I'm having problems with the headlights on my 51 F-1 that I recently purchased. (It's about 80% restored and upgraded to 12V)...The problem is that the headlights are VERY DIM on regular beam, and only normal when on high beam. I have cleaned all of the grounds that I could see, changed the Dimmer Switch, and the headlights are new halogens. My next move is to change out the altenator (even though it appears to charge the battery ok).
Any suggestions ?

 

The headlamps ground thru the body. Try for the heck of it, putting a ground wire from the body to the frame.Also measure the voltage at the headlamps to make sure it's a full 12V. Make sure the dimmer switch is wired correctly.

 

You might make sure that the light wiring didn't get routed thru a voltage drop for gauges or other appliances - like Barry said, check what you have at the light.

 

Like the man said- check the voltage at the headlight with the circuit under load (light burning). Your ground is probably ok if the upper beam is functioning properly. The low beam is slightly less powerful than the high beam (draws slightly less amperage). If you find the voltage less on the low beam than the high beam, start tracing and measuring back toward the light switch until you find the trouble. Remember, a bad connection may show normal voltage unless it is under load.

 

I got a deal for you. I'm having the same problem on my '51F1.
Whoever figures it out first....
Let the other guy know, OK?
Good Luck!
Erik
1951 F-1
#####.Louis

 

Update .....
I measured the voltage (under load - lights on).

Coming into the Dimmer switch: 12V
Leaving the Dimmer Switch (high and low beam): 12V

At the headlight on HIGH BEAM:
High Beam connector is 12V
Low Beam connector is 0V


At the headlight on LOW BEAM:
High Beam connector is 0V
LOW Beam connector is 7V

The voltage is the same from the headlights back to the
firewall entry. I haven't found the 7V inside the cab yet but that will be my next goal.

 

I do not see how the trouble could be in the cab if the voltage is good at the dimmer switch. Look for the trouble between the dimmer switch and headlight. My 53 has some bullet connectors on the fender aporn. I do not know if your wiring is custom, or what.It is a long shot, but you might put an ammeter on the feed line to the dimmer sw and see if the draw is higher on dim than high. It should be a little lower. If it is higher, see if something is connected to the low beam wiring that is not supposed to be, like park lights. It is more likely to be a bad connection and wherever it is it will be creating heat when running on low beam. Leave the lights burning on low for awhile and run your hands over the harness, looking for the hot spot. Play close attention to the plug on the dimmer switch.

 

That's a good idea and I'll try it.

If I find that there is more current when low beam is active, I'll probably just run a straight wire from the dimmer switch
to the lights to see if the voltage jumps back up to 12V and the current drops.
There is so much electrical tape on the harness going to the lights, it's easier to run new wires than it is to check all connections.

Thanks !

 

Why didn't you check voltage at the dimmer switch on low beam? If you have 7 volts there, it would be the dimmer switch. If it is 12v, it would be the wiring. You only checked the dimmer switch on high beam.

 

Hi Frank,
I checked both high and low beam outputs on the dimmer switch and they both measured 12+V.
(I verified this with a digital multimeter)

The healights measure the same 12V(High beam) and 7V(Low beam) on both lights, and regardless whether or not it's under load.
My multimeter probes are very sharp and I was able to probe into the conductor insulation and verify that the 7V is present on two wires (the wires going to both headlamps low beam connector).
As soon as the wiring harness enters the engine compartment, the 7V is present.
I'm positive (almost) the problem is somewhere on the cab side of the wiring harness, and I haven't had time to investigate.
My guess (and advice from this board) is that there is a bad connection, or the wiring is routed through some other switch or device.
Thanks

 

Oh...one more thing:

The wiring harness goes from the dimmer switch into a mangled mess of electrical tape, something I need to clean up anyway.

(Weekend job)

 

Ok,
It's fixed.

I went to Autozone and bought 3 different colors of wire (16 Ga) and wired the whole system from the Switch, to the Dimmer Switch, to the Headlights, and a new ground.

Dimmer Switch (with prongs facing you):
(Upper)Center pole--12V feed (input)
Right pole--12V High Beam (Output)
Left pole--12V Low Beam (Output)

Headlights:
Upper Center pole--Low Beam
Right pole--High Beam
Left pole--Ground to chassis or frame (If you are unsure the ground is good, it should have the same effect as the negative battery post)

The lights are now 100 % on both low and high beam.
Thanks !

Mike

 

 

I'll bet you bypassed the old Circuit Breaker that used to be in line. Cause of many a problem on 6 to 12 volt conversions.

 

I can't remember which way it is which but low beam is fused and high beam is direct with a fusible link or is it the other way around. I know they are wired differently from a protection standpoint and, as was stated above, there in may lay the problem.

Or some one ran the lowbeam thru the coil resistor.