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Issues with LED bulbs in taillights


Mar 4, 2019
Vancouver Canada
1997 Jeep TJ Sahara

I've been having some issues with my lights. First my headlights were dim and randomly would illuminate to their normal brightness while driving. This happened a couple times. One time while driving home, an alarm went off in the cab. I had no idea what it was. It would go off when I turned the key to electrics but when i turned the key further to ignition it would stop only while the key was in the ignition position then come back on when running. Restarted the vehicle a few times and it went away. Hasn't come back since.
However since then now my tail lights won't go on. The brake lights and the turning signals work fine. Headlights haven't had an issue since. I have had no hyperflashing with any lights.

So I recently wired in a new third brake light that's circular and has LEDs. I shortly afterwards put two LED bulbs in the top slot of each tail light. (There's 3 bulbs in each tail light. I replaced the top one). So the LEDs themselves are all working.

I guess I need a relay?


LJ Enthusiast, Retired USAF Weapons Loader
Supporting Member
Sep 25, 2018
South Carolina
First my headlights were dim and randomly would illuminate to their normal brightness while driving. This happened a couple times.
This sounds like your alternator is going out. When your lights were dim, it wasn't charging and you were just running off the battery. Then, for whatever reason, it started charging and you lights brightened up.

Were your gauges dim at the same time and then brighten up too when the headlights did?

I had a Chevy Beretta years ago that did that to me. I removed the alternator and had it tested at a parts store. It tested bad, new alternator installed and no more problems.
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Steel City 06

TJ Addict
Mar 12, 2019
Pittsburgh, PA
Agreed that the alternator is a likely culprit.

If the lights "randomly get brighter" during acceleration or higher rpm cruising, it's likely the alternator.

Do you have a voltmeter? Next time the lights dim or the alarm goes off, measure the voltage across the battery terminals. It should be about 13.6-14.6 volts with the engine running. Less than that indicates a likely charging system issue.
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