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Old 07-18-2016, 11:44 AM   #1
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1979 31' Excella 500
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Strange Tail Light Problem

We're on our way home from Alberta, CA to Michigan, stopped at a KOA for the night, and realized one of the tail lights on our renovated '79 Excella isn't working. Hoping to get some insight from the wise guys/gals here because this one has me stumped...

Using this diagram:



The Problem: Turn signal and brake lights do not work on the right side of the trailer.

What I've Tried:

1. Connecting TT male 7-pin plug to female 7-pin receiver - DOES NOT WORK

When connecting the trailer light plugs as you would normally, all lights work as expected except the R-Turn/Brake light.

2. Jumping 12v and R-Turn connectors on TT Plug - WORKS.

On the travel trailer male 7-pin plug, if I connect a jumper wire from the 12v connection to the R-Turn connector the tail light illuminates. Tells me the bulbs aren't burnt out and the wiring to the tail light is OK.

3. Voltage Meter on TV female 7-pin connector's R-Turn and Ground connector - WORKS


When I used a voltage meter between the GROUND and R-TURN connectors on the TV's female plug, I get on/off 12v (or close to it) on the meter's display in a cadence that's the same as the turn signal. Tells me the TV's plug is wired correctly for R-Turn.

4. Jumper wire between the R-Turn connector on the TV's female 7-pin receiver and the TT's male 7-pin plug - WORKS

If leave the TT and TV plugs unplugged from each other but connect the R-TURN connector from each via a jumper wire, I get on/off 12v (or close to it) on the meter's display in a cadence that's the same as the turn signal. The TV was hitched to the car at the time (creates common ground between TV and TT).

So.... I'm stumped. Seems like it has something to do with the plugs (scenario 1) but can't understand how that can be given the testing I did (scenarios 2, 3 and 4).

Help?
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Old 07-18-2016, 07:47 PM   #2
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What is the physical (mechanical) connection arrangement? Does one side have a single blade and the other side have two "blades" that are touching or nearly so that are spread apart by the single blade? If so, is it possible that the problem spot might be a poor connection? If you have the system I described you might try using a needle nose pliers to squeeze the two parts closer together.

In #2 of your description, did you just run a wire from truck #4 to trailer #6?
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Old 07-19-2016, 04:04 AM   #3
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When my left brake and turn signal would not function it was a bad ground. I stuck a jumper wire in the socket with the bulb the clipped the other end to the shell. This proved the problem and I fixed it.
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Old 07-19-2016, 04:20 AM   #4
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I just had a very similar problem. Are you using the original umbilical? Check the TV end of the umbilical for broken contacts. Looks like both your TV and trailer are working, the only other thing is the umbilical.
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Old 07-19-2016, 04:28 AM   #5
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I have the same problem from time to time. I clean and adjust the pins apply a dab of dielectric grease and I am good to go. The tools in the photo are very handy, I got mine from Mac tools

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Old 07-19-2016, 05:10 AM   #6
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I had same issue, broken wire buried some where deep in belly pan. A new wire was installelled and craftfuly run thru the inside of AS. All is good. It took several attempts over time to figure this out but the last man I went to out in IN knew exactly what it was. .
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Old 07-19-2016, 07:40 AM   #7
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Quote:
Originally Posted by kb0zke View Post
What is the physical (mechanical) connection arrangement? Does one side have a single blade and the other side have two "blades" that are touching or nearly so that are spread apart by the single blade? If so, is it possible that the problem spot might be a poor connection? If you have the system I described you might try using a needle nose pliers to squeeze the two parts closer together.

In #2 of your description, did you just run a wire from truck #4 to trailer #6?
You got it right: single blade connects to double blade. And yes, I just ran a wire from #4 to #6 in scenario #2. I will try the needle-nose pliers tip; thanks!

Quote:
Originally Posted by TheGreatleys View Post
I just had a very similar problem. Are you using the original umbilical? Check the TV end of the umbilical for broken contacts. Looks like both your TV and trailer are working, the only other thing is the umbilical.
Agreed, both TV and TT seem to be working; yes, it's the original umbilical. Checked for broken contacts... no such luck.

Quote:
Originally Posted by Naper View Post
I have the same problem from time to time. I clean and adjust the pins apply a dab of dielectric grease and I am good to go. The tools in the photo are very handy, I got mine from Mac tools
Great tip! Thank you!

Quote:
Originally Posted by Sbb View Post
I had same issue, broken wire buried some where deep in belly pan. A new wire was installelled and craftfuly run thru the inside of AS. All is good. It took several attempts over time to figure this out but the last man I went to out in IN knew exactly what it was. .
Yikes, I hope it doesn't come to that. Given scenarios 2, 3 and 4 work I am doubtful there is a wiring problem. Fingers crossed.

Quote:
Originally Posted by Dingo Girl View Post
When my left brake and turn signal would not function it was a bad ground. I stuck a jumper wire in the socket with the bulb the clipped the other end to the shell. This proved the problem and I fixed it.
Because the bulb works in scenarios 2, 3 and 4 I don't think it's a problem with the bulb and socket, but worth checking out nonetheless. Thanks for the tip!
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Old 07-19-2016, 01:46 PM   #8
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It's either the contacts in the umbilical cord plug or a bad ground. Look at the bulb closely when jumping pins 4 & 6 together to see if both filaments are lit. If a bad ground at the bulb electricity will flow through brake filament and out through tail filament and through all other running light filaments to ground. If you jump from pin 4 to 3 all running lights except tail filament will light because there is no circuit back to ground unless the right brake light wire is grounded. Sometimes you just need to scrape the socket where the ears on the bulb touch.
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Old 07-19-2016, 02:03 PM   #9
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Quote:
Agreed, both TV and TT seem to be working; yes, it's the original umbilical. Checked for broken contacts... no such luck.
I still have my money on your umbilical going bad. 40 years is an old cable, and you've ruled everything else out. Contacts could look OK, but still not be making good contact with the TV plug. Could be corrosion of the connections inside the cable. You're only looking at about $45 for a brand new umbilical.
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Old 08-18-2016, 05:55 PM   #10
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Well, I replaced the umbilical cables and still had the same problem. I eventually determined that when the running lights were connected that's when the right tail light stopped working. It made no sense. I tore the taillight apart hoping to find something, anything, that was clearly wrong. Nothing; everything looked fine. I flicked the bulb with my finger and low and behold a shimmer of light came out of it. I reseated the bulb with some bulb grease and now it works great! Wish I would have tried that first (duh)!
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Old 08-18-2016, 08:33 PM   #11
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Quote:
Originally Posted by trekerboy View Post
Well, I replaced the umbilical cables and still had the same problem. I eventually determined that when the running lights were connected that's when the right tail light stopped working. It made no sense. I tore the taillight apart hoping to find something, anything, that was clearly wrong. Nothing; everything looked fine. I flicked the bulb with my finger and low and behold a shimmer of light came out of it. I reseated the bulb with some bulb grease and now it works great! Wish I would have tried that first (duh)!
Just reading that hurt!

Glad you got to the bottom of it though.
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Old 08-18-2016, 08:50 PM   #12
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Quote:
Originally Posted by trekerboy View Post
Well, I replaced the umbilical cables and still had the same problem. I eventually determined that when the running lights were connected that's when the right tail light stopped working. It made no sense. I tore the taillight apart hoping to find something, anything, that was clearly wrong. Nothing; everything looked fine. I flicked the bulb with my finger and low and behold a shimmer of light came out of it. I reseated the bulb with some bulb grease and now it works great! Wish I would have tried that first (duh)!


Dielectric grease is your fiend.... oops friend.



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