I'm wiring the camper lights on my Fuso FG. Truck has a service body with single turn/brake light. Truck is Japanese so came with separate turn/brake lights so service body has a trailer adapter in the wiring to make the single turn/brake light work. All lights work fine on truck and service body. On camper tailights work fine. Right turn/brake works fine. Left turn/brake doesn't work. On the left turn/brake I have voltage coming thru the wiring (about 5 volts--same as right side and same as voltage on turn/brake light on the service body). If I put the left turn/brake light bulb in the fixture at the camper, voltage at the plug in for the camper drops to 0 but turn signals continue to work fine on the truck and service body. If I remove the camper left turn/brake bulb I can measure the voltage at the fixture, but as soon as I reinstall the bulb, voltage drops to 0. Why does installing the bulb cause the voltage to disappear? I've tried switching bulbs and it doesn't make a difference.
I'm not that great on diagnosing electrical problems and this one has me stumped.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

In two years of trying to teach electronics over the internet I have learned one important lesson: I cannot teach electronics over the internet. However, like the moth drawn to the flame, I keep trying.

Now I cannot solve all the problems you describe but you do ask one specific question I can try to answer. The best way to explain is to use the water analogy. Electricity is electrons flowing through a wire, similar to water flowing through a pipe. Voltage is the pressure. Amperage is the rate of flow (I realize you didn't ask about amperage). You have a situation where you seem to have pressure one minute and then it vanishes mysteriously when you try to draw any current through the bulb. What you have is a "high impedance" connection somewhere "upstream" from the bulb (or a high impedance ground connection "downstream"). High impedance means too much resistance. As a way of explaining, picture a garden hose connected to a faucet, and on the end of the hose you have a sprayer nozzle. You turn on the faucet and let the hose fill up. Now you turn the faucet ALMOST all the way off. You go the the nozzle and just press it a tiny bit and water sprays at high pressure. But when you press the nozzle a lot the pressure dies. You have the same situation in your circuit. Somewhere between the battery and the bulb there is a high resistance (bad) connection. AND IT COULD EASILY BE A BAD GROUND CONNECTION. The electricity doesn't favor positive vs negative. It wants a complete circuit. My advice is leave the bulb in, then start measuring the plus and ground lines all relative to a known good ground. What that means is have the black lead on the frame. Trace the plus side until you find good voltage. If it always reads good the trace the ground side until you find 0 voltage.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

I'll bet that you are using a different ground when probing the taillight bulb receptacle than the one used by the bulb itself.

I ran a ground directly from the bulb fixture to the negative post on the battery and the bulb still does not light. I assume this means that I must have a bad connection somewhere on the positive side.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

This is not too helpful but all I can add is that those turn signal converters will give crazy voltage readings which will be compounded by any grounding troubles.

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-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250

I bet that ground is hot.

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You don't need a weatherman to know which way the wind blows - Robert A. Zimmerman

The fact that I understood this should tell you your efforts are not in vain.

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1984 300TD

If voltage drops to zero when you put on a load there is too much resistance in the circuit like a bad connection or coroded wire. Best way to test circuits like this is with a test light. Spend about $15 and get a test light you can put a large bulb in like an interior light bulb. Using a voltmeter can be missleading. Also using a test light with a really tiny bulb can do the same thing.

Yes ! And I have no idea what is in one of those "converters". The trouble could be there, as KarTec just said. If you have a schematic of the converter I could GUESS what might be going on.

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

I finally had some time to work on this problem again. I used a light bulb to test the circuit in a variety of places. The problem was in the converter. Good amperage coming in, inadequate amperage going out. Replaced the two bulb to one convertor and everything is working fine. Have no idea how those converters work but something was awry inside. Thanks for all the help.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Those converters use diodes to "steer" the voltage from the input circuit to the proper output circuit.

Think of it this way...

If the two Input circuits (Stop/Turn Lights and Running Lights) are used to power one Output circuit (Stop/Turn/Running Light), you have to have something BLOCKING the activated signal from coming back and interferring with the source of the Input circuit's activation...

Diodes let electricity flow in one direction and impede/stop it from coming back to where it came...

Diodes also "open" or "short" - therefore, troubleshooting a black block of epoxy with input/output wires is pretty touchy, to say the least...unless you understand the basics of that type of circuitry, or you're able to read a schematic.

Generally, once that block of black starts acting up...ditch it and either modify the trailer light circuitry to mimic the towing vehicle's own, or just pitch it out and get another one...and if you're traveling great distances, buy one or two extra, just in case the one hooked up as you leave your homestead decides to "vacation" itself...

__________________

M. G. Burg
'10 - Dakota SXT - Daily Ride / ≈ 172.5K
.'76 - 450SLC - 107.024.12 / < .89.20 K
..'77 - 280E - 123.033.12 / > 128.20 K
...'67 - El Camino - 283ci / > 207.00 K
....'75 - Yamaha - 650XS / < 21.00 K
.....'87 - G20 Sportvan / > 206.00 K
......'85 - 4WINNS 160 I.O. / 140hp
.......'74 - Honda CT70 / Real 125
.
“I didn’t really say everything I said.”
. . . . . . . . . . . . . . . . . . ~ Yogi Berra ~

As I understand the converter, it's stopping the brake light circuit from trumping the turn signal circuit so that the turn signal will flash when the brakes are on.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Correct...when the car's circuitry sends the brake and turn signals (two separate signals) to the box, it senses the brake signal and activates both sides...then when the turn signal comes into the box, it senses the correct turn signal, and directs the on/off sequence to the proper side, over-riding the brake signal comand...and thereby allowing the turn signal to operate ALL THE WHILE blocking the correct turn signal information to ride the brake signal buss over to the other side and pulse the signal on the other side...lots of "logic-steering" diodes involved...

It's easier to throw the funky one out and buy new...

Or...

Just wire the trailer to match the lighting signals from the car towing it...

__________________

M. G. Burg
'10 - Dakota SXT - Daily Ride / ≈ 172.5K
.'76 - 450SLC - 107.024.12 / < .89.20 K
..'77 - 280E - 123.033.12 / > 128.20 K
...'67 - El Camino - 283ci / > 207.00 K
....'75 - Yamaha - 650XS / < 21.00 K
.....'87 - G20 Sportvan / > 206.00 K
......'85 - 4WINNS 160 I.O. / 140hp
.......'74 - Honda CT70 / Real 125
.
“I didn’t really say everything I said.”
. . . . . . . . . . . . . . . . . . ~ Yogi Berra ~