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'85 720 with No Brake Lights


djmikemouse

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I hope everyone is having a great Independence Day weekend! But the other day this lady followed me to my work and told me she had almost hit me twice because my break lights were out. After I took a look I realized that almost everything except the brake lights work. I have tail lights, turn signals and reverse lights but no brakes. I replaced the fuses and took a look at the switch for the light and it seems to be good. I do not have a voltmeter so I removed the wires to the plug in the back of the switch and connected them and still no lights. I'm thinkin of putting in a new switch just to see if it works but it doesn't seem like that's the problem. Any ideas?

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Well possibly they are both burned out????

 

 

On the inside of the right frame rail at or near the rear tail lights is a round plug that connects the lighting harness across the back to the frame harness that goes forward and into the cab under the passenger seat. As this is exposed to all manner of weather and debris thrown up by the rear tire I would look here first.

 

If someone has added a trailer plug look no farther.

 

Have someone hold the brake on and wiggle this plug and see if they flicker. I would unplug and plug it in several times to rub and clean the contacts inside.

 

 

Failing this get a test lamp ($5) and check the switch has12 volts on both sides when the pedal is pushed down. Now check that round plug on the Yellow/Red stripe wire. If it is there the problem is towards the tail lights. If no power slide the pass side seat back and locate the 10 wire plug on the carpet. Again Yellow/Red stripe should have 12 volts on the tail light side and 12 volts on the Yellow/Blue wire from the brake switch. If it's there but not at the back there must be a break somewhere along the frame.

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last time my brake lights went out I found the rubber plug that attaches to the brake pedal on the floor. Look at your brake pedal is the rubber pad still attached to the pedal ? Is the pad still in one piece? I had a pad break in 1/2 on a 240z once and the brake switch couldnt make contact with the pad. Good luck

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Alright so I did a bit of looking and realied the bulbs were burnt out so I had one on hand and was able to. I also wired up a new switch just in case. The new bulb turns on when I turn the lights on but when I push the button on the switch they do not shut off. So I took the bulb out and held the two wires to the bulb with no results. I took a new wire and held that and the red striped wire to the bulb and grounded the new wire. I was able to get the bulb to come on, so does that mean that the blue wire has a short in it? Basically my new problem is that the brake lights are now stuck on. I am not sure how to movemove forward at this point. Thanks for the input and happy 4th of July!

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Put the old switch back on. If the brake lights are on all the time your 'new' switch is likely on with the brake pedal up... it should be off withe the pedal up. Put the old switch back on. You've changed two things so now it's hard to tell which is causing the new problem.

 

 

If the bulbs were burned out, simply replacing them should fix the problem.

 

 

Check the bulb is properly in the socket. It's easy to get them in with the contacts touching the wrong things inside.

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Headlights off??? Then it must be the switch. Push the switch contact by hand if the lights go out then the switch is not being closed by the pedal in the up position. Look on the pedal, there should be a spacer that pushes against the brake light switch. Maybe it fell off. Tape something on there like a couple of pennies.

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The brakes lights turn on and off with the head and tail lights. I can press down the brake and manually push in the switch with my finger and nothing happens.

 

The brake and tail lights use  the same bulb, I think. Each bulb has two filaments. A dim one for running lights when headlamps are on, and a brighter one for brake lights. 

 

The bulb may be in the wrong position in the socket.  Look at the brass case and the socket. They only go in in one position and turn slightly to lock in place.

 

Jumper the two wires together on the brake switch the lamps have to come on.

 

Check this first...

If still no lamps then there is no power to the switch. DO YOUR 4 WAY EMERGENCY LIGHTS WORK?? If not the #12 fuse is blown. This the the 4th fuse to the left from the right side on the fuse box.

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In my truck there is big bulb for the tail light and a smaller bulb that I thought was the brake lights, unless both bulbs are for the tail lights. The two smaller ones were burnt out and when I put a new one in it stayed on with the bigger bulb. The bigger bulbs do flash with turn signals and my 4 ways do work. And I have moved the 4 way switch to make sure it is not interfering with the connection.

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You might give the harness a look for any obvious damage, it sounds like you've got a short to power.

 

My other advice is to go and get a cheap multimeter and learn to use it. Do a little research on basic electrical principles. It makes figuring out the little gremlins that come with the age of these trucks easier.

 

The bulb for the brake light is a dual filament, both filaments share a ground.

Do you know the bulb number you put in? Should be an 1157.

 

Your taillights are on constantly now, correct? Are both filaments lit up or just one? If just one, turn on the park lights. The other should turn on. Figure out what wire has power and trace it back.

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Alright so what I did was bypass one of the wires and went directly from the brake light switch to both tail lights. I now have brake lights but if the turn signal is on and I hit the brakes I no longer have turn signals. Any suggestions? Both turn signal and brake lights work but not at the same time!

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That's because the brake lights are fed through the turn signal switch on the steering column.  85/86 720s didn't use separate amber rear turn signals like all the other 720s so if you connect the pedal switch straight to the taillights you're bypassing that system.

 

If the turn signals work, then the wiring from the turn signal switch to the rear lights is fine.

 

That means the problem would be at or between the brake light switch and the turn signal switch in the column.  Check the connectors, but in some cases it's the turn signal switch itself that's causing the problem as the contacts inside wear out eventually, though it's rare to lose both sides at once. 

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  • 4 years later...

Hello. I did a bed swap from 1984 to 1986. When I had my 84 bed {tail connector is 5 prong) everthing works tail light, brake lights, reverse light, parking lights Lp lights hazzard and signal lights also works. Now I have a 1986 EVERTHING WORKS accept the brake lights (1986 is a 4 prong ) Note I did swap the electrical harness for thre taights fro the 84 to the 86 ones. Help

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My '84 FSM shows a 6 pin connector and 6 wires being used. Is that what you have???

 

Some earlier trucks share L&R signal bulbs with the brake signal. In the turn signal switch both L&R wires to the rear bulbs are connected to the brake pedal. When the turn signal is used that side wire is disconnected from the brake and the flasher unit connected and it flashes. When the turn signal lever is cancelled the flasher is disconnected and the brake signal connected again. 

 

This is perhaps what was done on the later '85/'86 as they share a similar bed different from the '84.

 

Try putting the '86 harness on. I believe the '84 brake light bulbs will not work but the turn signals should come on when the brake is used but still signal.

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On 2/21/2021 at 1:20 AM, Charlie69 said:

So I swap out rear harness due 85/86 is a 5 prong connector 83/84 is a 6 prong connector. put all new bulbs new brake light switch new fues still the same 

 

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First I need you to post the Manufacture Date of the truck from the drivers door jamb and the VIN so I can determine what year the truck is to determine what wiring harness you have.  The confusion I have is your info states a 1985 King Cab and you said you removed an 84 Bed.  Many of these trucks were built the year prior to sale date which would be the title date.  What I am saying is your 85 truck might actually have been an 84 model sold in 85 this would explain the 84 bed.  The other scenario is someone before you swapped beds.

1984 taillights, backup lights are the lights just above the bumper

image.jpeg.00606d397b99ed07aa30342cebfbc0c1.jpeg

 

1985/86 taillights with the backup lights combined in the taillight, the red lenses in the bumper are reflectors.  Notice the bed hooks on the 84 and the differences in the tailgates.

https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQgDTBmlscMlEnmNIz2fHRFwNkCyuDSndBzsA&usqp=CAU

 

 

 

 

Edited by Charlie69
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Not necessarily Mike in the USA it is determined by when it is first sold and titled so if an 84 sits on the dealers lot until 85 it is titled in the US as 85 not an 84.. This sucks when trying to figure out what parts are actually on the vehicle.  Many 79 720s were not sold in the US until 1980 so these 79s are titled as 1980s.

Edited by Charlie69
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