1978 Mercury Cougar

aprlshwrs30

Our '78 cougar will not hold a positive charge. It runs fine if we do not turn on our headlights. If we do, all power is drained from battery. We had alternator and battery checked and they tested fine. Could it be a short in a wire?

0patience

Well,
how was the battery and alternator checked?
With the engine off, you should have 12+ volts.
Start the vehicle, the battery voltage should come up above 13.5+ volts.
If you turn the healights on, how much does the voltage drop?
If you turn the heater fan on how much does it drop?
Leave the headlights off and turn on the heater, the turn signals and wipers, how much does the voltage drop?
It could be a short, could be the voltage regulator or several other possibilities.
Do all the exterior lights light up?

jgmilberg

While the engine is running pull the positive battery terminal. If the car runs with the battery disconnected the alternator is bad. The cause for the alternator getting smoked could be a bad cell in the battery, so remove the battery and have it put on a load tester. If is too low of a charge have them charge it and test it properly.

It really sound like the alternator can't put out the amperage needed to charge the battery and run the car.

Now that I typed all that another possibility could be a faulty voltage regulator. I am not sure what year Ford built that into the alternator, but it's another possibility.

The best way to figure it out is to have a shop test the entire system while the car is running. I know Auto Zone does this for free. Drive the car up there and have them put the machine on it. While they are testing turn on the headlights, that will tell you what is wrong for sure.

Let us know what you figure out!!