Charging system problem light came on
 

The battery shaped indicator light came on last night. I was able to complete my errand and return home safely. Now I retested the electrical system using a DVM. With the engine off, the battery was reading 12.34 V. With the engine on, the battery was reading 11.xx V.

Questions:

1. What would cause an electrical system to be at a lower voltage when the car engine's on?
2. Would the OBD-II computer code help to diagnose the problem?
3. Are there any other tests I can do to diagnose the problem?

I would prefer to leave the option of taking my car to the mechanic/dealership as the last option. Thanks in advance for replying.

Yup, what he said.

Also, from your DMM reading of the 12v battery at rest, you need to fix the problem soon, or you'll also damage your batt. I'd get it on a charger ASAP if you haven't fixed the problem yet. Starting batts aren't designed to be deeply discharged.

That's what I was thinking...

The large chains like AutoZone will do a load test free of charge (no pun intended).

Take this situation:

I've heard of cases where the battery goes bad.
The alternator makes an extra effort to charge it.
Meanwhile, the battery fails and is replaced.
But the alternator was weakened by the bad batt.
Then the alternator fails.

LSS, test both to prevent damage to either.

RH77

Based on my Haynes Repair Manual for my car, the alternator is connected to the EVR. The EVR is supposed to control the field coils of the alternator. Is it possible that the EVR is failing to provide enough current to create a high enough voltage? How could I tell?

Also, what's the going mileage that alternators might start failing?

MetroMPG,

Thanks. I haven't moved my car since I posted so the problem shouldn't be getting worse.

rh77,

A load test of:

1. The battery? (already tested to be a "good battery")
2. The alternator? I do not feel confident to removing my alternator.
3. The whole electrical system?

How does an alternator "become weakened" by a bad battery?

It is most.......likely that it is the alternator. It isn't to difficult, typically to change. However, on that car you have a couple of things against you. First, the serpentine belt has to have the tension released, to pull the alternator and then tensioned to reinstall it. Second, it is a bit tough to get at the bolts that hold it in, because even though their are only two bolts, they are on the back side of the engine, under the cowel.

The alternator is good for 70,000-150,000 miles. Anything over that and it is on highly borrowed time. I'd pull it and replace it, or take it to a garage you trust and have them do it.

I would charge the battery, as well, although if the car will still start on it's own, without a jump, I would just replace the alternator and then use it.

Gary Palmer,

Maybe this may be a factor.

Well, I don't know if tip toeing works, but it's worth a try. Maybe the Ford alternator is built better? I just know on my 94 Chrysler it died at a little over 150,000 and when I put new brushes in a Honda, at 150,000, they were on the far edge of short.