Re: low beam headlamps very dim

Posted by DavidPackard On 2023/10/5 18:30:49

Chris, you did say that both high beams are OK, and both low beams appear to be dull (far less illumination). Based on that input:

Since the high and low beams of any one bulb share a common ground at the junction block the likelihood of a grounding problem is reduced. Likewise, since the high beams are functioning normally the circuits to and from the headlamp switch are also likely to be functioning correctly. The inference is circuit 15 (gray) at the dimmer switch is functioning normally, meaning it has the correct voltage when the headlight switch is ON. That leads us to the circuits between the dimmer switch and the low beam connections on the terminal blocks, or perhaps the low beam side of the dimmer switch itself.

I would confirm system voltage on circuit 15 (gray) at the dimmer switch, and then having a helper cycling the switch, the voltages at both circuit 7 (brown) and 8 (light green). Since circuit 8C (light green) powers the high beam indicator we can conclude the circuit 8 (light green) is the high beam (the good one). I don’t remember if the switch is removed from inside the car, or drops away from the cowl. In either way you might be able to conduct all of the tests solo, but it is always acceptable to have a granddaughter help.

We interpret the measured voltages by comparing the high and low beam circuits at the output of the dimmer switch. Both voltages should be about the same when powered, however if the voltage at the low beam is lower than the high beam the dimmer switch or connections would be suspect. If the voltage at the low beam is higher than the high beam voltage the problem is likely between the dimmer switch and the splice that bifurcates the low beam circuit to the left and right junction blocks.

All of this assumes a single point problem, not a simultaneous problem at two locations (like both junction blocks decide to quit working, or the connection at both bulbs somehow have higher than normal resistance). Multi-point failures do happen, but are less common than single point failures . . . assuming the first single point failure can be detected by the operator. Considering the environment the dimmer switch is subjected to I would jump to a conclusion that corrosion on the low beam side of the either the connection or the switch itself has resulted in a higher than normal resistance, and that resistance had reduced the voltage to the both low beams. It doesn’t take a lot of voltage drop to dull the bulb light output.

dp