I have a 1964 F100 and have a weird problem , when ever i put a load ( headlights brake lights or turn signal) my interior lights flash along as if i’m having a serious voltage loss , my voltage gauge connected to the fuse box also drops and goes up with the turn signal , i thought it was a ground issue so i had added grounds to make sure and nothing. it’s very weird as everything works as should but it seems like something in the interior and maybe fuse box is cutting out , i tried using a power probe on the fuse box and noticed the voltage fluxing and also does it with key on engine off.

 

Has it done this since you owned the truck or is this something new?

Your description suggests the return path (ground) is unstable or shared across circuits that shouldn’t be interacting. The fluxing at the fuse block means the issue isn’t isolated to a single component. It’s systemic. Even if new grounds were added, the original path might still be active and flaky. If the fuse terminals or bus bars are oxidized, they can cause voltage drop under load. I'd recommend a voltage drop test across ground paths with a multimeter while activating loads. You could also do a bypass test by running a temporary ground from the fuse block to the battery negative and see if the pulsing stops. Also, inspect the fuse block terminals. Look for heat discoloration, looseness, or corrosion. Resistance checks from battery ground to other grounds might also show high resistance where it's not expected. All grounds should be common to chassis ground. Any ground that is not is suspicious.

If it's anything like other trucks I've worked on, then the original wiring harness is probably brittle, and the fuse box is due for a refresh. I’ve dealt with similar voltage weirdness before. Isolating circuits by pulling all fuses and replacing one at a time and testing in between can help as well. Patience and a keen understanding of the properties of electricity are helpful.

There is a bi-metallic, self-resetting breaker embedded in the headlight switch. It is designed to protect the headlight circuit only - not the entire fuse box or interior lighting. When overloaded (e.g., corroded contacts, high resistance), it heats up and opens, then cools and resets. This can cause pulsing, flickering, or voltage drop, especially if the breaker is weak or the switch is dirty. But typically this wouldn't affect other circuits unless something is wired incorrectly.

 

Hello , thank you for replying
Its something that i noticed last year and didn’t pay it no mind until i tried figuring it out now , can you go into a little bit more in depth on jumping a ground to the headlight switch? i noticed using a power probe that it’s getting a voltage drop on the key on side of the pd while the constant voltage doesn’t pulsate , i’ve attached some images of my headlight switch it looks very very dirty. i tried cleaning all my ends but haven’t touched the headlight switch/fuse box made into the switch any help would be great ! also ps the wires tapped into the box are my external gauges , i’ve already tried disconnecting them to see if one might’ve been shorting out and drawing too many amps or something but to no avail

 

Update!
I noticed the same color wire going from the fuse box inside comes out to the starter solenoid and then another one goes to the voltage regulator , i also noticed my charging while running was way too low , 11.8 - 12v which i am on a generator but id still too low , so i took it off and back fed the power and seems to not be flickering now , im gonna test the battery and see if its a defective battery. battery is a duralast and its about 5/6 years old if you have any inputs it would be appreciated

 

Here's what I'd do at this point. You can remove the headlight switch and test it independently if you have the tools.

The headlight switch connects to the fuse block so you have to release those two clips on the sides of the switch and then you can pull it from the fuse block to separate it.




Front and back sides of fuse block.

Then you can bench test the headlight switch: Apply 12V and load it with a test lamp or resistor bank. Watch for breaker cycling or voltage drop.

If you want to try a jumper first, you can bypass ground to the switch body. Run a jumper from battery negative to the switch case. If the pulsing stops, you’ve got a grounding issue at the switch mount.

If it were me - since I have extras - I'd quick replace the switch - just swap it out and see. If it’s original, it’s earned its retirement. New switches have tighter breaker tolerances and cleaner contacts. But if you can load test the switch then you can determine if it is the likely culprit or not. I now am suspecting it very well could be.

 

Currently in class but i will do that once i get home in a little bit , but im curious to see if i just remove the headlight switch and try activating the turn signal to see if that would work , but im gonna try to bench test the switch and then try jumping the ground to the fuse box to see, could you point out which pin / where to ground the jumper wire to? also what pin powers the headlight switch and where should i put the load on it? i’ve never done this so sorry if im asking alot just want to make sure

 

another question is couldn’t the voltage regulator also be an issue here since after disconnecting it, stopped pulsating at the fuse box?

 

Yes, it could be the voltage regulator. They have a current limiting relay inside of them. I don't see that symptom much since that high of current isn't caused by normal operations, but it is a possible cause of the quick on and off you are seeing in the voltage fluctuations. It is possible to bench test a regulator, too. The 1964 100-350 shop manual goes into the details of that.

 

with it running or off i’m only reading 12v at the regulator , according to the test it’s suppose to be within 0-10v but that seems strange to me

 

just attempted jumping the ground to the headlight body and to no avail , i think im going to buy a new headlight switch and try that as its so corroded where the rheostat is and causes the dash lights to only work in certain positions , do you have any places you recommend for that sort of electrical ? any places to get NOS ford parts?

 

Sorry, from my armchair here I'm not following what you are doing - but I will wish you good luck at this point.

 

One suggestion is to not Quote the reply unless there is some confusion as to whom you might be addressing the post to. Since it's just been you and I so far, no confusion. I missed half of what you wrote since you Quoted me so many times. Just food for thought.

I used to recommend Green Sales Company as the top source for NOS parts, but they have recently sold the business and I'm not sure who the new owner is and if they will sell parts in the same manner or ever sell to the public again. Time will tell there. NOS Parts Ltd (Rodney) in Texas is another good place and has about 8 headlight switches showing in stock. Or eBay. I'm seeing dozens of the switches there - some NOS some aftermarket.

 

hello i’ve been away from my computer due to school and work but i appreciate you responding , but since i posted this ive done a bunch of tests , i tried a headlight switch to no avail , i checked my grounds and voltage drop tested them and had .02v which im happy with , i checked the black and yellow wire from the starter solenoid to inside to the ignition switch and seems fine , one thing i did not like is the 75 amp fuse that is suppose to be inline is missing and they had cut and soldered a new piece of wire that has a newer style 40 amp fuse , i’ve kinda given up and ordered a new ignition switch from work and if it doesn’t work it’s going back together and just going to live with it , i’ve kinda exhausted all my resources to no avail

 

No 75-amp fuses on the light-duty trucks. The bigger trucks used a 70-amp circuit breaker, but the light duty trucks didn't. A person can add one, but the factory didn't install them.



These were designed to protect higher-load systems like dual battery setups and heavy-duty lighting.

Have you tried pulling all the fuses and checking if the problem persists and if not then adding back in one at a time and testing with each addition to help isolate which circuit it's on? I mentioned briefly in my post #2 of this technique. For electrical work I always use the 'divide and conquer' method that I learned 40 years ago. It's never failed me yet. Works with wiring, circuit boards, you name it. Even works well with humans... </chuckle>

 

I have tried pulling all the fuses on the “key on” side , but now that i think about it i didn’t pull the rear tail light fuse iirc , now that i think about it that could make all the difference if the rear tail lights don’t have propper ground or the lights are starting to go out , i’ll have to try that once my new flasher comes in , mine went out on me and i had to order a nos one off ebay because they didn’t even list one at local parts house but if it doesn’t work again i’ll probably put it back together and wait for the problem to get worse which isn’t ideal but might be the best course of action , it’s not affecting anything other than seeing my external voltage gauge and external gauges lights pulsate which annoys me but it is what it is , the other lights pulsate as well but no where as near as the external gauges