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I recently purchased a 1985 F-250 and have been spending the last few weeks bringing it back to life. This forum has been invaluable, I've perused many threads but this time I've hit a dead end and thought I'd try and seek some advice. Here is my situation:
I swapped every single bulb in the truck to LED (except headlights) and I swapped both flashers (hazard and blinkers) to LED compatible ones. I got every single light working except for one strange exception, the rear right tail light. When I pull out the hazard switch, all lights flash but the right rear, its dead. When I pull out the running light/head light switch, all lights function correctly. When I try the blinkers, all blinker lights stay on (solid) except rear right is dead. I am not familiar enough with these old trucks to know if the solid light is normal, is it like the rapid flashing of modern vehicles indicating it senses a bulb out? Also, there was a spliced in trailer plug (hack job) which I thought might have been the problem but now not so sure. I completely cut it out, re soldered the wires back to factory, and no change. Finally, I busted out the voltmeter and compared the left (working) light to the right one. As expected, when the running lights are on, 11-12v which makes sense. When the hazards are on, it fluctuates and is hard to read but the left (working) one spikes around 10-11v or so and the right one (not working) spikes only 5-6v. So it seems like it has power but its low voltage for some reason? I don't really know how to proceed now as I've exhausted every idea and test I could think of.
For troubleshooting it might be worth reinstalling the standard bulbs? Maybe it doesn't matter, but I dunno. Find the junction block connector for the stop and turn signals, study the wiring diagram, probe there with your test light to ground. This will split the circuit in half and hopefully isolate the problem better, i.e. Everything works there, though no juice at the tail, the fault is somewhere along the way back from that point. Keep in mind continuity isn't the same thing as being able to pass current.
Usually grounds & connectors are heavily corroded by now, and any splices especially. When the weird stuff starts happening, it's always flaky grounding. It doesn't take much resistance to cause problems, and the stop and turn lights are long wiring runs.
Tedster9, good points. As previously mentioned, same issue with reg bulbs before LED swap. And you're right, those wire runs are crazy long! It looks like the ground wire terminates behind the dash somewhere if I read the wiring diagrams right. I rewired a bunch of it too, cutting out the splices, and heat shrinking everything. I hoped that would be it but nope.
The other bit I find confusing is that both tail lights (brake and signal) + reverse are all grounded via the same harness. They all join up near the license plate into ONE ground wire. So how can they all work perfectly, including the same socket and bulb when the running lights or brake light is on, but then when the pulse signal through the other input in the socket and bulb, the signal is weak? Ok, maybe its the flasher, except that can't be either because it runs all the other lights and they work. My point is, I don't see what isolates this turn signal from everything else except two things. The (orange) signal wire and the alternate path in the physical socket. Ground is common to everything, flasher is common to everything. Am I mistaken? At this point, my money is on the socket itself... but I dunno.
Tomorrow I will test to voltage of the signal wire before the socket and see if its different.
Ok so the same thing was happening before the LED bulb swap right? If so this would have been when to try and fix it before the LEDs went in.
The lights don't work off turning on & off the ground and why the 1 wire for all the lights for ground. So if the others work the grounding point at fire wall and the wire back to the lights that work is good.
Also if I under stand right it is only the turn / brake light that does not work but the running light does on this side? If so then "I" would think the ground is good at this light but the turn / brake wire (same wire does both) is the issue.
If yours is like my 81 F100 you have a rear tail light wiring harness that goes across the back to both tail lights.
This plugs into the rear harness that runs from the tail light harness to the front and plugs into the main truck harness at the frame where the firewall & floor meet.
If you look from the engine bay side between the brake booster / master you should see a wiring harness come out of the firewall, part goes down to plug into the rear harness the other up along the fender to do the right side head light, turn / park, side marker, etc lights.
I would start at the rear plug for the tail light harness & rear harness. Pull it apart and check it good, clean as needed and check for voltage for all rear lights.
Think you would have 5 wires: 1 running lights, 2 left turn / brake, 3 right turn / brake, 4 back up, 5 ground.
If you have good power there then the issue has to be the tail light harness, if not move to the front plug, main harness / rear harness and do the same.
At this plug you will have a few more wires. Besides the 5 wires above you may have the back up light hot wire to the trany and back out to the rear, if auto maybe NSS wires, wires for the fuel tank(s) sending wires & valve switching wires if duel tanks. I would mainly be looking for left & right turn wires and running light wires at this point., forget the rest.
If still nothing you have to move inside and check the wires in & out of the turn signal switch as that is where the wires to the rear turn / brake lights starts at.
A bad turn signal switch can be a cause as that is where the wires to the rear, turn / brake lights starts at. So maybe start here and work back?
Dave ----
The first question; Do the brake lights work, both sides in the rear? And they are both the same brightness? If so, the rear bulbs and wiring are ok. The turnsignals and hazards use the same wires and the same part of the bulbs as the brake lights do. It's all co-ordinated in the steering column turnsignal switch.
You guys are awesome. I will be checking a bunch of stuff today but I just wanted to clarify, there are only two bulbs in the tail light. The reverse light just has a two inputs (ground and power) but the brake/blinker light has ground and two + inputs like this attached image. I'm sure I'm not telling you anything new, but I just wanted to explain my reasoning, and what I measured was the right pin was a steady 12v when the lights were on, and the left pin fluctuated in voltage when the blinkers were on, just not high enough voltage. Anyway, thanks so much for all the advice and I'll hopefully be reporting back soon with results.
You guys are awesome. I will be checking a bunch of stuff today but I just wanted to clarify, there are only two bulbs in the tail light. The reverse light just has a two inputs (ground and power) but the brake/blinker light has ground and two + inputs like this attached image. I'm sure I'm not telling you anything new, but I just wanted to explain my reasoning, and what I measured was the right pin was a steady 12v when the lights were on, and the left pin fluctuated in voltage when the blinkers were on, just not high enough voltage. Anyway, thanks so much for all the advice and I'll hopefully be reporting back soon with results.
Think of the 1157 bulb as two bulbs in one. Electrically you have 3 bulbs back there.
Well, what a crappy day today, thunderstorms constantly and I don't have a garage so needless to say I didn't get much done. I did however manage to check that the orange signal wire at the first plug (by license plate) has the same low voltage, so I no longer think its the socket. I saw the wire goes all the way down the driver side frame rail, so that will be fun to chase down. At this point, I'm fairly convinced tracing the orange signal wire will lead to the problem area.
Bulbs are fine, sockets are fine, grounds are fine, flashers are fine, just the signal on that orange wire...
Orange wire ends up at the steering column at the turnsignal switch. That orange wire is the turn and the brake wire in one. The turnsignal switch in the column selects whether the orange wire is put on the brake switch (center position) or is put on the flasher for turning. There is a plug at the bottom of the column, you should find the orange wire there.
I don't know if it was mentioned, seems you are already doing it, but always keep the bulb plugged in and the wiring all plugged in when checking the voltage. That keeps a load on the circuit and you will not get false readings.
You can see in the diagram below the orange wire is really a orange with a lightblue stripe. The lightblue might look white after all these years and it fades out.
Orange wire ends up at the steering column at the turnsignal switch. That orange wire is the turn and the brake wire in one. The turnsignal switch in the column selects whether the orange wire is put on the brake switch (center position) or is put on the flasher for turning. There is a plug at the bottom of the column, you should find the orange wire there.
I don't know if it was mentioned, seems you are already doing it, but always keep the bulb plugged in and the wiring all plugged in when checking the voltage. That keeps a load on the circuit and you will not get false readings.
Originally Posted by Franklin2
You can see in the diagram below the orange wire is really a orange with a lightblue stripe. The lightblue might look white after all these years and it fades out.
Dave, that is fantastic information! I have heard of the turn signal switches failing pretty frequently, or at least the cause of issues reading through other posts so maybe that is where it originates, I will have to check there next. I have the entire dash torn apart so access to everything is super easy at this point. Yes the orange wire I refer to does indeed have the light blue stripe.
Edit: I misread the response, thought you said it happened only after you messed with LEDs, LEDs can still contribute to the solid on problem as described below though:
If you don't have the problem with standard bulbs but do with LEDs it is not the wiring. So I skipped a few posts.
LEDs are polarized meaning they only function properly in one direction unlike your old filament bulbs.
Also make sure you got the right bulb. 1157(two prong) 1156(single prong) I has a similar issue on my front turn signals because the book I quoted said 1156 and the prong was wide enough to contact the prongs in the socket and it stayed constant on.
Quick inspection showed two contacts in the plug and I swapped for the correct bulb.
Also if your led bulb polarity does not match the socket polarity then it will do funky things. Take the bulb out and flip it.
If none of that works, and you put old filament bulbs back in and they work, trash your LEDs and get new ones.
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