Brake Lights Don't Work but Running Lights Stay On

[Papasmithnc]

Hello,

 

I hope someone can help me here.  86 720 Kingcab.  134k

 

PROBLEM

My running lights stay on all the time unless I disconnect the battery. The brake lights don't work and, the turn signals don't work. Strange thing is, the running lights will go out when I take out the brake fuse. I am guessing they are on the same circuit. However, they are not on the same circuit as the turn signals as far as I know.

WHAT I HAVE DONE

After checking fuses and brake switch, I carefully disassembled the combination light switch from the steering column and found that it had old some old grease on contacts and some minor corrosion. I then cleaned all contacts, put dielectric grease only where appropriate, put a multi-tester on it to check operation after re-assembly, put it back on the truck and viola, no difference. All wires look good leading to the switch. This truck has been under some kind of shelter most of its life and is better than average shape.

The thing was, the signal lights worked until I loosened the light switch assembly. All that is required here is, to loosen one compression strap around the steering column via one Phillips screw. WHAT?? Didn’t touch anything else. Battery was disconnected.

WHAT I PLAN TO DO

I going to look at the relays and grounds more closely now.

 

In addition, plead with members here to provide direction in tracking down this elusive problem. If anyone has an idea, please let me know. As of now, this truck is out of commission and I want to get it back on the road where she belongs.

 

Thanks,

Dave in NC

[Aibast]

Might be just a ground issue. Make sure the lights in the rear get a good ground.

[gearboxx70]

I'm no expert on this, but check out the turn signal switch in the column. I've had 3 vehicles with odd rear lighting problems. I traced all 3 back to the turn signal switch at the column. Most of these switches are mostly plastic which gets brittle after so many years of use and heat cycles. Small cracks near contacts can add up to big lighting problems.

Another thing to check would be a trailer light pigtail. My old Ford pick-up had issues with both the trailer pig-tail and the turn signal switch at nearly the same time. Talk about odd lighting! I eventually made the trailer pig-tail magically disappear with a special tool I like to call wire cutters. And I used the free option for the turn signal switch... I hang my arm out the window.

 

Oh, and definately check ALL the grounds in the lighting circuit!

[DanielC]

I am not familiar with 720 trucks, only 521 trucks, and I am really familiar with them,  But some basic info, will cross over.

 

This I cannot stress enough, and I am going to yell.

YOU MUST CHECK AND CONFIRM THE GROUNDS ARE GOOD!

 

Pickups are kind of unique in the fact they have a lot more places for the grounds to go bad on the taillights.  The taillights on a 521 are grounded to the bed.  The bed in turn grounds to the frame.  The frame in turn grounds to the engine, or the cab, and the engine finally grounds to the battery.

 

This is how you can check and confirm the taillight ground is good.  This example is for a 521, but the same ideas and concepts work.

This is a picture of a 521 taillight.  The yellow wire on the taillight assembly frame is one I added to test the taillight ground.

You cannot see it in the picture, but the lamp sockets have a ground wire that goes to the taillight frame. 

  

I suspect your 720 has a similar wire on your taillights.  It will probably be black.  I think 720 trucks have the the taillights in the corners of the bed, and you can remove the taillight assembly, and find a black wire that goes to the bed sheet metal from the taillight assembly.

 

The yellow wire goes to the front of the truck, by the battery.  I did not have a long enough yellow wire, so I spliced a blue wire to that yellow wire.

This picture shows a volt meter checking the voltage between the taillight frame, and the negative battery post.  In this picture, the brake lights were turned on, by jumping a wire across the brake light switch.

The reason the volt meter is showing about one half of battery voltage is because the taillight frame has lost its ground, and the lights that I turned on, in this case the brake lights, were using the backup lights to ground.  In this picture, you can see both brake, and reverse light glowing dimly.

 

The way to fix this problem on a 521 is usually to run an extra wire from the taillight frame to the frame of the truck. And in the engine compartment run an extra wire jumping around the rubber in a motor mount, from the frame, to the engine,  I did that.  After those changes, this is what the voltage on the taillight frame was.

 

This is what the brake lights looked like before.

This is what they looked like after the additional grounds.

 

Now, to your truck.   Remember, I said my truck was using the reverse light to ground the brake light, and because of that both lights glowed dimly?

On your truck, the brake lights may be using the taillight for the brake light ground.  Because the brake light light is about 25 watts, and the taillight is about 5 watts, the taillight lights up to almost full brightness, and brake light does not barely get warm enough to glow.

 

Now, all that I explained just checks for the ground problem.  It is very possible that you may have other issues too.   But you MUST confirm you have good grounds first.

[wayno]

Give this a try, cycle the 4 way switch several times that is on top of the column, everything seems to go threw that switch, if that don't work, find another one and try that one, have you used that 4 way switch lately?

[datzenmike]

Dave

 

Power is supplied to the light switch at all times, just like the horn and interior light. It doesn't matter if the key is on or not. Power to the running lights/tail lights has to come from the light switch when turned to the first and second position. If they are on in the OFF position try unplugging the light switch under the steering column. If they go out it's likely the switch.

 

If your turn signals are working, are they? then I doubt the ground is bad. The brakes and the turn signal share the same bulbs. Back up, tail lights signal and brake lamps use the same ground. If turn signals NOT working move the 4 way emergency light switch slightly toward the ON position. This is a common problem with 720s. The emergency switches are not very good when placed in the off position they are supposed to connect up the turn signals to the lamps but often the switch jumps right past the contacts. This is why you move the switch slightly towards the ON position.