My girlfriend’s car–a 1997 Mercury (Ford Escort type) was going schizo. It wouldn’t turn over, but I was able to jump it. So, I replaced the battery. Then, the battery light kept coming on, so I went ahead and replaced the alternator.
So, it’s running fine for a few weeks, then on the way home from my place (260 miles away) she tells me that the battery light comes on, and her dashboard and headlights are dimming when she slows down. As I told her before I replaced her battery and alternator not to cut off the engine, she fills it with gas and immediately afterwards, everything is just fine.
What the heck am I missing? It can’t be the battery or the alternator–I just replaced both. It can’t be the wiring, they just don’t go bad. . . period. It can’t be the fuel filter 'cause that would just shut the engine down. . .
. . . just what haven’t I seen? I’ve got the tools that I can fix it, I just can’t diagnose it!
It sounds to me like the alternator is working, given that the lights stay bright when she is traveling fast (and when the alternator is producing the most current). Did you tighten the alternator belt enough? It may be slipping at slow speeds (although I would think it might slip more at high speeds…), which would mean it isn’t really charging the battery. All WAGs, though, I’m afraid, and maybe it’s time for her to take it to a shop…
I’m thinking there’s a short to ground somewhere draining the battery. Or at least sucking enough power that when the engine speed is only at an idle, there’s not enough left for the instrument panel. I would say the alternator but since you’ve already replaced that… I’d take a real good hard look for any wires that may be rusted/detached/worn insulation etc.
It can be the battery or the alternator. Just because they’re new doesn’t mean it’s impossible for either (or both) to be faulty.
It can be wiring. It does go bad sometimes. In fact, some Ford models are notorious for having problems with the alternator harness plug.
Make sure the battery is fully charged (if it’s not, slow-charge it until it is). Rest voltage should be about 12.6.
Start the engine. Charging voltage (measured at the battery) should be 14-14.5 (this varies some depending on numerous factors).
Starting wiggling connectors at the battery and alternator, looking for any related change in charging voltage. Tap the alternator and watch for same.
With luck you may identify the problem. If that doesn’t do it, then more sophisticated equipment and/or techniques will be needed - or more luck if it’s an intermittent problem that doesn’t act up when you’re checking it.
Well tofergregg, Joe and Gary you all raise good points, some of which I’ve already racked my brain for, some of which, I haven’t.
Personally, I’m thinking she’s got a short somewhere in the engine too. I threw a Fluke across the terminals of the old battery, and got 14.5 volts. I installed both new, and got the same thing (the only change was that she could start her car without a jump).
I’m thinking she has a short somewhere in the engine, but I won’t be sure until she calls me back tonight to tell me she got home. I politely asked her to kill the engine, let it sit for half an hour, and then start 'er up again.
I just can’t think of what’s causing the electrical problems!
A short? Probably not. Short circuits tend to either blow fuses or let smoke out of the harness. (Sometime knows as a thermal event or a fire.)
Alternators can go bad in stages. Rebuilt alternators can be bad out of the box, or fail very shortly thereafter.
Follow Gary T’s suggestions.
The only things I would add is
The voltage of a fully charged battery is 12.6V, but you have to remove the surface charge. To do this after charging the battery or running the car turn on the key, and headlights. Leave the lights on for 30 seconds. Turn the key off, and wait about a minute and then measue the voltage.
Do you know how to do a voltage drop measurement?
My personal feeling is that either there is a bad alternator, or a poor connection causing a voltage drop.
I haven’t seen a voltage regulator that wasn’t built into the alternator in a long time.
I’m betting that you’ve got something drawing a significant amount of current, even when the car is turned off. It could be almost anywhere in the electrical system, though it’s not likely to be in anything that switches off when you turn off the key. As long as it keeps discharging the battery it’s going to keep killing batteries, since lead acid batteries chemically self destruct if they get discharged too much.
If the battery light is still consistantly coming on, you can try removing fuses one at a time and running the car to see if that makes the problem go away or not. If it does, then you know the fault is on the circuit for the fuse you just removed. Note however that there are other circuits in the car which aren’t fed from the fuse box. They may come directly off of the wiring inside the engine compartment, and may be protected with fusible links instead of fuses.
If you have an ammeter, you can check the circuit by disconnecting the battery and putting the ammeter in series. If it’s drawing enough current to dim the lights there may be a fairly significant fault current, so make sure you use an ammeter that can handle a lot of current. You basically do the same thing, just keep popping out fuses until the current in the ammeter drops.
A wiring diagram for the car (like in a chilton’s or hayne’s manual) will come in really handy at that point.
I had a similar problem with an old buick. In my case it happened to be the car’s computer that fried, which was easy to diagnose since the check engine light came on but it wouldn’t give a fault code. Plus, the engine started running rough and gave all the other systoms of a computer controlled engine running in cripple mode (low gas mileage, stuttering at low speeds, etc). I don’t think you have a blown computer since you don’t have the other symptoms associated with it, but I think you have basically the same situation. Some failing component is drawing enough current to kill the battery but not quite enough current to blow the fuse. Could be anything from a cooling fan to the radio to who know’s what, but something is either dead or dying. Anything else busted in the car, like maybe the power antenna on the radio?