Troubleshoot Dim Headlights Issue

Since we have had so many questions about dimming headlights when the engine is at idle, I have decided to explain the problem here and hope it will help allot of you out.

The headlights (and all of the other lights on the car) go dim for only one reason, that’s because the system voltage drops below the normal voltage of 14 volts. Voltage is electrical pressure just as water pressure makes the water come out of a faucet. The greater the pressure (voltage), the faster the water comes out. Loose the pressure (voltage) and the water come out slower. Loose electrical pressure and the lights get dimmer. Amperage is electrical volume and since your headlights and standard electrical options are less than the amperage (volume) capabilities of the alternator, voltage should remain in the normal range (14 to 14.4 volts). So, why does the voltage go below normal? First thing to be sure of is that the wiring is not restricting the flow of current to the system. This can be done using a digital voltmeter. Connect the positive(+) of the meter to the battery stud of the alternator and the negative (-) of the meter to the POSITIVE of the battery. Run the car with the lights on (to create a load) and read the meter. You should not read more than .3 to .7 (that is tenths) of a volt. Do the same with the negative side of the charging system. Connect the negative (-) of the meter to the alternator case and the positive (+) of the meter to the NEGATIVE battery terminal. More than that and you have wiring that is not allowing normal flow of electricity. (Hint, did you check the bulkhead connector for a hot or burned connection?)

Now for the more probable cause of your problem. The same type of Chrysler alternator was used for many years and all had the same physical shapes. However, the electrical characteristics changed over the years. Before 1969 the alternator was a rather low output (37-42 amps) and was used with a mechanical regulator. The rotor installed in these alternators had a field current of 2.3 to 2.7 amps. In the later 70’s the windings in the rotor was changed and had a much higher resistance. Over the years the alternators have been rebuilt so many times that the rotors generally don’t get tested for the correct application and a higher amp rotor could be installed in an alternator that it was not designed to be in. A higher output alternator will give you more amperage capacity, but only at higher engine RPM’s and will usually not do as good as a lower amp alternator at low engine RPM’s. Later model high output alternators are designed to give good output at low & high RPM’s and the engine idle is usually 200 -300 rpm’s higher than your classic car. So the moral of the story is that there is a good possibility that the alternator on your car was not rebuilt with the correct parts to operate as it should on older cars. If the same alternator was installed on a 70’s car with an electronic regulator and a few hundred higher engine RPM’s, you would never have a problem.