'71 521...No Brake Lights

[Gnostichumor]

Hey everybody, I have a 1971 521 Pickup and the brake lights have suddenly quit working again. I should preface this by saying that I am relatively new to working on cars and very new to electrical issues. 

 

A little history on the lights: I've already replaced the brake light switch on the pedal (the lights quit working last month and this was the issue) and I've tested the new switch again this time. It's putting out voltage. When I jump the wires from the switch however, I get no light from the brakes. 

 

I've also had to rewire the blinker light harness as the old contact had rotted away. I just replaced the contact and bulbs and they work fine.

 

I believe the bulbs are ok because the tail lights work fine, there is just no change when the brakes are pressed or the sensor wires are jumped.  The fuses on the box seem to be fine also and I've already cleaned up all the contacts on the fuse box just to be safe.

 

My thought is that there is a grounding issue somewhere along the way to the brake lights. Like I said, I'm new to electrical work and so I'm not sure the best way to go about seeking out a bad ground or figuring out what is wrong with the lights past what I've already done.

 

Any suggestions?  Thanks so much.

[banzai510(hainz)]

521s have a corrossion issue around the fuse bax area.

 

 

Do your dash lights work?

I got pulled over by the cops and said I had no brake light or maybe the runner ghts were not working. I replaced a fuse  and it was fine.

 

maybe replace all the fuses 1st as they can break inside on the ends but look visually good.

 

i assume you looked at the bulbs already and ck to see if by chance they popped.

 

when you cked the switch under neath the pedal one side should have 12volts.If you have the 12volts then you know the fuse is good and the fuse box,  the other side you should be able to ohm close to ground or have a resisitance reading of some sort.

 

 

when my brake lights dont work its the switch not screwed in right for the plunger to make the contack work

[datzenmike]

If you have power on both sides of the brake switch then the problem is between there and the rear lamps.

 

If the license, marker and running lamps work then the ground is ok.

 

If both brake lamps are out then I would suspect the harness between the rear lamp harness and the cab or  the connector plug at either end.

 

 

The 620 has a connector under the passenger's seat and the frame harness comes up through the floor. Can't remember how the harness is on the 521.  Find the brake light wire and check it has power when the brake is depressed. Then check the other end at the tail light harness connection.  

[DanielC]

Here is a 521 wiring diagram.

 

The stop lamp switch should always has power on one side.  This switch may be a pressure switch, in the brake line junction, on the fire wall, above the starter, or a switch on the brake pedal arm.

The brake lights are independent of the turn signals, and a wire from the stop lamp switch just goes to the brake lights.   If you have a brake pedal switch, there are connectors under the glove box that connect the cab wiring harness to the engine room harness.  With both brake light switch locations, there are connectors between the engine room harness, and the bed harness.

Any of these connectors could have some corrosion on the contacts.  I believe the entire brake light wiring is a green wire with a yellow stripe.

 

It is very common for the taillight assembly to lose ground.  The brake light lamps have a short harness, that connects to the tail light frame.  The tail light frame should ground to the bed.  The bed should ground to the frame of the truck.  The frame of the truck should ground to the engine, and the engine should have a good connection to the battery negative.

[Phixius]

have a multi meter?  Fuse block is a good place to look also.  they often get needles and leaf gathered there and hold moister.

I had to replace the fuse block.

 

 

this was the process I used to track down mine. 

 

 

I First checked the brake switch under the dash... jam screw driver in derr...

 

its good...next...

 

 

tested the harness from firewall back... I jumper the harness to test the rear harness. if it doesnt work still, then problem will be there..

 

good.next...

 

 

so, down to under the dash to the brake switch...

 

using the multi meter I unplugged both sides of the harness and checked continuity of the wires. and found that.

there it is... corroded connection inside the plug. 

 

 

now for those that dont know, to get these buggers out. need to poke somethin in there and hit the little latch that holds it in place...

 

 

 

 this thing holds it in the plug...

 

I also had more...

 

 

 

[banzai510(hainz)]

Phixuis

Good photos

[Gnostichumor]

Thanks so much everybody! This community is so great.

 

I'm going to be spending part of today going through the processes everyone outlined to find out what's wrong.

[maxima_tyler]

hey guys, im resurrecting an old thread so i dont have to make a new one. the OP never posted his fix, so im hoping someone else can.

 

Im having an issue where all my lights are working except for my brake lights.. Even the night time lights turn on which is the same bulb as the brake light. i've checked power through the brake switch under the dash, like its shown in Phixius' post, and im getting 12.84V through the switch, but no brake lights. From the wiring diagram it looks like the only parts of the circuit are the fuse, switch light bulbs and some green/yellow wires. all of which seem to be ok. the bulb is new, and the grounds at the tail lights look good. if im getting 12.84V through the switch does that mean the circuit is ok? what could be the issue?

[DanielC]

521 brake light wiring is really simple.  The brake light switch has power all the time the battery is connected, and the brake light fuse is good.

What happens to the tail light when you put the truck in reverse, key on, engine off?   If the tail light goes out, you have a tail light grounding problem.

 

Change the bulb.  It may be new, but bad.  you can also test the bulb with a piece of wire, and touching the contacts on the bulb, white holding the sude of the base of the bulb on the negative battery terminal.  Check for power on both sides of the brake light switch with the pedal pushed down.  If you have power then, there is a connector for tail light wiring by the starter, connections under the glove box, if a brake pedal mounted brake light switch, and the only other connector is the tail light assembly harness into the truck's rear harness.

 

There is no interaction between the turn signals, and brake lights, other than a common ground.

[maxima_tyler]

the bulb is good. i tried it in a different socket, and it worked.

would i still get 12V at the brake light switch if there was a break in the harness between the switch and the bulb? im still learning electrics, but i thought if the circuit was not complete (i.e. broken wire) it would not read any voltage at all...?

[Lockleaf]

You state that the driving light is the same as your brake light. This isn't quite correct. Yes they are the same physical bulb, but there are two competely separate filaments in it. It operates as two different bulbs in one housing. So unless you have tested both filaments individually, you haven't fully tested the bulb.

[banzai510(hainz)]

1157 bulb?

 

if you got 12volts on the wire that comes ON when the switch is released then I think its not getting s ground.

[DanielC]

12 volts should be on one wire of the brake light switch ALL the time the battery is connected, charged, and the brake light fuse is good.

[maxima_tyler]

alright guys i am thoroughly confused with my brake light problem. here's what ive tested and found.:

 

- all bulbs are good. all lights work normally except for brake lights. both filaments in brake light bulbs work.

- if i connect positive power to the green/yellow wire in the plug in the engine bay, the brake lights work.

- if i connect positive power to the green/yellow wire in the plug under the passenger side dash by the glove box, the brake lights work.

- if i unplug the brake light switch at the pedal and apply positive power to the connector going to the brake lights, the brake lights work.

- if i unplug the brake light switch positive wire and apply positive power through the switch, the brake lights do NOT work.

- if i unplug the brake light switch at the pedal entirely, and jump those two wires with a multi meter, the brake lights do NOT work, but i do get 12.85V through that jump.

 

so i think my brake light switch is probably dead, but with that last pointer it doesnt seem to be my only problem.... there is something going on between the battery positive/fuse and the brake light switch. i've already changed the fuse, and the horn and turn signals which work off the same fuse still work. i dont see any breaks in the wiring harness under the dash either. these lights were working before and i dont know what would have changed...

 

any suggestions before i pull out what remains of my hair???

[DanielC]

There are two wires going to the brake light switch.  One is always hot, the other goes to the brake lights lights.

If you jump the two brake light switch wires together, and the brake lights come on, the switch is bad.

[maxima_tyler]

There are two wires going to the brake light switch.  One is always hot, the other goes to the brake lights lights.

If you jump the two brake light switch wires together, and the brake lights come on, the switch is bad.

 

I tried this and the brake lights did not come on.

 

i am getting 12.8v through that jump but no lights, so is it possible something is wrong at the fuse box? i put an new fuse in, but that didnt change anything... that is the only connection point i havent tried to jump around yet.

 

if there was a break in the wire between the switch and the light, the multi meter should show 0V, right?

[banzai510(hainz)]

if you get 12.8 volts then how the fuse is bad  when you measure one side of the switch???????? if one side has 12volts and the meter is grounded then power is on one side of the circut. YOu follwe that one side till you lose the 12volts

 

DO THIS!!!!!!!!!!!

take the switch put the meter to the OHM mode and see if short and open when switch is depressed. pressed should be Open released should be shorted

 

 

you can still measure 12volts with a broken circut if you just measuring the + side of the battery. The switch is open when the lights are off but there is still 12volts at the  one side of the switch

[maxima_tyler]

Man, sometimes its the freakin simplest things guys, it makes me go crazy!

 

i spent a couple hours last night trying to chase this problem and i didnt make any headway from where i was at before. Since this electrics had beat me down so much i just said f@ck it and plugged the brake switch back in, and wouldnt you know it, the brake lights started working! after i tucked away all the wires, i tried it again, and no brake lights. after a little crying i put some pressure on the brake switch tabs and the lights came on, no pressure on the tabs and they go off..

 

so something inside the brake switch is loose. i ordered a new switch off ebay last night, hopefully that comes soon and it fixes everything. i was hoping to get this fixed for the long weekend. doesnt look like thats gonna happen now, but at least i found the problem.

 

 

 

 

My question for you guys now. if it was just the switch that wasnt working, why didnt the lights turn on when i jumped power around the switch? it showed 12.8V but no lights. i tried checking the resistance like Banzai suggested, and maybe im not using the multimeter correctly, but the meter always showed -1 or +1 in any combination of wiring and switch on/off or no switch at all.

[banzai510(hainz)]

I dont know what +-1 is on your meter.  Put if you put the leads together and short the leads whateverthsat reading is is s ahort and is your reference to go by what a short is.

 

As for you bypassing the switch Maybe its the leads(connectors) to the switch that are intermittant.& not the switch.  The crimp mayb be corroded.  The Japs know how to wire a Battleship So im sure they can wire a truck. Most times by now its just a corrossion issue broken wire ect.... Means it can be found. Many times just rseating the plug will do it!!!!!!!!!!!!!!

 

My 521 the switch unscrews over time if the Jam nut is loose and I have lights on all the time issues or the plate falls out causeing the same thing, Lights ON