with the R and F terminals.

KC John

I didn't think it could be a bad alternator right out of the box, but I've seen it happen with other items. I guess I'll go get another alt tomorrow and see if I still get the same results. Thanks everyone for all your suggestions.

Revfan

I'm subscribing!

dplotkin

Not true. The battery is needed to function as a filter as the alternator delivers rectified but unfiltered DC current. It also functions as a cushion to deal with sudden load changes in the system. A working alternator is most unhappy running without a companion battery.

Based on the facts you have provided it seems obvious your alternator has no output. While running even with a tired battery you should still see 14 volts+\- across the battery.

In the old days a failed alternator replacement generally included the regulator too, as these were inexpensive mechanical and consumable devices. Today solid state regulators last longer but are easilly taken out by a shorted field coil in the alternator, for example.

So I would replace both and check your connections carefully while you are at it. A splice, for example, in the heavy red wire put there by someone in the cars past could have failed (happened to me once).

Dan

KC John

Thanks Dan, that was the information I was looking for. I knew the car should stay running if the alt was putting out something. Now that I know my regulator is solid state, and you say it is possible to be taken out by a shorted field coil in the alternator, I feel like I'm on the right track.

I'll pick up both today, thanks so much.

65GGvert

Your regulator may not be solid state. If it is, someone changed it to that.

KC John

I had to drill out some rivets and found out this is a solid state regulator. Is there any way to check to see if it's good or not?

magicv8

IMO putting in a new alt and not being able to run fast idle without a batt failed the reg, or else your harness has a problem

dplotkin

That is a solid state regulator and even though it doesn't look like a over grilled pork chop I'd still replace it. A bad alternator can burn them up and the reverse is just as true, a failed regulator can burn up an alternator. In the old days they were usually replaced together.

Replace the alternator & regulator together. If your wiring is OK, this should take care of it.

Dan