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Old 06-13-2024, 04:31 AM   #1
mike42
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Default Stop Lights 1940 Ford

Morning….

I’ve replaced the stop light switch a couple times in order to get the stop lights to function. They went out again and it doesn’t appear to be the switch this time. I pulled off the two wires on master cylinder and joined them together with a little pigtail and the lights still don’t go on. I also noticed that when I turn on the headlights the tail lights don’t go on anymore but the headlights do. Does dash light switch control stop and taillights ?

Thx Mike
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Old 06-13-2024, 05:36 AM   #2
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Default Re: Stop Lights 1940 Ford

The stop lights are only controlled by the brake light switch and should come on even if the ignition is off.
The tail lights are controlled by the head light switch.
From your description it sounds like either both bulbs are burned our or the wiring to the tail lights got disconnected. Check for voltage at the rear bulb sockets and diagnose from there.
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Old 06-13-2024, 06:03 AM   #3
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Default Re: Stop Lights 1940 Ford

If you check the two wires at the stop light switch one is energized and the other not. It sounds as if the rear lights have lost their ground. If the license plate light is lit this will confirm this. The stop lights are not connected to the light switch.
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Old 06-13-2024, 08:28 AM   #4
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Default Re: Stop Lights 1940 Ford

Thx guys ! I’ll follow up tomorrow on this. Much appreciated!

Mike
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Old 06-13-2024, 09:05 AM   #5
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Default Re: Stop Lights 1940 Ford

Just went to the barn real quick and checked license plate light. Turned on the headlights and the license light “does not” come on. So does that mean I’m not grounded in the back or a bad wire connection somewhere ? I have to leave for the day but will check back later. Thx !
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Old 06-13-2024, 02:10 PM   #6
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Default Re: Stop Lights 1940 Ford

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On the firewall on the driver side of the engine room there are a bunch of connectors between the dash wiring harness and the chassis wiring harness. The wire to tail lights and tag light are in that bunch. If you can still determine the wire colors, it is the all black wire. The bullet connectors between the harnesses can get pulled loose or corroded to lose connection. I would start there.
When you pull it apart, check to see if there is voltage on the wire coming from the dash. If no, the problem is a connection back under the dash, maybe on the light switch itself. If yes, clean all parts of the connector and wire ends and see if that fixes your problem.
If not, there are connectors at the left rear of the chassis where the wire splits to go to the tail lights and tag light.
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Old 06-14-2024, 02:38 AM   #7
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Default Re: Stop Lights 1940 Ford

Okay thx I’ll check into it today. What I can say is that when I connected the two wires off the master cylinder there was a little spark and the alligator clip got hot as hell so I think there’s juice going to the brake switch !
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Old 06-14-2024, 10:50 AM   #8
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Default Re: Stop Lights 1940 Ford

Mike, if the alligator clip got hot, I would suspect a short somewhere going toward the back of the car. I would inspect the sockets to make sure the hot lead for the bulb hasn't moved over if the wire is fine. Did you say you had tail lights but no brake lights? If the tial lights work the grounding is good. I mostly suspect the wire between the brake switch and the back has a short somewhere, maybe near the exhaust.
Let us know what you find.
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Old 06-14-2024, 05:39 PM   #9
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Default Re: Stop Lights 1940 Ford

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Okay thx I’ll check into it today. What I can say is that when I connected the two wires off the master cylinder there was a little spark and the alligator clip got hot as hell so I think there’s juice going to the brake switch !

Of course, that little spark means that there is something consuming that current. In other words, something is operating, whether correctly or incorrectly. I'd suggest that the switch itself is possibly grounding internally. Take both wires off of the switch. Check EACH lug on the switch with your ohm meter for signs of being grounded. If one of them IS grounded, you've found your "sneak circuit".

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Old 06-15-2024, 12:00 AM   #10
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Default Re: Stop Lights 1940 Ford

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Did you say you had tail lights but no brake lights? If the tial lights work the grounding is good.
No. In addition to no brake light, he said headlights come on but no taillights and no license plate light.
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Old 06-15-2024, 12:03 AM   #11
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Of course, that little spark means that there is something consuming that current....I'd suggest that the switch itself is possibly grounding internally.
He got that spark by connecting the two wires to each other. Brake switch was by-passed.

This one is a head-scratcher. I have no idea.
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Old 06-15-2024, 06:06 AM   #12
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Default Re: Stop Lights 1940 Ford

No license plate light appears there is no power. Maybe dirty contact on the circuit breaker. For temporary test purpose, jumper the circuit breaker and see if you have lights.
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Old 06-15-2024, 07:36 AM   #13
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Default Re: Stop Lights 1940 Ford

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No license plate light appears there is no power. Maybe dirty contact on the circuit breaker. For temporary test purpose, jumper the circuit breaker and see if you have lights.
Headlights are also protected by that circuit breaker, and his headlights do come on.
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Old 06-15-2024, 08:42 AM   #14
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Default Re: Stop Lights 1940 Ford

Ck the harness along the frame rails going to the rear of the car. Mine had several bad places.
Paul in CT
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Old 06-15-2024, 09:00 AM   #15
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Default Re: Stop Lights 1940 Ford

A simple test light goes so much further in looking for electrical issues than guessing or looking for sparks; just saying.
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Old 06-15-2024, 01:09 PM   #16
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Default Re: Stop Lights 1940 Ford

Thx guys. I had to go out of town for a couple days. A good buddy passed. I’ll be getting back into this now that I’m home. I’m not an electrical guy so this is new to me ! I’ll struggle through though. I sincerely appreciate all the comments and help and will look into everything mentioned and report back.

Thx Mike
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Old 06-15-2024, 01:10 PM   #17
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Default Re: Stop Lights 1940 Ford

PS - does someone make a good wire harness that runs to the taillights ?

Thx
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Old 06-15-2024, 01:52 PM   #18
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Default Re: Stop Lights 1940 Ford

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PS - does someone make a good wire harness that runs to the taillights ?

Thx

Contact Michael Driskell at 3rd Gen Automotive, BELOW. He supplied ALL of the excellent harnesses for our '37 coupe.

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Old 06-16-2024, 05:35 AM   #19
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Default Re: Stop Lights 1940 Ford

Coop thx. He rebuilt the distributor last year I think.

I found one clothed wire that was bare at the rear end of frame. I also tried some 1054 bulbs and still nothing except the headlights come on. I’m thinking about running a couple new 14 gauge wires, as the ones I’ve got are spliced in several spots.

When my father in law died he left a bunch of items for checking electrical things, but I don’t have a clue on how to use.

Should both taillights be grounded ?

Thx…..Mike
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Old 06-16-2024, 08:29 AM   #20
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Default Re: Stop Lights 1940 Ford

Check the voltage going to the switch and taillights. I had an issue with low voltage. Use a meter
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Old 06-16-2024, 09:20 AM   #21
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Default Re: Stop Lights 1940 Ford

I’m trying out all this stuff my father in law left me for checking stuff !
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Old 06-17-2024, 05:08 AM   #22
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Default Re: Stop Lights 1940 Ford

Man oh man……looked at all the wiring from master cylinder to the rear of car and there are so many different connectors over the years I have no idea what’s what ! I want to try to run new wires from master cylinder but hard don’t know which wires I should connect to in the rear. What a mess !
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Old 06-17-2024, 06:13 AM   #23
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Default Re: Stop Lights 1940 Ford

Get a complete wiring harness from Tyree Harris or one of the several other suppliers be done with electrical wiring problems. Extra wires can be added for accessories like turn signals.
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Old 06-17-2024, 09:55 AM   #24
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Default Re: Stop Lights 1940 Ford

C & G shows the tail light harness.
https://cgfordparts.com/tail-light-h...91a-14405.html
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Old 06-17-2024, 10:05 AM   #25
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Default Re: Stop Lights 1940 Ford

Mike, This wiring thread will help you re: tailight wiring.
https://www.fordbarn.com/forum/showt...WIRING+DIAGRAM
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Old 06-17-2024, 10:07 AM   #26
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C & G shows the tail light harness.
https://cgfordparts.com/tail-light-h...91a-14405.html
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Old 06-17-2024, 11:10 AM   #27
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Default Re: Stop Lights 1940 Ford

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Man oh man……looked at all the wiring from master cylinder to the rear of car and there are so many different connectors over the years I have no idea what’s what ! I want to try to run new wires from master cylinder but hard don’t know which wires I should connect to in the rear. What a mess !
Don't despair; while wiring can be intimidating, if you look at it one circuit at a time it will be easier understand and proceed.
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Old 06-17-2024, 04:59 PM   #28
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Default Re: Stop Lights 1940 Ford

Unless you have turn signals there is really only 3 wires that run to the back. Fuel gauge, stop lights and taillights. My suggestion for someone that has little knowledge would be to find the 2 wires to the left rear taillight. Run a new stoplight wire to just that brake light same for the taillight wire. Get that one light to work then you may have a better understanding of how it works. Every light in the back t's from the left rear. New harness maybe needed but is no good if you don't know how it hooks everything together. Mike
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Old 06-18-2024, 01:01 PM   #29
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Default Re: Stop Lights 1940 Ford

Thx to all. I’ve been in contact with Tyree as I heard he makes a good harness. I did get the stop lights to work. Found a bad wire and bought a new switch so I can at least drive during the day. No taillights when I turn on the headlights though.

Rich….. it is intimidating as hell when you don’t know what you’re doing !

More later and thx again for all the help !!

Mike
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Old 06-18-2024, 01:35 PM   #30
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Default Re: Stop Lights 1940 Ford

Ok, good progress. You had two separate independent problems, coincidently happening at the same time.

Rear taillights and license light is a solid black wire, fed by a screw terminal on the side of the light switch. Suggest you start at the switch, to see if the black wire is still attached/tight at the switch. (That same terminal screw also has a black/blue wire feeding the instrument panel dimmer switch. Do the instrument lights come on?)
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Old 06-19-2024, 05:15 AM   #31
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Default Re: Stop Lights 1940 Ford

Kartrae…….I’m looking into what you suggested.

Jay…….I’m going to into our suggestions also.

I just hate working on something that I’m not comfortable with ! Damn !

Thx all……Mike
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Old 06-19-2024, 07:55 AM   #32
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Default Re: Stop Lights 1940 Ford

Jay……when I turn on the headlights the dash lights also come on. The dimmer switch also works. Does that indicate there’s no problem at the switch ?

Thx
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Old 06-19-2024, 10:31 AM   #33
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Jay……when I turn on the headlights the dash lights also come on. The dimmer switch also works. Does that indicate there’s no problem at the switch ?
Yes, most likely if the dash lights work the switch and connection there is OK. (this assumes no one has done some unorthodox wiring, like connecting the dash wire to the switch's headlight terminal. Your dash lights should come on with the parking lights....Let us know if they do not.)

So now you have to move to the next likely suspect, which unfortunately will require getting under the car:
The rear body harness connects to the front harness above the left frame rail just under the headlight foot-dimmer switch. The bullet connector there could easily be loose or corroded. Find the rear harness in the left frame rail under driver's door and follow it forward to find three individual wires with bullet connectors. The solid black wire is the taillight wire. Carefully pull the connector apart and use electrical contact cleaner and wire brush to clean both sides of the connector. Then push back together and see if that solved the problem.
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Old 06-20-2024, 02:10 AM   #34
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Default Re: Stop Lights 1940 Ford

I’ll check the parking lights this morning. The wires near the dimmer switch are a bundled up mess, but will check it out.

Thx
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Old 06-20-2024, 03:53 AM   #35
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Default Re: Stop Lights 1940 Ford

Jay…..if I can’t find that black wire in that bundled mess is it possible to run another wire off the light switch where the black wire originates ?

Thx
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Old 06-20-2024, 09:32 AM   #36
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Default Re: Stop Lights 1940 Ford

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Jay…..if I can’t find that black wire in that bundled mess is it possible to run another wire off the light switch where the black wire originates ?

Thx
Possible, but probably more work. Working under the dash on the switch is not easy. Probably need to remove switch from dash to work on it, which is a tricky procedure in itself. Then you would need to get the new wire thru the dash and find a path to down under the car. And then you have a RubeGoldberg wire in there that doesn’t belong. Not recommended!

Yeah, lots of wires in a tight location under the foot dimmer. I would first try to follow the rear harness forward to feel the three wires coming out of it to see if you can feel one of them that feels loose or broken.
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Old 06-20-2024, 10:29 AM   #37
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Default Re: Stop Lights 1940 Ford

Understand ! I don’t need anymore mess than what I already have. I’ll check along the harness going back.

Thx
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Old 06-21-2024, 06:02 AM   #38
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Default Re: Stop Lights 1940 Ford

I’ve got to leave things alone for awhile. I’ve been obsessed with this and all my yard work has gone to hell. At least I’ve got stop lights now and I never drive at night so the taillights will have to wait. Thanks to everyone for the help. I printed everything up so I can refer to it at a later date.

Mike ����
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Old 06-23-2024, 08:45 AM   #39
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Default Re: Stop Lights 1940 Ford

Just to update…..I took out the glove box and radio speaker grill in order to get a clearer shot at light switch. I attached an alligator clip to a 25 foot wire used the other end to touch the black wire on switch. Bingo…..all lights came on ! So I now know I have a problem somewhere in the line running back to the taillights. That may be another issue, but I at least know where to look.

Thx again to all for the assistance.

Mike
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