Rear Brake Light Problem

[nvious62]

i have a 1994 silverado that had a 5.7 350 tbi in it. The motor went bad so i put a 305 in it. The brakes worked fine before the swap now im having problems. The problem is that when the truck is off all three brake lights will work. When key is switched to the on position or the truck is started they will work intermitenly. all the other lights work fine all the time it is just the brake lights. i checked all the ground straps on the motor checked the connectors on the brake lights at the back. also after i did the swap when driving up the road the the parking brake light in the dash will flash on and off but when the brakes are applied the light will go out then when proceed to take back off it will start to come back on and flash. Sometimes it will be real dim sometimes its bright. i would greatly appreciate any help.

[71GMC_3/4T]

I misread your post. I thought both of the turn signal indicators would light up in the dash. Now I see you said it was the brake light. This is how the codes are presented, by the number and duration of flashes.

Sorry if I caused any confusion...

 

When you say all 3 brake lights work intermittantly, you mean they sometimes don't light up when you press the brake? Sounds dangerous, and I really can't understand why an engine swap would cause that particularly.

 

Bobcat's on the right track. Find out what the code is and fix that. Maybe the other problem will go away too...

 

Jim

[bobcat5544]

First off I need to know what the codes in the system are. get a piece of wire or a paper clip. under the dash is the diagnostic connector insert one end into the top row last slot to the right (A) and the other end into the bottom row 2nd slot from the left (H). Turn the ignition key on watch the abs light it will start to flash. it show flash once then flash twice then stop then flash once and then flash twice and stop and then do it once more...after it does it 3 times the next time it flashes you will need to count how many times it flashes

 

example: (flash,flash,flash) =3 puase (flash,flash) = 2 stop and repate 2 more times

the codes # would be 32

[nvious62]

yea i pulled the codes but the only ones i got was cooling temp sensor and knock sensor i know those wouldnt not have anything to do with my two rear brake lights. the problem is the two rear ones basically will only work when the truck is shut off. if the key is turned to the on position or running the majority of the time they do not work but every other light that i have works front and back

[bobcat5544]

yea i pulled the codes but the only ones i got was cooling temp sensor and knock sensor i know those wouldnt not have anything to do with my two rear brake lights. the problem is the two rear ones basically will only work when the truck is shut off. if the key is turned to the on position or running the majority of the time they do not work but every other light that i have works front and back

 

 

The reason for that is you are only getting codes for those to sensors is you are checking codes for the PCM by useing Slot A and Slot B. To get ABS codes you have to use Slot A and Slot H

 

[nvious62]

checked to see if there was any abs codes and there was none. did some more trouble shooting today checked all fuses and the ground back by the tail lights all was good. so unplugged the t for the hitch and just hooked the connectors together still no lights when its turned to the on pos or started only when the truck is off. so hooked the volt meter up to it when the truck is off i am getting twelve volts on when i probe a and d together and twelve volts on b and d but as soon as i turn the switch on i have no power at all on any of them. this is all on the plug that runs from the front to the rear for the lights

[bobcat5544]

I was rereading everything you posted. You first stated both rear and high mount brake light does not work when truck is on or running.

Then you stated later on in a different post just the 2 rear brake lights are not working when truck is on or running.

 

which is because it makes a big differance

[nvious62]

the third light works all the time on or off its just the two rear that will not work when its on or running sorry if i confused u but all the help is appreciated

[bobcat5544]

next thing I need to know. With the truck running and brake pedel pressed down do the turn signals and/or hazzard lights work any differently.

[nvious62]

the turn signals work normal normal blinking and normal clicking noise the hazards i did not check i have just realized that they was tied in there but i can run downstairs and check it out and let u know

[71GMC_3/4T]

Another thing that might help is determining whether or not the rear turn signals are working or not. They use the same bulb filaments as the brakes I believe.

 

The brake lights are "processed" inside the turn signal switch. And turn signals are unpowered with the IGN off. Brake lights will work any time, same with hazards.

[nvious62]

just rechecked the turn and the hazards they all work normal with the truck running and pedal pressed

[71GMC_3/4T]

I'd say put the old engine back in it and see if that helps...

 

If you have your foot on the brake and signal right, does the right rear flash, and the other two (hi-mount and left) bulbs stay on? How about for a left turn?

 

Are you looking at the bulbs, or just verifying the lights are lit from the outside? The reason I ask is that bad grounds can cause all sorts of weird stuff, and if it's the park light illuminating and not the brake/turn filament, you could be chasing your tail for a while.

[nvious62]

wish i could its shot. no i havent actually pulled the bulbs out to watch them but i can tell that the parking lights stay on and when you turn the signals on that they blink and the parking lights stay light but still all the while the pedal is pressed the only brake light that is lighting up is the third one

[bobcat5544]

I know you said you have checked all the grounds, but the problem your having has the same characteristic

of a bad ground.

 

This is what you to do. Under the hood on top of the drivers side fender well the abs module mounted there. there are 2 connectors pluged in to it. The connector that has 6 wires coming out of it there is a black wire in slot A. Which ever way works for you I need you to ground that wire. Once grounded see if the brake lights work.