I have installed a new battery & new.

  • 156,000 MILES
I have installed a new battery & new altenator. The car runs fine but the Alt. Light comes on and sometimes for days then goes out. Also sometimes the car won't start so I just tap the ground or positive cable located in the engine compartment and it starts and runs for weeks then it does it again. Please don't ask me what the Alt. Out put is, it is what it's supposed to be. The Alt is GOOD & the battery is GOOD. THANK YOU.
Do you
have the same problem?
Saturday, February 23rd, 2013 AT 4:31 PM

1 Reply

Okay, so if we're supposed to take it on faith that everything is good, there's only a few possibilities. I can tell you from a lot of experience working on my own Chrysler products and other peoples' cars, I have assumed the same thing numerous times and been wrong. If the numbers are good, you should be willing to share them. Your alternator could have a bad diode and output voltage will be perfect, ... Right between 13.75 and 14.75 volts. What will be low is output current. With a full-load test it will be exactly one third of its rated value, and that's not enough to run the entire electrical system under all conditions. If you won't tell us what the output current and voltage are, we'll have to assume they're good and possibly overlook an obvious cause of the problem. It's a very poor mechanic who assumes new parts are working correctly. Experienced mechanics know the first thing to check is what they just worked on.

The place to start is by measuring the voltages on the two smaller wires on the back of the alternator. That must be done with the engine running and while the "battery" light is on indicating the problem is occurring. One wire will have full battery voltage on it. The other one is the key. It should have less than battery voltage but not 0 volts. 0 volts indicates an open brush inside the alternator. That's an inexpensive repair. Exactly the same battery voltage on both wires indicates a break in the circuit between that control wire and ground. There will be at least one connector to look at, then it goes to the voltage regulator inside the Engine Computer. The regulator typically causes extremely little trouble so start by inspecting that wire. If that circuit is working correctly, you'll find between 4 and 11 volts. The lower the voltage, the harder the alternator is working. If that voltage is correct and the "battery" light is on, we're back to suspecting a diode.

For the no-start, I don't know what the other symptoms or observations are like head light brightness or noises from the starter, so look at the nuts for the jump-start points and be sure they're clean and tight. Next, follow the smaller positive battery wire to the fuse box and be sure that nut is tight. Do the same where the smaller negative battery wire bolts to the body, if that is different than the jump-start terminal. If all the lights and gauges go dead at the same time that it won't start, look at the cables where they're attached to the battery. Due its location and the fact one of the cables is stretched very tight, vibration can cause the center bolt to work loose.
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Saturday, February 23rd, 2013 AT 4:56 PM

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