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Old 05-04-2014, 03:27 PM   #1
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Weird tail light problem. REDBEAR HELP!

I've got a weird problem with my LED bus tail/stop lights. With the engine on and headlights on my rear brake lights are always on except for the center stop lamp which works normally.

Troubleshooting results:

Engine off, headlights off - brake lights work normally.
Engine off, headlights on - tail lights & brake lights work normally.
Engine on, headlights off - brake light works normally.
Engine on, headlights on - tail light is bright, no difference when brake pressed.

I checked all the grounds I could find then checked for voltage on the brake light wire:

Engine off, headlights off - .04v un-pressed, battery voltage pressed (brake lights normal)
Engine off, headlights on - .04v un-pressed, battery voltage pressed (tail and brake lights normal)
Engine on, headlights off - 2.25v un-pressed, battery voltage pressed (brake light normal)
Engine on, headlights on - 2.50v un-pressed, battery voltage pressed (brake light on with no pedal press)

I pulled one of the tail lights (a Dialight 40 Series LED tail light) and tried connecting it directly to the battery and it worked properly. Next, I wired a standard two filament stop/brake light bulb into the circuit and it worked fine in all situations.

What I'm getting out of this info is that the extra .25v on the brake light wire with the headlights on is enough to trigger the LED brake light.

Here are the options I can think of at the moment:

1) Hunt down the source of the voltage on the brake wire (the proper and likely the most nasty option).

2) Knock down the voltage somehow (a resistor or a really long wire run?).

3) Replace the LED arrays with incandescent bulbs.

Anyone have a better idea?
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Old 05-04-2014, 03:46 PM   #2
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Re: Weird tail light problem. REDBEAR HELP!

Quickest, easiest fix IMO would be to wire everything to the center lamp that is working properly.

Unless that one is a incandescent is why is still works properly.

Brake lights are the most easy to rewire from scratch. Start a wire from the brake light switch, run it to the rear brake lights, add grounds, Done.

If it's still doing it, run power directly from the battery through a fuse to the brake light switch.

Brake light switch is mounted under the dash where the brake peddle lever makes contact with it.

Nat
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Old 05-04-2014, 09:19 PM   #3
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Re: Weird tail light problem. REDBEAR HELP!

Thanx for the suggestion Nat.

I had a look at the three lights and they're all on the same circuit. My guess is that the center light has a higher trigger voltage than the other two.

I have some 12v relays that I stripped out when I originally re-wired Roachie. If the relay activates at over the 2.50 volts on the brake light wire it *should* do the trick.

Fingers crossed
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Old 05-04-2014, 09:34 PM   #4
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Re: Weird tail light problem. REDBEAR HELP!

Roach, are the directional lights using the same lamp as the brake lights? If so, are they affected in any way? Also, is the center stop light on its own circuit or is it tied to the stop lamps? Just curious. I'm sure that the use of a couple of relays will solve your problem. The led lamps need an inline resistor to function properly. Some come with one built in and others don't. "Somewhere in the USA" knows a lot about these things and would be a good one to ask. Good luck, Jack
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Old 05-05-2014, 01:32 AM   #5
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Re: Weird tail light problem. REDBEAR HELP!

You should not see voltage on the line without the brake being pressed.

I may not be right, but my first guess is a bad battery ground to the chassis. Most vehicle batteries have two grounds. The main ground is to the engine for the starter and the alternator charge. There is also either a ground braid from the engine to the chassis, or a second wire from the battery to the sheet metal for the power return of all the lights and accessories.

My step-daughter once had a Cavalier that had an intermittent braid from the engine to the body. When the resistance went up, the factory anti-theft thought the car was being hot-wired and would lock out the starter. I ran a piece of #6 wire from the battery ground terminal to a quarter-inch sheet metal screw in a fresh hole drilled in the sheet metal and the problem stopped. I grounded to the welded unibody, and not a bolt-on part like an outer fender that might be insulated by paint.

My clue was that the line on your bus went higher when the alternator was charging, and turning on the headlights made it even higher. More current drawn through an elevated resistance would create a greater voltage drop. If the engine block was the zero reference, the added voltage through the headlight bulbs would raise the voltage at the top of the resistance to the chassis.

An extra battery ground wire, or cleaning up the grounds you have can't hurt, and might solve your problem.
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Old 05-05-2014, 10:53 AM   #6
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Re: Weird tail light problem. REDBEAR HELP!

Thanx for the replies guys!

I just finished installing the relay and the lights now work normally. It's a work around and not a proper fix so I'll keep looking for the problem.

Jack - The turn signals are separate lights on this bus and even though they are also LED they work fine. The problem lights only do stop/tail duty.

Redbear - I had cleaned up the battery bank & engine grounds when I first got the bus but I'll have another look. At least I know where they are. I learned the hard way to look at the grounds first but in this case was only looking for grounds on the wiring harness itself. Your insight on the alternator/engine ground makes sense.

Assuming my grounds are found to be good, will using a relay to trigger the brake lights be a problem down the road? I'm using one of those 4 pin Bosch style relays and wonder if the repeated cycling will wear it out quickly.
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Old 05-05-2014, 11:50 AM   #7
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Re: Weird tail light problem. REDBEAR HELP!

Roach, if thats a 30 amp relay it will out last both of us
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Old 05-05-2014, 01:02 PM   #8
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Re: Weird tail light problem. REDBEAR HELP!

Roach, that Bosch relay will probably outlast the life of the bus
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Old 05-05-2014, 03:04 PM   #9
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Re: Weird tail light problem. REDBEAR HELP!

Great info guys!!!!

As usual, Skoolie is the Information Destination.
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