Electrical Issues while accelerating

BLEASDALE

MEMBER
1999 DODGE INTREPID
6 CYL
2WD
AUTOMATIC
132,000 MILES

So today I purchased this vehicle for my cousin. Everything looked fine with it besides a minor oil leaks and a few cab lights out. I test drove it in a gated community (25mph) and it ran fine. After I purchased it and drove it home I noticed something I have never seen a vehicle do before. First while accelerating quickly the headlights will get brighter, then after the transmission shifts they will return back to normal. Not just that but when accelerating from red lights up to around 50 mph, around 3k rpm or 45 mph the radio will cut out, and the back dome lights will come on for around 3 seconds, while this is happeneding the dashboard display lights will go from brighter to darker, as well as the headlights. By the time I got home the driver side headlight was out (not sure if it was just old or a result of this). Im currious on if this is an alternator issue, battery issue, or some circuit that is loose. Please help me, I would hate to give her a car that is having these issues.

3 Replies

CARADIODOC

MECHANIC
33,873 POSTS

This sounds like a charging system problem, but don't be too quick to blame the alternator. The voltage regulator controls system voltage and he lives inside the Engine Computer. That circuit causes very little trouble. When there is an over-charge condition it is more common to find a grounded control wire between the alternator and the computer.

To start the diagnosis, you need to monitor system voltage while the problem is occurring. You'll need to connect a digital voltmeter to something that has full battery voltage. That can be the cigarette lighter, a fuse inside the car, or a fuse under the hood. If you go under the hood, you can clip the meter under a wiper arm. You should see 12.6 volts with the engine off and the battery is fully-charged. With the engine running, the voltage must be between 13.75 and 14.75 volts. It should remain steady regardless of engine speed.

If the voltage is too high or it goes higher and higher as engine speed goes up, you'll need to measure the voltage on the smaller wire on the back of the alternator. There's two of them in a small plug on the back of the housing. The alternator is on the left side of the engine near the bottom. One of the two wires will have full battery voltage. It's the other wire we need. It will have 0.0 volts, full battery voltage, or something in between. It's the something in between, typically 4 - 11 volts, that is normal. For your symptom I'd expect to find 0.0 volts. That would point to that wire is shorted to ground. The cause of the problem could be in the alternator itself, but that would be very rare.