Dim Headlights on 40:

[40ford]

I have posted on this problem before but I am back at trying to figure this problem out!With both headlights out and car not running I am getting about 5.50 at the pig tail.If I install one headlight and it burning I check the other pig tail and it is about 4.5.Is this normal?I have installed new headlight switch,dimmer switch,good battery,new regulator,generator has been checked,battery cables new and big.When driving car at night headlights are weak and when I put on brakes the dash lights go dim and headlights get even weaker.Any suggestions on this problem?I refrain from driving at night because lights are crappy!

[marko39]

You should have battery voltage at lights. You must work backwards from lights to see where your voltage loss is. Do you have wiring diagram? Is this a new wiring harness?

[deuce_roadster]

You don't mention grounds. Make sure you have good (clean shiny) grounds everywhere including between your fenders and the body and the body and the battery. What you describe happens often with fresh restorations where everything is painted everywhere.

[slowforty]

Mr Deuce Roadster is Correct. I put a better ground on my 40 headlight and watched them get brighter when the ground improved

[JayChicago]

Theoretical guess here:

Original post said: "when I put on brakes the dash lights go dim and headlights get even weaker". That makes me think that when brake lights go on the entire ground bus (the body of the car) is being charged, reducing its ability to take-on ground voltage from the other lights. So why is the ground bus not immediately discharging to the battery as it should?

I would suggest cleaning/tightening the battery-to-body cable connections.

[big job]

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Originally Posted by JayChicago

Theoretical guess here:

Original post said: "when I put on brakes the dash lights go dim and headlights get even weaker". That makes me think that when brake lights go on the entire ground bus (the body of the car) is being charged, reducing its ability to take-on ground voltage. So why is the ground bus not immediately discharging to the battery as it should?

I would suggest cleaning/tightening the battery-to-body cable connections.

I agree looks like body has either no ground cable or a weak ground cable.
Grounding is a lot like lightning it will seek a path of ground usually where
you do not want it. I have seen smoked - melted, from speedometer cables
to E brake cables to choke / throttle cables cause the system is 'gasping' for
ground. Just look at these new cars ground wires all over cause most of the
vehicle is plastic. Wood and fiberglass boats = a mile of grounds. My best
friend is many jumper wires, a test light and volt meter. I grew up with
6volts and my father got into road rage with blinding mirrors pulling over
then following said car blasting him with his high beams LOL causing a mad
mommy good ole days

[40ford]

I can't figure it out.I have about 6.20 volts going to the headlight switch.Don't look like there is about 5 volts going out?I got about 5 volts at the dimmer switch.For some reason the voltage at headlight pig tails went from about 5.50 to 5 volts.I have unplugged all my other lights one at a time with no change in voltage at headlights?I thought some other light was draining voltage real bad.I am not an electrician expert so I am still lost.At battery I have new woven ground to firewall and then another from there to nut on head?Is this the way a 40 should be set up?

[Kube]

Originally Posted by 40ford

I can't figure it out.I have about 6.20 volts going to the headlight switch.Don't look like there is about 5 volts going out?I got about 5 volts at the dimmer switch.For some reason the voltage at headlight pig tails went from about 5.50 to 5 volts.I have unplugged all my other lights one at a time with no change in voltage at headlights?I thought some other light was draining voltage real bad.I am not an electrician expert so I am still lost.At battery I have new woven ground to firewall and then another from there to nut on head?Is this the way a 40 should be set up?

Be 100% certain that where you have those grounds, the area beneath is free of paint. I "hide" an external tooth lock washer between my cable ends the adjacent surface(s).
The dimmer switch is often a culprit in this circuit. You might try bypassing it for the moment and see how the results may change.

[Kurt in NJ]

So you are loosing voltage in the headlight switch, is it reproduction, or original ford--- the cheating cure is to put heavy power wire to relay as feed, and use weak headlight switch to work relay

[JayChicago]

I agree, seems like a bad headlight switch if there is 1+ volt drop through the switch. But I don't think that explains the other symptoms, which seems like a whole-car ground problem.

OP said " At battery I have new woven ground to firewall and then another from there to nut on head" I would try re-doing that connection at the firewall, trying to get the best metal-to-metal contact there.

[philipswanson]

Check your voltage from the negative (-) on the bulb (center) socket to the positive (+) directly on the battery. I am assuming you still have a positive ground. You might need a length of wire to make those connections. If you get a full 6 volts, you have a bad ground.

Also, what gauge wire are you running to the headlights?

[40ford]

Headlight switch came from Carpenters.I still looks like to me it is the switch also.I guess I could just by pass the switch with jumper wire and see what I have.It still bothers me about the battery gauge not reading in the green going down the road about 60mph.May be the gauge though but it is NOS.

[FlatheadTed]

A fire wall ground is good but your lights are connected to the fenders so a ground wire from the battery (or close ) to the bulb holder is better , Yes not original .set up a tempey ground and touch it on the reflector at night and see how bright ,if it improves it . Modern cars have a separate ground ,Ted

[Aarongriffey]

Are we sure the car has 6 volt headlight units? Not 12?
Take one out, leave it plugged on to the wires and run a jumper wire from the ground terminal on the back of the sealed beam unit to a good ground on the car to see if the makes the light brighten.

[JSeery]

I'm mostly repeating what has already been posted, but I would make up some good jumper wires and start doing some checking. See what you get with each.

Jumper across the light switch from the hot wire to the wire going to the dimmer switch.

If that isn't the issue, try a jumper around the dimmer switch.

Next, try a jumper from the headlight ground stright back to the battery.

Etc, until you find the problem(s).

Grounds can always be a major issue, but the lighting switch reading seems suspect.

[40ford]

Originally Posted by Aarongriffey

Are we sure the car has 6 volt headlight units? Not 12?
Take one out, leave it plugged on to the wires and run a jumper wire from the ground terminal on the back of the sealed beam unit to a good ground on the car to see if the makes the light brighten.

Both of the headlight sealed beam bulbs are 6 volts.I have tryed another ground wire to bulb and got no change in brightness.

[joe 1950]

Every thing mentioned is very good info but what is the generator putting out for voltage at 1500 rpm you should be @ 7.5 volts with every thing turned off check your generator output also on my 50 cpe i have the pos cable to the cyl head as per factory and i also installed a large braided strap from bell housing to body good luck keep us informed

[FlatheadTed]

Hope I am not repeating ,Cold solder out let wire on the light switch maybe ,Can he do what you and others said on post 12 ,run a hot wire to back of the bulb , look for change ,

[40ford]

I think I will just get another cable and run it from maybe a hog head bolt to the frame under car.I don't think this would hurt and it may help?Would it matter if it was just a cable or should it be a woven ground strap?

[1931 flamingo]

"not reading in the green going down the road about 60mph."


What's the fix for this ?? I also have this problem. Gen and regulator have been gone through.
Paul in CT

[joe 1950]

doesn’t matter if it’s cable or braided strap but get it big enough mine goes from the bell housing to the body not the frame body is mounted on rubber mounts

[JSeery]

Originally Posted by 1931 flamingo

"not reading in the green going down the road about 60mph."


What's the fix for this ?? I also have this problem. Gen and regulator have been gone through.
Paul in CT

You would first have to determine if it is the Generator/Regulator output or the gauge that is the issue. Sounds like you have checked the Gen/Reg, so most likely it is the gauge. Is the wiring to the gauge and the gauge ground in good shape? I would take a voltage reading at the gauge and see what you get, should be somewhere a little above 7 voltage with the engine rpms up. (this all assumes were are talking a voltage meter and not an ampmeter here)

[JSeery]

Originally Posted by 40ford

I think I will just get another cable and run it from maybe a hog head bolt to the frame under car. I don't think this would hurt and it may help? Would it matter if it was just a cable or should it be a woven ground strap?

I'm a big believer in grounds and they are often the problem with electrical issues. But the results you are posting appear to be more than a grounding problem (it is also possible and common to have more than one electrical issue). You really need to troubleshoot this and determine what the issue(s) is/are.

Get a fairly heavy gauge, long wire and connect it to the ground on the battery. Now recheck your voltage reading into and out of the light control switch (and other devices, such as the dimmer switch, input to headlights, etc). The voltage at any of your check points should be very close to battery voltage, in other words, there should not be much voltage drop in the wiring or any devices such as the light switch. The voltage from the battery should be >6 volts not running and >7 volts running (above idle).

Between using a dedicated ground directly to the battery for voltage checks and jumper wires to bypass suspect areas you should be able the isolate issues in short order. Additional grounds are always a good thing, but you still need to isolate any issues and resolve them.

[Aarongriffey]

The ground cables can be a woven ground strap. Or a regular insulated cable. Doesn’t matter.
Six volt systems need at least a size 1 for both battery cables. I like to use larger.
Put a ground strap or cable from the frame to the engine or transmission might give the starter some extra pep. Sure wouldn’t hurt but I doubt that is the dim light proble.

[40ford]

I was real curious and checked the charging voltage on my truck today.It is a stock 1952 F-1 with everything pretty much new including the wiring harness.With the truck at high rpm I am getting 7.50 at the battery.Should a 40 do the same thing.The lights on my truck are just about as good as if they were 12 volt!

[aussie merc]

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your on track you also need to check what is called stored voltage that is what is avalible at the battery after it has sat isolated for 24 hours a battery in good nick will only drop about a half volt and then stablise so if you charge at 7,5 have 7 after stopping eng it should stabilse at around 6.5 volts this also will tell you if you have any standing current draws ie clock, boot or bonnet lights that are remaining on etc a battery thats not quite right will give many false signals before failing any dought have someone loadtest it

[TomO]

A voltage drop test measures the resistance in a circuit and is more accurate way than using an ohm meter or measuring the voltage at the end point of the circuit. An ohm meter is hard to read when the circuit resistance is less than .05 ohms, but a voltage drop test will show how much voltage drop wcan occur with .005 ohms.


A voltage drop test is done by connecting a volt meter between the source of the voltage (battery) and the end point of a circuit while the circuit is in use (lights on or starter engaged).


To test the headlight circuit on an 6 volt pos ground car, you would connect the COM lead of the volt meter to the NEG battery post and the + lead to the connector closest to the last point in the circuit. On a Ford, it would be the bullet connector by the right headlight, using a paper clip or other thin wire to probe inside the bullet connector. The total loss should be less than 0.5 volts . To isolate the source of n\the excess resistance, you would move the red probe back one connection at a time. Each bullet connector should not drop more than 0.05V. The circuit breaker, dimmer switch and the headlight switch should not drop more than 0.1V each.


To test the grounding circuit for the headlights, connect the COM lead of the voltmeter to the grounding pigtail in the light bucket and the POS lead of the meter to the POS battery post. With the lights on, the reading should be less than 0.1V.


If you need more help, I check the Early Ford V-8 Club Forum regularly.

[40ford]

Tom,I just don't understand what is going on.I checked my voltage on the hot wire going into headlight switch this morning with car not running and sat all night and I get 6.28 volts and I check the voltage going out to headights and I get 5.51 volt?I jumped over the switch and the jumper wire clips got real hot fast sending out 6 volts to headlights?Like I said before this is a new switch.

[Ken/Alabama]

There is a large wiring connector located close to the headlight switch. I found that connector getting hot and was the cause of my poor lightning issues. Finally removed the connector and crimped and soldered the wires together. No more getting hot and good lights.

[40ford]

Ken,I don't exactly know what wire you are talking about.Is it close to the instrument panel or part of the wiring harness?

[TomO]

Your post says that you have approximately 0.8V drop in voltage across your light switch. If you did the measurements correctly, your light switch is bad. The max drop across the light switch is 0.1V.


If it is a new switch, send it back to the vendor and request a replacement. If it is an original switch, you can try operating it many times and then recheck it. This operation will clean the contacts if they are the cause of the problem. The original switches can be disassembled and repaired.


I suggest that you do a voltage drop test as I described in my previous post. It is a much more accurate test than trying to read the voltage like you are doing. It will also help isolate the problem to the connection or component that is making your lights dim.



I have no idea why your jumper wire clips got hot, unless you were using 18ga jumper wire. You need a 12ga jumper wire to test the headlight circuit.

[1940 Convert]

If you aren't familiar with doing a voltage drop test there are several videos on Youtube. Like Tom says that is probably the quickest/easiest way to narrow it down if you can't get results with using a set of jumper cables to give temporary connections. I've often had luck with just giving a new temporary ground to body or chassis with a full size set of jumper cables.


That kind of weird that it shows this as my first post but still has my private messages and join date. I haven't logged in in quite a while.

[40ford]

Tom this is what I am using.I still have the old I think original switch.To me it looks like it has to be the switch because I am not getting 6 volt to dimmer switch?
12066761_dcd_85654_pri_larg.jpg

[JSeery]

Your jumps look like they are too light a gauge size for headlights. But that may be irrelevant at this point as it appears your switch is bad. If you want to bypass the switch while you are waiting on a new one, fabricate a short jumper of heavier gauge wire.

[40ford]

Originally Posted by TomO

Your post says that you have approximately 0.8V drop in voltage across your light switch. If you did the measurements correctly, your light switch is bad. The max drop across the light switch is 0.1V.


If it is a new switch, send it back to the vendor and request a replacement. If it is an original switch, you can try operating it many times and then recheck it. This operation will clean the contacts if they are the cause of the problem. The original switches can be disassembled and repaired.


I suggest that you do a voltage drop test as I described in my previous post. It is a much more accurate test than trying to read the voltage like you are doing. It will also help isolate the problem to the connection or component that is making your lights dim.



I have no idea why your jumper wire clips got hot, unless you were using 18ga jumper wire. You need a 12ga jumper wire to test the headlight circuit.

Is there a way to tell if switch is original?How do you clean the switch?The switch kinda looks like it has been taken apart before because of markings on tabs!

[aussie merc]

new or otherwise that switch is no good and has bad contacts and at best should only be used to run a relay otherwise needs to be replaced Bad contacts cause heat too much heat dosent end well

[40ford]

I called Carpenters this morning and told them the switch I had was defective.They are sending me another one no questions asked!Great service there!

[40ford]

I changed all my battery cables to size 00.Ran extra grounds from motor to frame and body.Car certainly cranks much faster.I hope this new switch cures my problem.I also ran extra ground wires at headlight buckets.I used this ground strap that Advance Auto sells which works real good for this.
5701635_rnb_60213_pri_larg.jpg

[1940 Convert]

I had some ground problems on my 40 also. Too much paint under the ground strap was keeping it from making good contact. Those universal ones make good supplemental grounds and don't look out of place.

[JSeery]

Originally Posted by 40ford

I changed all my battery cables to size 00.Ran extra grounds from motor to frame and body.Car certainly cranks much faster.I hope this new switch cures my problem.I also ran extra ground wires at headlight buckets.I used this ground strap that Advance Auto sells which works real good for this.
Attachment 438825

Be sure the metal is bare and clean under the ground straps. I put a little grease on both surfaces to help with rust and help with conductivity. I am not a fan of di-electric grease for this application, but that is just me, many use it. Di-electric grease is specifically designed to block current flow. With tight connections there is enough metal to metal direct contact that this is not much of an issue, but just doesn't seem like the right way to go to me. I use an electric contact grease that is designed to promote current flow, but it is more difficult to find. For a ground connection normal grease will work. Di-electric grease is designed to protect multi-pin connectors in wiring harnesses and needs to block current flow between pins located close to each other. Good stuff for the designed application.

[40ford]

Ok,I got my new headlight switch and it was in a vintique box?I noticed there was even a part number etched on it.Installed and I got 6.27 volts going in and 6.27 going out,problem solved here!Have not checked at dimmer switch yet.I did notice with lights on the connections for headlight pig tails on both sides get very hot!

[JSeery]

Originally Posted by 40ford

I did notice with lights on the connections for headlight pig tails on both sides get very hot!

Heat normally = resistance, so if they are getting very hot you would assume there is excessive resistance or current draw.

[40ford]

I checked the voltage at dimmer switch and don't have but about 4.50 volts.I am losing a lot of voltage from the headlight switch to the dimmer.I know the new switch is putting out 6.2 volt now?Would under the shell on firewall be the next place to check?Ok,I need to back track -I traced the wire from switch which has red markings on it(hot wire I guess)to the small box under dash where ballast resistor is and the red marked wire and a black hook up and with lights on they are "extremely hot" at small nut?Not sure what the purpose here is for???

[deuce_roadster]

There is a circuit breaker inside the square box with the holes in it to protect the wiring if there is a dead short. Perhaps that is the problem. The replacement is usually that whole piece of insulation board with the ballast resister and circuit breaker. When the circuit breaker is in action due to a short your headlights cut in and out.

[40ford]

I am really lost here.I am no electrical expert!I have 6.27 volt going in to switch.When I turn headlights on this drops to 4.21?Is this normal?

[marko39]

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Check battery voltage with lights on. You should have same voltage at switch and lights.

[aussie merc]

simply a bad connection is resistance and causes heat if any connection is hot its bad and needs to be looked at. 6volt systems suffer badly as it dosent take much resistance to eat up 6 volts if you need more help PM me

[40ford]

With headlights on car not running I have 6.06 volts at battery and 4.72 going into headlight switch?

[marko39]

What voltage do you have in and out of the circut breaker? Trying to find the culprit here.

[40ford]

which would be the in and out?Would that be the red marked wire going in?Is this with everything turned off?Ok,I checked the two post.With headlights off I have around 6 volts on both posts.With headlights on I have 6 volts at one and about 4.74 on other?

[JSeery]

Do you have a wiring diagram? The Yellow wire with the Red tracer is the wire from the Circuit Breaker to the Light Control Switch. The wire you are calling black is actually Black with a Blue tracer and it does from the Circuit Breaker to the interior Pillar Lights. The wire going into the Circuit Breaker should be solid Yellow and comes from the Starter Solenoid.

The Circuit Breaker should be opening up if there is excess current draw in the lighting circuit. It is possible that the points in there are stuck and keeping it from working or that it is providing excess resistance when there is a large current draw. Need to be careful, but you could connect a HEAVY gauge wire jumper between the two Circuit Breaker terminals and see if that fixes the issue. The problem here is that if the problem is not at the Circuit Breaker and you remove it from the circuit, there is no protection for the rest of the circuit wiring. So, you would only want the jumper on there long enough to check it out.

[TJ]

Among the good advice you are getting, have you checked all the connections to make sure they are clean and tight? Make sure all connections are clean and tight at the circuit breaker. Then check all of the bullet connectors at the headlights and tail lights. You are getting as lot of resistance(heat) which usually means loose and dirty connections. I suspect the dimmer switch as it's mounted on the floor where dirt and moisture can enter the contacts on the switch and cause corrosion. Also how old is you're wiring?

[40ford]

Originally Posted by JSeery

Do you have a wiring diagram? The Yellow wire with the Red tracer is the wire from the Circuit Breaker to the Light Control Switch. The wire you are calling black is actually Black with a Blue tracer and it does from the Circuit Breaker to the interior Pillar Lights. The wire going into the Circuit Breaker should be solid Yellow and comes from the Starter Solenoid.

The Circuit Breaker should be opening up if there is excess current draw in the lighting circuit. It is possible that the points in there are stuck and keeping it from working or that it is providing excess resistance when there is a large current draw. Need to be careful, but you could connect a HEAVY gauge wire jumper between the two Circuit Breaker terminals and see if that fixes the issue. The problem here is that if the problem is not at the Circuit Breaker and you remove it from the circuit, there is no protection for the rest of the circuit wiring. So, you would only want the jumper on there long enough to check it out.

I am afraid to do the jumper might create other issues.With headlights on at the breaker I have 6 volts coming in and 4.74 volts out if I am checking correctly?

[40ford]

Originally Posted by TJ

Among the good advice you are getting, have you checked all the connections to make sure they are clean and tight? Make sure all connections are clean and tight at the circuit breaker. Then check all of the bullet connectors at the headlights and tail lights. You are getting as lot of resistance(heat) which usually means loose and dirty connections. I suspect the dimmer switch as it's mounted on the floor where dirt and moisture can enter the contacts on the switch and cause corrosion. Also how old is you're wiring?

I have a new dimmer switch and not getting the voltage to it.Wiring does not look that bad,but I know that don't mean anything!Not sure if it is original or not.Car sat in shed for 25 years.

[JSeery]

Originally Posted by 40ford

I am afraid to do the jumper might create other issues.With headlights on at the breaker I have 6 volts coming in and 4.74 volts out if I am checking correctly?

If you have 6v into the CB and 4.74 out there is a major issue with the CB. You could try checking the resistance through the CB. But, IMO, you ether need to check it or replace it. Not sure how easy it is to find a NOS CB, but be careful about new parts that include the ballast resistor, most of the new ballast resistors are total junk.

[TJ]

I think before you go any further you should replace the wiring. You say it doesn't look that bad, what ever that means. If you are not sure about the condition or whether it is original or not I would opt for new harness.

[deuce_roadster]

Also, those stock bullet connecters can be pulled back a bit looking like they are connected but actually are not. There are a bunch of them under that metal wiring cover where all the wires come together. I would double check that all of them are indeed making a connection.

[40ford]

Maybe this is part of the problem.I don't think this is original wiring is it?This is going to my tail light.
IMG_1619.jpg

[TJ]

Looks like original wiring to me. If the rest of your wiring looks like that replace it.

[JSeery]

Looks like a replacement harness to me. They use an interwire with modern insulation covered with a cloth winding. The outer cloth covering is just decoration. The wires look fine. For appearance you could replace the wiring, but don't see how what is shown is an issue.

[aussie merc]

you could bypass the factory circut breaker with an aftermarket one you just need to know what the amperage is

[JSeery]

Not a bad idea.

[40ford]

I think the amperage would be 30 amps?

[JSeery]

Originally Posted by 40ford

I think the amperage would be 30 amps?

I don't know the original CB amperage, but 30 amps should work. It will open up if it is overloaded which is what it is intended to do.

Just looked up a reference, 30 amps.

[40ford]

My problem solved.It was the circuit breaker.The one I took out looked very bad and corroded.
CIRCUIT BREAKER.jpg

[deuce_roadster]

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Good job, and you learned something about your electrical system!

[JSeery]

Good job!