Headlight fuse blows

KarlJS

KarlJS

Senior Member
When I revived my 39’ 1/2 ton truck there was a 20 amp fuse for the headlights. I replaced wiring with new reproduction cloth wire and changed all bulbs so it is now a 12 volt truck. Turned on headlights and 20 amp glass fuse blew, is this the correct fuse? Also the fuse that blew was a cheap kit from Harbor Freight. Thanks for all the help and remarks..
 
The fuse doesn’t know or care whether your system is 6 or 12 volts. The fact that the amperage draw exceeds the fuse value indicates a direct short in your lighting system. Likely places to start looking are at the dimmer switch, along the frame rails if you have dual exhaust, at the bullet connectors, and at the light sockets. One or more of those locations has been compromised and is blowing your fuse.
 
My '41 car would occasionally blow the fuse on my replacement type headlight switch. That was near disaster on a dark night on a crooked road.

I found the short was where the wire bundle wraps down under the frame rail behind the dimmer switch. The edge of the frame box had worn through wire insulation. I split a length of neoprene tubing and ran it along the edge of the frame box to insulate the wire bundle from the metal.

Also be sure you have a good grommet where the wire bundle comes through the firewall.
 
Same here , same place on a all stock 41 tudor. The factory hi beam switch is very close to the frame and floor .
 
Yup! Found and replaced the fuse with a “good” fuse. Now I have headlights, taillights and brake lights. But…when I step on the high beam switch the fuse blows. Could the switch be bad? Wondering how bad I need high beams any way?
 
I would say it is unlikely that it is the switch itself. More likely it is in the wiring from the dimmer switch to the front headlights. In looking for the short, I would work my way back from the right front headlight bullet connectors.


With the headlights turned off, disconnect the high beam bullet connector on top of the frame just behind the radiator; if you can still detect the colors of the wires, the high beam should be black/red and the low beam black/green. If you can not tell the difference, disconnect them both. Check the resistance in the circuit back towards the dimmer switch. It should be open circuit; that is infinite/max ohms. If open circuit then check the resistance to ground through the headlamp filament. The bulb filament should register 6 to 12 ohms. If there is a short in either direction, it will read near zero ohms, and you should pursue that path to find your short. Look for cracked, worn or frayed wiring insulation around the bullet connectors, under mounting clips, and passing through sheet metal.



If the short was back towards the dimmer switch, go to the bullet connectors on top of the frame, right behind the radiator on the driver side. Disconnect the high beam (or both) wires going from there across the front cross member to the passenger side. Look for the short towards the passenger side; if zero ohms the problem is in the piece that goes across. If the cross wire is a good open circuit, check the ohms back toward the dimmer switch; you should still see the short. Then disconnect the high beam (or both) going up to the driver side headlight and check both directions of that circuit to see if the short is in the wires to the headlight or in the wires going back to the dimmer switch.
If the short is still in the direction of the dimmer switch things get tougher. The wires are buried in a bigger bundle and the dimmer switch connections are hard to get to. Disconnect the high beam wire (or both) from the dimmer switch and check for a short back towards the headlights. If the short is in that direction, you should probably replace that whole wiring harness. If it is not check the high beam terminal on the dimmer switch for short to ground. Click the dimmer switch and try again. If either gives you a short, replace the dimmer switch. It would be best to have someone click the dimmer switch while you watch the needle on the ohmmeter. If it just twitches, the short could be intermittent, but long enough to blow a fuse.

If not, that leaves one other potential culprit. That is the wire from the dimmer switch that goes back up through the firewall to the high beam indicator light in the dash. Check that one for short versus 6 to 12 ohms for the bulb. Check the wire bundle going through the firewall for fraying. Check that the bulb and its socket are properly installed

Good luck.
 
Karl,
I had the same problem when I "upgraded" my headlights. The new ones were offshore from O Reillys. Found out they were wired different and when I depressed the dimmer switch it went to ground and would blow the 20 amp fuse. Put in a different headlight and see what happens.
Also one time, I got new headlights but one was a 12v and one was 6v from the supplier. The 6v of course would blow the fuse.
Hope this helps.
 
I unscrewed the high beam switch from the floor and just let it hang by the wires, when I press it to change from low to high and back the fuse does not blow…. Wondering if grounding out?
 
Letting it hang would change the position of the wires relative to the frame to which the wiring harness is attached. Look for any open insulation along those wires.
 
Switches wear out too. I'd use a short stop type circuit breaker instead of a fuse. Ford used circuit breakers for a long time. They will reset so you can get home in the dark. The short can be repaired in the daylight. If it pops a breaker with a knowl fault, you just avoid the fault till you can fix it.
 
I am wrong but the floor switch is shorting if you disconnected it to ground and it works. But once its connected to ground it doesn't work.


New is not perfect, nor I'm I right.
 
Tinker said:
I am wrong but the floor switch is shorting if you disconnected it to ground and it works. But once its connected to ground it doesn't work.


New is not perfect, nor I'm I right.
Click to expand...


A very astute observation. If the switch housing is no longer connected to ground, a short within the switch housing would no longer cause the fuse to blow.
 
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