Trailer lights problem - JeepForum.com
 
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post #1 of 9 Old 09-06-2021, 01:00 AM Thread Starter
jgc98
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1997 ZJ 
 
Join Date: Jul 2006
Location: Denmark
Posts: 106
Trailer lights problem

Since I got this 98 JGC, I've had the opportunity to drive two boat trailers. Old trailers often have missing lights/bulbs, but these two trailers show the same behavior so I think the Jeep is the culprit.

Turn signal and brakes, they light up fine. But the normal rear and side marker lights are problematic on both trailers. They light up for a few seconds, then turn off, and the Jeep's own rear lights do the same and "Rear lamp failure" shows. If I disconnect the trailer wiring, lights come back, sometimes immediately, sometimes after a restart.

So it's a Jeep issue, not a trailer issue I think. And it's not a fuse, because lights come back. Is there an overload function? It should be noted that this Jeep has DRL via a module built in after it came to Denmark, so this problem could be due to that non-Jeep module. But first, is this overload/coming back part of the OEM Jeep wiring?

Thanks


97 Jeep Grand Cherokee Ltd 5.2

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post #2 of 9 Old 09-06-2021, 05:44 AM
yjsaabman
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I would be suspect of the trailer light module itself. The factory Jeep ones seem to be problematic, and if everything on the Jeep is OK until a trailer is connected that's where I'd be starting.

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post #3 of 9 Old 09-06-2021, 06:46 AM
wingless
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The factory trailer wiring on the ZJ has a relay pack to make the vehicle "immune" to the trailer.

On my '94 it is tucked up underneath the passenger side cargo window wrapped in a padded blanket.

It sounds like the vehicle was modified to add trailer wiring. If so, then properly installing one of those trailer wiring modules should fix the problem.




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post #4 of 9 Old 09-06-2021, 09:27 AM
jeepjeepster
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Our 94 with the factory wiring use to do this and the 7 pin plug on the Jeep was bad. Seems I remember something was shorting out only when a trailer was plugged in. It made the VIC say rear lamp failure and I think it eventually blew the trailer wiring fuse.

1994 ZJ-I6 w/270,000 mi
1998 ZJ 5.9 w/66,000 mi Slate Black
2004 KJ w/108,000 mi
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post #5 of 9 Old 09-06-2021, 04:11 PM
RedRiverT
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I think there is a circuit breaker in the factory trailer wiring relay bundle, and if so it has to be the type that resets after it has cooled off a little or after power is cut off, whatever makes them reset.
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post #6 of 9 Old 09-12-2021, 11:09 PM Thread Starter
jgc98
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Location: Denmark
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Thanks for suggestions, now I know where to start digging!

97 Jeep Grand Cherokee Ltd 5.2

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post #7 of 9 Old 09-16-2021, 09:59 PM
RedRiverT
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Quote:
Originally Posted by wingless View Post
The factory trailer wiring on the ZJ has a relay pack to make the vehicle "immune" to the trailer.

On my '94 it is tucked up underneath the passenger side cargo window wrapped in a padded blanket.

It sounds like the vehicle was modified to add trailer wiring. If so, then properly installing one of those trailer wiring modules should fix the problem.


A more in-depth explanation is that the relay pack powers the trailers lights separately from the vehicle's lights.

The relay pack gets power straight from the battery, through a big fuse and maybe another relay, and then the purpose of the multiple relays in the pack is to copy what the vehicle tail lights are doing by sending the power from the battery into the appropriate trailer light wire, ON - OFF or constant ON.
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post #8 of 9 Old 09-17-2021, 10:51 AM
LouC
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On the '98 your factory style trailer relays and wiring will be on the opposite side from the pic above, you remove the spare and then the interior panel and its right there. My '98 did not come with the factory tow package but I added it when it was about 2 years old so I remember doing this job. I have not towed anything with it in years, I might just hook it up to my boat trailer to see if it all still works lol.
Also I have found that the 4 way and 5 way boat trailer connectors are very suseptable to oxidation, I've had to clean/replace these over the years. Even on a brand new trailer that my brother bought this summer, we had the same thing!
And if you are using an adapter to adapt the 4 flat to the 7 round, you can have problems with those too. Older boat trailers with no brakes or drum brakes will have the 4 flat, newer ones with disc brakes will have the 5 flat (one wire is added to power the back up solenoid for disc brakes).

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post #9 of 9 Old 09-17-2021, 09:56 PM
Rapidrob
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My '98GCL trailer light module is on the LEFT side behind the spare tire. It was bad and caused all kinds of problems. I put in a newer type and the trailers ( 2) work perfectly now.
You can use a meter and test each plug socket in the rubber female socket trailer light cable. With NO trailer hooked up, you should see no less than 12 VDC at the plug with the various lights turned on as if the trailer was hooked up, and ignition on. The blinkers will run up to 12 VDC and many of the cheap meters may noy be fast enough to give you the true total voltage. don't think this measurement is bad,it most likely is the meter. If the meter has MAX / MIN setting use it! If you see no voltage, or lower than 12 VDC on any plug socket, the module is bad.

'Nam Vet
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