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Old 10-25-2024, 09:46 AM   #1
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Brake light wire getting 8vdc all the time!

On my 1994 Fleetwood Jamboree Rallye Ford E350 when the lights are on as I like to ride, the brake lights BARELY show over the taillights.

Got new LED Brake/Taillights and still BARELY can see brake lights and still cannot see them at all when the sun is on the back. (BTW, had to change flasher to electronic version because of fast flash) Anyways... today tested the voltage, Brakelights are 8.4vdc all the time! And probably goes to 12vdc when brakes applied (no 3rd party help at the moment). When driving w/o lights on, brakes and turn signals show great!

I've got a work around by adding tail lights separately and current tail/brakelight assembly both being used for brake lights.

Getting to the question... any hints on where to look for the issue?

Thanks, Phil
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Old 10-25-2024, 10:27 AM   #2
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I would first look for corrosion in and around the brake light sockets. Both of them. Then look for rust around the ground connection if you can find it. I'd run a temp jumper wire from the ground wire to a nearby chassis part for testing.
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Old 10-25-2024, 11:41 AM   #3
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Let me ammend the above to say All the time when engine is running 8.4vdc.
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Old 10-25-2024, 12:36 PM   #4
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Well, let's establish what measurements you've already done.

What is the voltage elsewhere in the RV?
What is the voltage across both sets of batts with the engine running?
What is the voltage from the Batts + terminal to nearby clean chassis metal?

If it's already been localized to the brake lights, do you have a trailer connector? What's the voltage there? (I ask about that because there are nearby splices that connect to the RVs wiring that could be the problem).
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Old 10-25-2024, 01:52 PM   #5
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Tomorrow, gotta go, grandson arrived and hungry... plus we have some other things scheduled for today...

THANKS for your help!
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Old 10-26-2024, 09:20 AM   #6
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I think I know the problem, the headlight switch! One of the old style pull types, pull 1/2 way for running lights and all the way for headlights.

I've had a problem with it since I got it 3 years ago, the dashboard lights would go out as I increase/decreased brightness. I was thinking it was just an old potentiometer with some corroded spots. But yesterday as was doing the testing, it started 'crunching' badly and I thought 'there's another hundred dollar problem'. It's only 30 years old, I wish my 65 year old hip and fingers cost a hundred dollars to repair or even a thousand to get rid the arthritis pain.

As I was running through a million voltage checks yesterday: everything off, just ignition turned on, running, running with lights on, on shore power... all those on the brakelights, taillights, and turnsignals on three battery systems (chassis, generator and separate house batteries) , I noticed I had NO voltage at all on the taillight wires at any time! I figured I must have had a bad crimp on a buttsplice somewhere and I'll find it today.

After we got through with the grandpa/grandson thing about midnight (a star party for amateur astronomers), I surfed the ford forums and found several examples very similar to my own, or worse, no taillights at all. Several of those were remedied with a new headlight switch.

I always thought I had taillights, because I always run with my lights on and the 'tailights' looked 'on' due to the 8vdc that was on the brakelights as soon as the ignition switch turned on!

It only took 3 hours of surfing between 2a til 5a. Yeah my eyelids are dragging right now, but I'm excited to get a new switch and get it installed.

I'm a little embarrassed to say I'm a life long electronics tech, navy ET1, but it NEVER occurred to me that it would be a problem like this, given the symptoms I had. So hopefully I'll have an extra set of tailights to put on the shelf.

Thank you so much for suggesting the exhaustive voltage checks! And until I read up on this issue, I was going to expand on those checks today to include the front running light. I've NEVER heard of a problem like this, I was thinking it would be a bad chip somewhere, but this thing doesn't have the fancy computers like the ones these days, it has a very early version of electronic ignition called a TFI module (Thick Film Ignition).

I'll let you know if it actually fixes the problem, I surely hope so.

Anyways THANKS again for the help!
Phil
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Old 10-26-2024, 10:29 AM   #7
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As a life long ET/Eng myself, much time spent as a test engineer on battery powered fork lifts, I've found that some chores in debugging just make me go, "Ugg, not fun". Then I have the same issues with my knees with mild arthritis, hips can bark at times too. I can barely get up off the ground without help these days. Crawling around under things like the dash or the chassis isn't the most pleasant of tasks and I don't look forward to having to do it. Luckily, my hands are free of arthritis (but did require 3 surgeries for trigger fingers).

Anyway, I would think that if it was the headlight switch, you'd also find low voltage at other places, like the front of the RV. But if it has given problems in the past, changing it out would help overall. That might fix some other problem.

Just looked up the '94 Bounder's schematic for the Freightliner chassis and the COMMON power source comes from the headlight switch. It's common to the front and rear, and running lights. So if one set has 8V, they all should as they are all tied together.

The individual control wires for stop, turn, running, come from the turn signal switch. Which gets the power for individual functions from the hazard relay, the flash relay, etc. Those all go to the fuse panel. So I guess in '94, they didn't always go through relays.

So I'd inspect the turn signal switch, maybe spray contact spray inside it for testing.
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Old 10-30-2024, 06:08 PM   #8
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I have not found the problem!

I pulled the flasher and brakelight switch and it's still there!

I had already ordered a set of brake / taillights with reverse light before I found this stray 8v on the brakelight circuit.

I went ahead and installed those as just taillights / reverse lights and on the other set of LED lights I have the brake AND taillights wired to show just brake lights.

Yeah, not what I wanted to do, but it has had this problem for the 2 years I've had the RV and at least now they can see my brakelights come on.

Taillights and reverse on the left, brakelights on the right.

TTYL, Phil
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Old 10-30-2024, 06:17 PM   #9
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But did you test the turn signal switches? According to the other similar posts I read, it's often the problem. Or did I miss something in your reports?
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Old 10-30-2024, 06:42 PM   #10
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When things don't add up, such as getting what appears to be some cross feed, I like to go to work with a simple 12 volt test light. One with a simple incandesant light bulb so it will draw a bit of current.

I suspect you have a bad ground issue. Finding a decent ground can be difficult so here is a simple trick to use while testing.

Grab an extension cord and plug it into one of the outlets in the RV. Then using an alligator clip or some way to connect to the ground hole of the extension cord. With this setup you can make some reliable tests.

First, test your setup by going to your batteries and see how bright the tester is.

Then go to your tail lights see how bright it gets while checking the hot side of your lights.

IF the test light is bright, find the ground side of your light and put the test light on that. IF you have a bad ground, the test light may glow dimly, all the while the existing RV light may show a bit brighter.

I understand you measured 8.5 volts with a meter. But not knowing what you were using for a ground it can be misleading.

The test light can be used as a (signal injector). Just take a scrap piece of wire and connect it to your test light alligator clip, then connect the other end to the positive of your battery.

Now when you poke the test light onto any ground, it should light up strongly.
You can now use it to inject voltage as well.
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Old 10-31-2024, 08:45 PM   #11
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SLIDE UNDER and Start w/ good VISUAL on all GROUND WIRES/ SCREWS. Even some 3-yo vehicles have corroded/ broken braided ground straps; MY recent bad ground a 24yo screw popped the head and left wire dangling in the air.
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Old 11-01-2024, 11:25 AM   #12
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<<I'm a life long electronics tech, navy ET1, but it NEVER occurred to me that it would be a problem like this,...>> and me, an old, out of practice AE, (used to be good) when first bought rig, spent a week chasing wires until finding a blown fuse that I swear I checked x3 times previous. But had already replaced HL and Multifunction for other issues. In my defense, tech life turned into 50+yrs management vs hands on. Another, was replacing Boost solenoid, disconnected both batteries and still had voltage, decided needed to be "another days project" when wife pointed out.... "still plugged in" :-( :-) LOL
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Old 11-01-2024, 04:25 PM   #13
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so.... bypass the pot in the headlight switch. Still a problem?

Every time I've had issues with LED lights its because somewhere along the way one of the sockets is wired backwards. Start removing bulbs util the problem goes away.

And grounds? Yeah, that's a possibility.
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