No Brake Lights

[AbandonedBronco]

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.

[87SLbronco]

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.

[87Roughneck]

Originally Posted by AbandonedBronco

The brake lights in my '81 just went out...
I'm not sure where to start digging into this.

Really?

Originally Posted by 87SLbronco

Check the brake light switch on the brake pedal?

Yeah, try that.

[AbandonedBronco]

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.

[AbandonedBronco]

I'm having difficulty finding it in my Chiltons, can someone tell me where the ground is for the brake lights?

[BroncoJoe19]

Really?

First thing to check, is your bulbs.

I think that they are grounded behind the left tail light assembly.

[bronco6696]

Bulbs.. Its behind the drivers tail light.. Another thing to check is where there is a brake light harness screwed into the t fitting on your brake line below the master cylinder on the frame..

[Steve83]

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.

[87Roughneck]

Originally Posted by AbandonedBronco

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.

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.

[BroncoJoe19]

Originally Posted by 87Roughneck

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?

Yeah, that's how I interpreted that statement too.
I'm thinking that he should have zero volts across the two terminals. If he is getting anything at all, that means that he has a short to ground, (and as you point out, not a good one).
[edit] my thinking was convoluted and wrong... he should have 12 volts across those two [end edit]

[Steve83]

Originally Posted by BroncoJoe19

...he should have zero volts across the two terminals. If he is getting anything at all, that means that he has a short to ground...

No, that would mean he has high resistance in the switch; enough to create a 2.5V drop where it should be 0.

A short to ground would blow a fuse; a poor short (high resistance) to ground creates a hot spot, and may also blow a fuse.

[BroncoJoe19]

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.

[Steve83]

Originally Posted by BroncoJoe19

Steve says that it is not behind the left tail light.

Yep - not there.

[BroncoJoe19]

Originally Posted by Steve83

Yep - not there.

LOL... what are you friggin nuts!?
I laughed so hard it brought tears to my eyes :)

[AbandonedBronco]

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.


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.

[Steve83]

9.5V on the bulb side says the switch isn't connected to the bulbs; it's weakly connected to power. This is for a later truck, but the brake/turn/hazard circuits are basically the same as both of yours:



Did you try working the hazard switch yet?

[AbandonedBronco]

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.

[Steve83]

Originally Posted by AbandonedBronco

...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.

Originally Posted by AbandonedBronco

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.

[AbandonedBronco]

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.

[AbandonedBronco]

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.