Weird tail light problem

[hawkeyepoole]

I noticed that the right turn/brake light on the rear of my M35A2C wasn't working. I assumed that it was a burned bulb, so I pulled the cover and swapped the bulb to the other side to see. It worked just fine. So then I figured for some reason, there's no power to the socket. Maybe a broken wire somewhere.

I put a voltmeter in the socket and found 24 volts flashing on and off just like the left side. I tried swapping the bulbs. Both work, but neither works in the right side. I cleaned the socket, tried adding a bit of solder to the center contact of the bulb, in case that was an issue.
NOTHING!

There is 24 volts at the socket, yet the bulb will not light! How can that be? I put my voltmeter contacts on the socket and see the voltage pulsing with the flashers on.

Weird as ****, and I'm at a loss! I'd hate to have to buy a tail light when this one appears to work, except it doesn't....

 

[dburd72]

Bulb

Did you check for a good ground at the side of the socket. If so you need to perform a voltage drop on both sides of the circuit

 

[Danl]

Danl M915A1

you are reading volt on your meater because . you are holding one end of your test lead to a ground sorce Jest like you should do. now switch your meter to oms on uper end meters you can get a tone when you touch lead together on this setting . now put one lead on the side of the light socket, and the other lead to ground you should get a tone or oms if you don't get a tone than the light socket is not getting a ground

 

[hawkeyepoole]

I've almost thought about posting a video. I checked ground between the ground on the truck (the frame) and the outside of the socket. I checked that I'm getting 24 volts when I have the minus on the shell for the bulb socket and the plus on the center contact. I pulled the circuit apart behind the light and verified voltage is there as well. It's like there's some magic that is preventing the bulb from lighting despite there being voltage in the socket!

 

[Shoprat 19th Engr]

Run a ground wire from the frame to the out side of the socket and see if the light works if it does check the little strap on the socket as this is the part of the ground for the light

 

[Danl]

Danl M915A1

the light Bulb is no good

 

[gungearz]

Consider the bulb is not making a good contact with the socket. While having your accessory switch off or the positive battery cable off your battery.. Spray your contact surfaces off with a little brake cleaner, either use some steel wool or a small piece of fine grit sandpaper (200 grit or finer) and rough up both contact surfaces. More or less to get them bright again. Give it another shot of brake cleaner. Let dry for 10 minutes. Then take some dielectric grease and give the socket and bulb contact a light coating. Also, the bulb socket maybe worn some and maybe requires a little rerounding with some pliers or you can change out the socket assembly all together.

 

[1 Patriot-of-many]

Pull everything out of the casing, a few phillips head screws, I found at the back it was all rusted and screwed up. I ran some wires to ground and all fixed. I agree with others, bad ground.

http://www.steelsoldiers.com/deuce/78908-turn-signal-ground-2.html

Pay particular attention to page 2 of that thread.

 

[SoundGuy]

I know I am a little late to the party, but I figured I would explain what you are seeing in case someone else is reading this.

What you have is Voltage but no current. Your meter does not really put a load on the circuit, but the lamp does. With the lamp the voltage is dropping very low. Your wires turn into resistors with all of that corrosion.

The other possibility is a pitted contact on either the socket or lamp. It looks very burnt. Your meter makes contact, but the lamp does not. It gets worse with time as the pitting causes arching which leaves a burn mark that produces pitting. And round and round you go till light you do not have.

 

[bigbry67]

You are losing either power or ground when the circuit is loaded, and you need to figure out which side you are losing by performing a voltage drop on either side . The DVOM will not "load" the circuit by providing enough resistance for current to flow like the light bulb does.

 

[cranetruck]

Make sure both mounting bolts are grounded on the assembly, a must for plastic housings.

 

[hawkeyepoole]

I'll try those things while I'm at the hangar today later. (it's 2am Sat) I have belts to replace as well. It'll be interesting to see what the issue is. I'll see about pulling the light apart to check for rusted parts...

 

[JOEDEUCE]

Hello, I had a similar issue at one time, tested with a volt meter seen voltage, seen 24 volts, made sure I had a good ground to the light bracket....still didnt work.... scratched my head a few times .......it wasnt until I checked to see how well my light bracket was grounded to the frame, that I figured it out...... On the meter I was reading voltage but the ground between the light bracket and the frame was poor enough that it wouldnt light a bulb....
I had other issues as well that made my diagnosis a little more difficult... My rear lights were badly corroded, so I replaced them..... My turn signal switch had corrosion inside the switch... I took it apart and attempted to clean it and would have been successful, but somebody had been in the switch before and boogered it up, so I bought a new one....
Between corroded lights, a faulty turn signal switch and bad grounds , I was chasing my tail at first , but I figured it out.

 

[JOEDEUCE]

lights

Sounds like your issue is definately ground related, as other members have pointed out already.... Keep us updated on your progress

 

[hawkeyepoole]

thank all of you! I will check this business about the ground as it seems the only logical thing going on here! I won't have time until a few days from now, but after the parade I'm in on Tuesday, I'll see what I can find out!

 

[hawkeyepoole]

OK. BlueDeuce and I went to the hangar today and worked on the truck. We replaced the compressor belt, but I'll post that on a different thread. We checked about everything we could on the tail light issue. He and I both verified that we are seeing 24 volts (pulsing) with the flasher on, coming from the proper wire in the harness. I forget which number it was... We tried completely replacing the light bucket with a different one. We verified that we could make every light in the bucket work, except we could not get any bulb to flash with the brake/flasher wire.

Bluedeuce volunteered to climb under the truck and check the wiring harness connection up near the cab. It was clean, no broken wires and he pulled it apart and put it back together.

The flasher works fine as three of the four lights flash. If the right front light wouldn't flash I would blame the flasher.

Before I pull out a TM and play electrical engineer has anyone got a real good idea how I could have voltage, but not enough current to flash the bulb or light it with the brake pedal? Where else are there connections to the right rear brake/turn bulb circuit?

TIA

 

[Shoprat 19th Engr]

Try to see if you get voltage on the socket with the bulb in the socket if you do it is a ground problem for sure then try a peice of wire from the outside of the socket to the frame rail

 

[SoundGuy]

Measure with a volt meter between the negative on the battery and the ground on the lamp with the lamp lit. It doesn't matter which probe you put where and you can just use a wire to extent the negative of the battery. In a perfect world this would be 0 volts. If is is more than a few volts then you defiantly have a ground problem.

If not, then it is the positive wire.
Do the same thing using the positive battery terminal (+24 volts. Not the dog bone) and measure from there to the brake lamp positive as close as you can get to the bulb. Make sure to do this with the lamp lit using the brakes, not the flasher. You can jump across the brake switch to keep the brake lights lit. Again, in a perfect world this would be 0 Volts. If it is more than a few volts your problem is on the positive voltage side.

Once you find which side your voltage drop is, you can move the probe until you find where the voltage is dropping. I wish I was closer as I would be glad to go and help you solve this.

 

[hawkeyepoole]

Thanks SoundGuy. I'll see about trying that next weekend when I can get back to it.

A little side info: when I was returning the truck to the Vintage Flying Museum hangar where she lives (Meacham Field, Fort Worth TX) I discovered that straight line winds had destroyed two hangars inside the airport and blown one of our 3 ton hangar doors off the rails! it was too dark to get pics, but maybe someone will get pics before the repairs are complete. These doors are 20 feet high and 15 feet wide, made of steel with steel corrugated metal covers.....

 

[hawkeyepoole]