Passenger brake light not working.

[NateDogg024]

I'm out of ideas here can anybody please help. I've checked, checked, and rechecked and I cant seem to find out why only the passenger side brake light wont work.

I have four different circuit boards that I'm testing with, two LH (left hand) and two RH (right hand), all four work in the LH plug (tail, blinker, brake, and revers) and everything minus the brake light work in the RH plug, so I'm fairly confident all the circuit boards are okay.

I've tried many combinations of blubs and still no brake light from the RH side. I have two different wire harnesses and both are acting the same way (one is in vary bad shape and the other I have looked over and made good, with new connections and soldered).

I've used my volt meter and have traced voltage all the way from the pedal to the light sockets, on both sides, and all wires; and I have 11.98-12 volts all the way back on all the wires that correspond to there functions.
I've also used my test light and I have a light on, as well as voltage, when the tails should be on(brown wire), blinker when on(RH dark green wire LH yellow wire), and brake lights when on(same dark green and yellow wire), but, help me understand this, when I put in a bulb no brake light on the RH side.
I have nearly zero ohms with power off (00.4 ohms, so I'd call that zero) on all the ground wires to the frame rail. With a fresh bare metal contact to the frame.

After all of these checks and passes why no RH brake light?

Could this be my problem???
When power is on and testing is being done at the light sockets I have 11.98 volts on both + (positive) sides of the light socket (tail and brake) and resistance on the - (negative) side of the light socket. The LH side reads just over 9 ohms and the RH side reads over 13 ohms. Why is this?
When power is off zero ohms to ground. When power is on high ground. I should probably know this but I'm stumped. What am I missing???

Thanks for taking the time to read any and all suggestions will be appreciated.
Nate

 

[hookedup50]

Any luck with this problem? I have this problem on the drivers side. I change boards and they burn out in a few days or last a long time. I am gonna order earlier lenses and sockets and solder them in to eliminate the crap circuit boards. That's what I read on here. I wish I had an answer.

 

[NateDogg024]

No luck just yet.
I do have an update:
The higher ohm readings must have been an error on my end.
Tonight i tested the fact that the circuit board has 12 volts at the bulb socket. With the LH brake light on, the bulb out of the RH socket, and 12 volts on the dark green wire, as soon as the bulb gets put into the socket the voltage drops to zero on the dark green wire. So were is the voltage going when the bulb is in place? This happens with ever circuit board I have. There must be something going on with the dark green wire but what?
Any suggestions on where to look?
I also tried a new blinker relay and no change in functionality.

 

[NateDogg024]

Any luck with this problem? I have this problem on the drivers side. I change boards and they burn out in a few days or last a long time. I am gonna order earlier lenses and sockets and solder them in to eliminate the crap circuit boards. That's what I read on here. I wish I had an answer.

Hooledup50: Can you link me to a thread on the upgrade your talking about?
what year do you get the sockets from and are the earlier lenses a simple bolt on upgrade?

 

[schiker]

Does the LH brake go dim with RH bulb in socket even though it doesn't light?

Sounds like not enough amperage to light the RH bulb like a weak connection or bad wire etc.

You mentioned you tested with your test light but it might not pull enough amperage to accurately test power to the circuit. Make a test light with the bulb that goes into the socket.

You can also test by making a jumper to bypass the circuit board test with the actual bulb not your test light.

Last time I was working on my reverse lights I used alligator test leads from Northern or Harbor freight to clip to lights.

 

[NateDogg024]

Does the LH brake go dim with RH bulb in socket even though it doesn't light?

Sounds like not enough amperage to light the RH bulb like a weak connection or bad wire etc.

You mentioned you tested with your test light but it might not pull enough amperage to accurately test power to the circuit. Make a test light with the bulb that goes into the socket.

You can also test by making a jumper to bypass the circuit board test with the actual bulb not your test light.

Last time I was working on my reverse lights I used alligator test leads from Northern or Harbor freight to clip to lights.

The LH light does not dim at all. It's operation looks to work just fine weather the RH bulb is in or not.
I've also been using my DVOM along with the test light to check voltage.
I have not yet checked current flow.

What I'm not understanding is why there is 12 volts at the bulb socket but as soon as the bulb is pressed in the voltage is lost. I'm watching this happen with my meter.

 

[hookedup50]

I will look to find the link, it is somewhere in this section. I believe the Fermanator mentions it to someone with similar issue, and describes this fix. I seen a thread on it as well, but might be on GMT400 site.

 

[schiker]

Voltage is potential it tells nothing of ability to flow current. Its like water pressure. Say you have a garden hose and the spigot is just barely open if you leave a nozzle closed at the other end of hose it will build pressure (may take a while). But if you open a big nozzle at the end it will have very little to no pressure (all the restriction is at the barely open spigot).

V=I x R If there is a weak connection that cannot support current ie no appreciable current then voltage will drop. A DVOM does not pull enough amperage to test a circuit because its internal resistance is very high. They usually only show you if there is potential.

Incandescent light bulbs draw more current when they start as the element is cold. Current heats up the bulb and it glows to make light. When they glow hot the resistance goes up a lot. The amount of current required to warm up a bulb is dependent on its wattage. You can't really test a bulbs resistance cold to tell its wattage as the amount the element heats up and changes resistance might be big. You have to measure the current flow and calculate the wattage while burning.

This is why your test light might work but a bulb doesn't.

I know when I was trying to diagnose my reverse lights I would get voltage everywhere that would make sense when I used my DVOM but lights would not come on. So I started using an actual bulb to test for power. I can't remember what fixed it but eventually found it.

Have you taken a small screw driver and cleaned out any hardened dielectric grease from the circuit board socket. The GM stuff gets hard with age and being dielectric grease it does not conduct electricity well. At one point I picked and scraped it all out and cleaned the ports then put a new dab of synthetic food grade grease in the socket after it all worked to retard corrosion.

 

[NateDogg024]

I will look to find the link, it is somewhere in this section. I believe the Fermanator mentions it to someone with similar issue, and describes this fix. I seen a thread on it as well, but might be on GMT400 site.

I've been looking on both forums and cant seem to find the mod your talking about, if you can locate it that would be great.
thanks

 

[NateDogg024]

V=I x R If there is a weak connection that cannot support current ie no appreciable current then voltage will drop. A DVOM does not pull enough amperage to test a circuit because its internal resistance is very high. They usually only show you if there is potential.

Yes I'm familiar with Ohm's law and all of it's properties, but, correct me is I'm wrong, if there is no voltage measured then there will be no current flow. Current can only be measured in series and I don't have a set of alligator clips, so that is why I have not yet measured current (it can be a pita with out them). Current can also be calculated be without voltage no calculations can be made or measured.

My problem that I'm seeing now is, 12 volts is showing up at the circuit board light socket via the drkGreen wire; once the bulb is pressed into the socket the voltage disappears.
I will be checking the brake light switch on the pedal after work today to see if the voltage is missing up front. I haven't examined it to thoroughly because the driver side is working as it should. Maybe it is the pedal switch. I do have a spare switch that I can try hopefully I will get lucky and it's that.

 

[NateDogg024]

Update:
Well I feel like a dumb ass, but this just goes to show that you can never rule anything out when it comes to electrical problems without checking it first. The three way plug the allows you to put between the factory harness and plug in a trailer light harness was bad. I must eat crow now because you were right schiker

Voltage is potential it tells nothing of ability to flow current.

Lights are all working now!

 

[Will L.]

Crow shmow- the thing is working. Just buy Shriker his winner winner chicken dinner and enjoy the end of the electrical fight!

 

[schiker]

Don't feel bad and I am glad you found it. No winner chicken dinner for me just trying to help and point out some mistakes I have made. Looking at a DVOM only using voltage and resistance measurements it has humbled me a few times and probably will again. A current measurement or full current load test can show you things sometimes.

 

[hookedup50]

This thread has helped solve my same issue, next time it happens. Thanks Schiker

 

[schiker]

I was talking to someone the other day and thought of a more picturesque way to explain how the DMM can fool you just using a voltage reading on an unloaded circuit. In case you are trying to explain it to someone on another similar issue.

If I wire a black box up with a 12V battery inside to a couple of external lugs. You measure ~12V from 2 different lugs to ground with a DMM. BUT that doesn't tell you much of anything. I could have wired to the lugs with 1/0 welding wire or 22 ga speaker wire. Measuring voltage and sometimes resistance on a line without current flowing doesn't really tell you how much current can flow when there is load on the circuit.

 

[M.Novak]

I had the same problem with the backup lights on my '79 K 10. Got 12+ volts everywhere i checked, but the bulbs would not light up. After much trial and more error, the trouble was found to be a bad switch on the 4 speed tranny. Pulled the harness off the switch, put a jumper in the harness and the lights were on, so, a new backup light switch fixed it. I can only guess that the contacts in the switch were fried enough that only

 

[WarWagon]

To add to the above:

This is the same issue we see when the lift pump quits due to burned contacts in the OPS. 12v with no load, but, plug the lift pump in and the voltage falls to 7v. The 5v is lost across the OPS contacts and the lift pump won't work on 7v mainly because there isn't any current available.

Same with a light that won't light. You loose the entire 12v with any current (like when the bulb is plugged in) across the bad connection. Sometimes you can use an IR temp gun to find problem wiring or connectors that are hotter than the surrounding stuff.

 

[M.Novak]

Don't know what happened above, it should have said that the contacts in the switch were fried enough that only voltage could be measured downstream of the switch and not enough current could be passed through the burnt contacts to light up the bulbs. I guess I've been lucky with the lift pump and OPS issues. I've hade my '94 diesel truck since early '96 and never had any problems with the OPS, never replaced it yet. I've always spent the extra money and put a GM lift pump back in it when it quit. I came from that "other" GM Diesel site that's now defunct and the general consensus was that the aftermarket LP's draw more current than the GM pump and that's what burns the OPS out. I can only agree with that statement up to this point because it's what has worked for me without fail. She's waiting on a new engine right now, one or more of the main webs broke back at the end of March and the crank broke. Has 516,336 miles on it. Less than 25 miles on the new IP. Me and my son always called it "old reliable"! That was the first time it ever let either of us down with a major breakdown.

 

[WarWagon]

Don't know what happened above, it should have said that the contacts in the switch were fried enough that only voltage could be measured downstream of the switch and not enough current could be passed through the burnt contacts to light up the bulbs. I guess I've been lucky with the lift pump and OPS issues. I've hade my '94 diesel truck since early '96 and never had any problems with the OPS, never replaced it yet. I've always spent the extra money and put a GM lift pump back in it when it quit. I came from that "other" GM Diesel site that's now defunct and the general consensus was that the aftermarket LP's draw more current than the GM pump and that's what burns the OPS out. I can only agree with that statement up to this point because it's what has worked for me without fail. She's waiting on a new engine right now, one or more of the main webs broke back at the end of March and the crank broke. Has 516,336 miles on it. Less than 25 miles on the new IP. Me and my son always called it "old reliable"! That was the first time it ever let either of us down with a major breakdown.

Curious as to if the harmonic damper was in good shape? Can't complain about 1/2 million miles out of it!

You should post some pics on the carnage thread:

http://www.thetruckstop.us/forum/threads/6-5-carnage.39865/

 

[M.Novak]

The balancer and the pulley are both in good condition, they'd been changed before when the pulley started knocking. Checked em both when we put the new IP in it. Did it the old fashioned way and took the water pump off to get to the IP drive gear bolts. It got a new WP as well when it went back together. It's not apart yet, the reason I'm 99% positive about what happened to it is about 4 years ago I got a piece of the main web with that machined face about half the size of a dime out of the oil drain pan while changing oil. I told my son who at the time was driving it to college 100 miles from home to take it easy on it because it wasn't long for the world. Good thing he lived there and only made that drive twice a week. It lasted way longer than I ever thought it would. Then the IP was giving us trouble for the last year or so, it had about 200K or more on it with a relocated PMD from Heath Diesel, don't know if I can say that or not, but there it is. I'm thinking the new found power it was making with that new pump is probably what did it in. Timing was probably advanced a little too much since we didn't have a way to check it. I drove it and it felt good and wasn't real touchy when accelerating like you would expect with too much timing dialed in, so I figured it was alright until we could get it up to a shop to check it and do a TDC relearn. Oh, and it still runs, just knocks like heck. I drove it on a trailer and brought it home from town and then drove it back off and parked it in front of my shop. I think the crank broke under the main cap on whichever web(s) that split. When I do get it out and apart I will try to post pictures in the carnage thread for everyone to see.