Sweeping needle on volt meter? Help....

[darkredturbo]

I searched around for a while, but I couldn't locate anyone with my exact problem.

Yesterday I was driving my '89 Caravan when suddenly the needle on the volt meter started to sweep wildly back and forth. I was pulling into a parking lot anyway, so I stopped, waited a minute, and it returned to normal. Then, on my way home the voltage dropped to about 1/3 from the left (a bit above the "red" zone) and my check gages light came on. It would fluctuate a little bit as I drove. I started it up after work and the voltage was right back in it's normal spot (I'm guessing that's 14v--norm, rght?). I took it to Sears on my way home from work to have an alt. test done because the voltage dropped as I drove it. First, they said I needed a new battery (doesn't suprise me), then they did the test and the alt. started at 14v and then dropped to 12v. They suggested replacement. I was going to take the alt. off my other van tomorrow, but today, the gauge was fine all day!

I read quite a few posts on kaput voltage regulators. Could this be my problem? Maybe a shorted line? Or do I just need a new alt.? I'm new in the electronics dept.

I guess I could just change the alt. tomorrow and see, but it's only 31 degrees tomorrow, and I figured I'd ask for a few suggestion before I freeze myself.

Oh, it was also snowing quite a bit yesterday. Could water have affected the system somehow?

Thanks,
Ian

[charger R/T]

Yes it could be the alt and yes it could be in the control of it (the regulator). It could also be a loose wire or one broke and intermittant. There are quite a few threads on the charging system here and a few differant ways to check everything. Some of the ways people post to check this works but can be confusing to a electrical novice. It does sound like a connection or intermittant wire issue. First I would hook a volt meter to the battery start the engine and carefully move the harness around while keeping an eye on the meter. Shake it all the way back to the area of the battery and computer. You may luck out and find a spot that makes it go crazy. The way the system works is actually quite simple. First some mopars run a ground wire from the alt to ground, some don't. If yours does check it. I believe the only ones that run a ground wire have bushing in the mount system of the alternator.
Now that the ground is out of the way the rest is the same to all cars. There are three wires on the alternator. One large wire that goes to the battery. It has power on it all the times even with the engine off. It has a fuse link in it near the battery or is fused in newer cars. Check this good it can do what you describe. The other two wires is where some peoples descriptions can get confusing. (I hope mine isn't to you) These are the field wires. These control how much the alternator will charge. It seems common to find these wires loose. There is a set of windings in the alternator and each end of this winding hook to these two wires. One of the wires gets power (+) when the key is turned on and gets full battery power at all times with the key on. The other one gets a ground to make the system work. It gets it from the computer and the computer varies it to control output voltage. So if the computer needs a little charge the computer gives it a little ground if the computer needs to up the voltage it will give it more ground. Both of these wires can be checked while hooked up but this is where I think it gets confusing for the novice. I would unhook both (make sure they are not hitting anything and turn the key on) with a volt meter (neg term on battery ground) see which one has battery power (+) the meter should read the same as your battery. (a fully charged battery should be around 12.5 volt) Now put the postive lead of the volt meter on the positive terminal of the battery and remove the one on the negitive terminal. Use it to check the other wire it should read battery voltage (some cars computer will not put the ground on this wire with the engine off so if it does not read battery voltage it can be checked later with the engine running) Turn the key off and at this point you can switch your meter to ohm and hook it up to the alternators field wire terminals on the alternator you should get a reading. When I go this far to check these systems I take a piece of black tape and mark the wire that gets (+) power on it for future use. This helps if testing the system with the wires hooked up. Hopefully you found an issue and fixed it. Hook it all back up and see what it does. It also does not matter which little field wire goes on which terminal. I know this got long but I also hope it didn't confuse you and is helpful. Post back if you find something.
Tim

[darkredturbo]

That was great Tim. Your instructions were very easy to understand. I will check the fuse today, but if it is something else, I will do all of that tomorrow, for it is to be 38 degrees! I'll keep this thread updated.

Ian

[darkredturbo]

Ok, so since the van was running fine this weekend, I checked everything but could not find the problem. Then, this evening I was on the highway and the whole thing started happening again. I pulled into a gas station and realized that besides the two field wires and power source, there is also a small ground near the bottome of the alternator (or it is when I'm looking down into the van. This ground had no bolt and had come loose! As soon as I reattached the ground, everything was working properly again.

Thanks for all the help. The problem was much more simple than I had expected, but everything you had me do really taught me something about my electrical system!.

Ian