No brake lights - walk me through this....

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gcrump01

gcrump01

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1980 FJ40 that has no break lights. I'm not 100% sure but I think they worked a month or so ago when I installed the wiring harness. Brake pedal was in then but no fluid, etc., but I'm pretty sure I tested the depth of the brake switch back then.

Anyway, now I have brakes, but no brake lights. The bulbs appear to be fine. I've removed the switch.

1. How do I test the switch? Is there anyway to bypass it to prove that the switch is the problem?

2. Where should I have power? At wiring harness where the switch plugs in? And at the brake lights?

thanks.
 
Under the dash, up at the pivot for the brake pedal, unplug the wires from the switch. Set a multimeter to measure resistance (the little omega symbol) and touch one lead to each contact on the switch pigtail when the pedal is depressed.

Repeat this test with the pedal not depressed.

Depressed, the multimeter should register some number. Hopefully, a small value (less than 1 ohm would be good). When the brake pedal is not depressed, the multimeter should not give a reading (open circuit).

HTH
 
Pulled the brake switch & tested w/ multimeter as you guys instructed. went from no reading to a small value as I wiggled it. Pretty soon one of the wires came completely loose. I'll get one ordered.

IMG_3498.webp
 
The best way to start is to determine if you have a ground problem or a switch/wiring problem. This cuts the number of things to check in half.
To do this, connect a wire to the battery negative and run it back to the rear of the vehicle. You can use a long jumper cable or just a wire with an alligator clip on it.

Then using a test light, connect the test light negative to this wire and connect the test probe to the positive terminal on the bulb socket while applying the brakes.
If the bulb lights up then that shows that you are getting voltage to the brake light and the problem is in the ground.
If the bulb does not light up, then that shows that you are not getting voltage to the brake light and you should look into the switch, fuse, wiring.
 
New brake switch installed and I confirmed I have power to the plug at the brake light and only while the brakes are pressed.
So I think it is safe to say that the brake switch is functioning properly.
IMG_3513.webp


Then I can confirm with a multimeter that I have continuity from the corresponding tab on the plug to the post inside the bulb socket. It's the one on the right. The one on the left is the running light & the running lights work.

IMG_3512.webp


So, I have power to the harness side of the plug. I have continuity from the light side of the plug to the socket. When I plug the light in, I have no power to the bulb socket.. It only plugs in one way & the green wire with white stripe goes from the plug to the bulb socket.


So what the heck is going on here? I can only assume that the harness side of the plug is corroded and not making a good connection????

Any way to clean the female side of the plug? Any other ideas? @Coolerman any thoughts?
 
The potential cause of your problem is limited by the tests you've done. You've eliminated a few things but have a few more to test.
You know that you have voltage at harness side of the plug.
To make the light work you need the plug to connect properly to the socket terminal, the bulb terminal to make a connection to the socket terminal, the bulb ground to make connnection to the socket ground and the socket ground to be connected back to the battery negative through the body/frame connections.

Of those four things, you can pin it down to two of them by doing the test that I recommended a few posts ago. ie. Get a good test ground back to the light and see if a test light works when that is used as a ground.
 
Thanks. I'll do that test tomorrow. The thing I don't understand is that using the test probe to the harness plug, I have power, but not at the bulb terminal. The test probe is grounded to team same frame bolt. I was assuming that this eliminated the ground issue. I thought I was essentially testing power along the same wire at different points using a consistent ground point.
 
Btw @edwjmcgrath im not arguing with you or your suggestions. Just asking questions to try & understand what's happening here. Really appreciate the help.
 
Few things to check:
Make sure the light is wired correctly. Aftermarket lights (though that appears to be a good quality OEM light) have been know to be mis-wired or to have poor crimps.
The next thing, is the light housing MUST be grounded in order for the lights to work. I can't remember if the 1980 lights had the ground wire for the lights in the plug? If it does, verify your rear harness chassis ground is connected. It's located about mid ways on the harness near the tranny.
Don't rule out a bad bulb until you have verified it works.
 
gcrump01 said:
Thanks. I'll do that test tomorrow. The thing I don't understand is that using the test probe to the harness plug, I have power, but not at the bulb terminal. The test probe is grounded to team same frame bolt. I was assuming that this eliminated the ground issue. I thought I was essentially testing power along the same wire at different points using a consistent ground point.
Click to expand...
Ok, I misunderstood what you have checked. You have power at the female brake light pin of the harness connector at the rear of the light fixture using a test light as shown in your picture. You also measure zero ohms from the male pin at the back of the light fixture to the light socket pin.
Have you checked for power at the socket terminal using the same test light-or just that the bulb does not light? If it is the former then it sounds like a bad light fixture. If the latter, I'd check with a test light (wrap all but the very tip with electrical tape so you don't blow a fuse). If there's power there, look into the bulb. Triple check that it's the correct bulb type-no substitutions. Try a different brand.
 
Coolerman said:
Few things to check:
Make sure the light is wired correctly. Aftermarket lights (though that appears to be a good quality OEM light) have been know to be mis-wired or to have poor crimps.
The next thing, is the light housing MUST be grounded in order for the lights to work. I can't remember if the 1980 lights had the ground wire for the lights in the plug? If it does, verify your rear harness chassis ground is connected. It's located about mid ways on the harness near the tranny.
Don't rule out a bad bulb until you have verified it works.
Click to expand...

It is a Toyota light - or I've been scammed.
IMG_3514.webp


The light is grounded through the harness with a tab attached to the light.
IMG_3515.webp


I don't think the ground is the problem though. Using the test light, I'm grounding to a bolt on the frame. I can turn on the parking lights and light up the test light when touching the left prong inside the bulb housing. I can also light the test light touching the female part of the wiring harness plug (for the brake light). However, I cannot light the test light touching the right prong inside the bulb housing with the light plugged into the harness. I do have continuity from the right prong to the corresponding male tab on the harness. Just doesn't make any sense..
And neither brake light works - all testing to this point has been done on the P side, since power goes there first.
 
I would look closely at the guts of the harness connector itself. From what you say, the only thing that could cause those symptoms is if the female and male terminals are not touching when you plug them together.
 
Racer65 said:
Do the turn signal and reverse lights work?
Click to expand...
Yes, all other lights work great on both sides. I think I'm going to take the male plug from on old light & wrap it w sandpaper and run it in and out of the female connector a few times. Just try & clean it inside. Any reason not to do that?
 
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