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

MorningÖ.

Iíve replaced the stop light switch a couple times in order to get the stop lights to function. They went out again and it doesnít 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 donít go on. I also noticed that when I turn on the headlights the tail lights donít 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 Iíll 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 thereís 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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Default Re: Stop Lights 1940 Ford

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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.

Coop


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Old 06-16-2024, 05:35 AM   #19
mike42
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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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