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No Brake Lights

10K views 41 replies 8 participants last post by  Broncplow 
#1 ·
Hi all,
The brake lights in my '81 just went out. I haven't had a chance to check the bulbs yet, but I can't imagine both would burn out at the same time (although, I suppose it's possible). I know they were both working a few weeks ago.

The running/tail lights still work. Just no brake lights when I press on the pedal.

I checked the fuses and they all seem Okay.

This is my daily driver, so I know it's just a matter of time before I get pulled over.

I'm not sure where to start digging into this. Any guidance? Thanks.
 
#2 ·
Check the brake light switch on the brake pedal? It's possible that it worked it's way out of adjustment. I know on my bko if Im under the dash for whatever reason and am not careful I can screw with mine just enough so the switch doesn't make contact when you hit the brakes.
 
#3 ·
#4 ·
Really what?



Either way, the switch is working. However, I ran a voltmeter across it and I'm only getting 2.5v instead of the 12.5v my '84 Bronco is getting.

I'll start tracing it down, but if anyone has any ideas that could speed the process along, I'd really appreciate it.
 
#9 ·
So, how do you know that the switch is working?

You got 2.5 volts when you went across the 2 terminals of the switch? Is that correct? All that tells you is that you don't have a good voltage source to the switch or that you don't have a ground on the other side of the switch, which is through the turn signal switch, brake light bulbs, etc. You still need to find out which is the problem.

Attach one lead of your meter to a good chasis ground and check for voltage at the brake light switch. If you don't have battery voltage at one terminal, go find out why not. If you do have battery voltage at one terminal, then check to see if there's a good ground on the other, and if not, then chase that problem down.
 
#5 ·
I'm having difficulty finding it in my Chiltons, can someone tell me where the ground is for the brake lights?
 
#8 ·
Before anything else, try working the hazard switch a few times.

Joe
Only the defrost grid is grounded at the L taillight; all the taillights are grounded in the engine bay.

6696
The brake light circuit doesn't pass through anything on the frame; what you're seeing is the '93-only SCCDS.
 
#12 ·
lol... this is one of those things that would take minutes to test, and hours to describe how to do it with all the possibilities.

Regarding the ground. Steve says that it is not behind the left tail light. The only location listed in haynes is for a 1992 and newer and that is at the driver's side upper radiator support. The older trucks it doesn't give the location of the grounds.

For a 1981 the black wire to the bulb is the ground. You might try running a separate ground to it to see if that makes a difference.

Again... test your bulbs. If one goes out, the other one has just as many hours on it and is likely to fail soon after the first.

My mind gets fuzzy when I try to wrap it around voltage drop off, but one thing for sure... Steve was right when he said that I was wrong. I was thinking that the brake pedal switch supplied the ground. Nope!

To the suggestions that 87Roughneck offered, I would like to add. Check the brake switch pins for voltage. One should have battery voltage. Then check the other with the pedal depressed. It should also have battery voltage, or just a little less. This is to test the switch, if you do not have the same voltage going through the switch, it is bad.
 
#15 ·
Thanks for all the guidance everyone. The less hunting and pecking I have to do is very much appreciated. It's 100 - 110° out, so crawling around inside my Bronco is like working in a sweatbox and the less time out there, the better.

I have two Broncos side-by-side, the other is an '84, so it's nice to compare since its brakes work just fine.

On the '84, if I remove the plug to the switch and check the voltage across the plug, I get 12.5v. If I repeat that on the '81, I get 2.5v.

If I check the plug on the '84 with the negative terminal for the voltmeter on a good frame ground, and the positive to the plug, one wire gets 12.5v and the other gets 0v. (Remember, this is with the plug unplugged from the switch.)

If I repeat this on the '81, one wire gets 12.5v, but the other (which was 0 on the '84) gets 9.5v. Both sides had voltage when it wasn't even connected to the switch. :confused:


Would bad bulbs cause voltage problems?

I understand the basics of electronics, but I'm not the best at diagnosing what things mean, so thanks for any help.


Heh, and no, Steve, I'm not seeing any ground there.:toothless
 
#17 ·
I did, the hazards work fine.

Thanks for the wiring diagram. That's a little easier to follow than the Haynes manual one.

Are you saying that the connection to the bulbs might have come apart/loose? Would this be behind the taillights?

I'll try tracing the wires by color and see what I can find.
 
#18 ·
...the hazards work fine.
That's not the purpose - working the switch several times may clean the contacts, allowing the turn/brake lights to perform normally. If so, buy a turn/hazard switch.

Are you saying that the connection to the bulbs might have come apart/loose?
No, the fault is most likely in the turn switch contacts, or the hazard switch contacts.
 
#19 ·
Okay, thanks. I'll give that a try. May just pull the steering wheel and clean them. I've had that apart several times (although probably not for a year or more), so I know my way around it a bit. Hopefully that's all it takes.

Thanks for the guidance.
 
#20 ·
Well, I turned the hazard lights on and then checked the voltage to the brake switch. Every time the hazards "click", the voltage spikes up to 12v and then drops back down to 2v. I'm guessing that's not normal...

Also, when the hazards are on, the front lights blink, but the rears don't. Out to do more digging.



EDIT: Nevermind, that makes sense that the brake voltage would spike with the hazards. It's flickering them off and on. :rolleyes:
 
#22 ·
I'll go check that right now.

In the mean time, I cleaned the ground on the brake lights and changed the bulbs for good measure.

The driver's side works perfectly now, but the passenger side is getting 0.1v at the bulb.



I've had an issue before where if my passenger side brake light went out, I'd jiggle the key and it'd start working again. Could the ignition lock cylinder be to blame? Seems odd, but it's fixed it before. It's just not doing it now.
 
#23 ·
Tested the same wire right out of the column connector and it's also 0.1v. Does that mean I need a new turn signal switch to fix it?
 
#24 ·
0.1V isn't significant, but which side of the connector are you testing? The dash, or the column? As that diagram shows, the ig.sw. doesn't directly affect the turn signals, and it has no effect on brake/hazard. The lock cylinder is purely mechanical - if wiggling it has any effect on anything electrical, it's either coincidental, or it's moving some wiring in the column.
 
#25 ·
I tested the wires at the brake lights first with a sure ground, and when the hazards were on, the passenger side alternated 0, 0.1, 0, 0.1 (whereas the driver's side alternated between 0, 12, 0, 12...)

I then found the same colored wires that went up the column and into the turn signal switch and they did the same thing. The wire that was the same color as the driver's side brake light alternated between 0 and 12, and the wire for the passenger side alternated between 0 and 0.1v.
 
#27 ·
That's what I'm thinking. Everything (as far as I can tell) checks out right up to the turn signal switch.
What are the whiskers? Not sure I'm familiar with that.
 
#29 ·
Since those are incorporated into the turn signal switch (I've messed with that cam before), I just decided to forgo checking those whiskers and bought a new switch. I'll install it tomorrow morning and report back. I imagine it'll fix it right up.
 
#30 ·
Well, I swapped the old turn signal switch for a new one and all my problems went away. The old turn signal was only sending voltage to one side and not the other. I suppose 30 years of wear on contacts will do that. :)

Thanks for walking me through tracking down the issue. I imagine I would have eventually got there, but it would have taken me days instead of hours. I really appreciate it.
 
#31 ·
Quick question:

All the new turn signal switches I saw for sale, including the one I bought, had an extra wire with a bulb on the end of it. My original switch harness didn't have this bulb. Anyone tell me what the bulb is for? (I'm assuming it's a dash light of some kind.)
 
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