Loosing taillight voltage with trailer?

[Bryan]

the vehicle was not running

[Seangie]

Bryan -  you'll have to do some good old chasing wires here.  Pull out your manual if you have one.  Some of the older bus gurus might be able to give you a better idea where to start.  Check to see if there are any numbers or labels on the wires.  If you have a toner you can tone out the wires.  Might be best to disconnect your batteries while you do this.

Keep it going...you'll figure it out!

-Sean

Fulltiming somewhere in the USA
1984 Eagle 10S
www.herdofturtles.org

[gumpy]

... when I check the voltage on the wires running to the bus lights, they are only 9 volts, doesn't matter if it's markers, blinkers, breaks, etc, it's all 9 volts. To eliminate a ground issue, we run a wire directly to negative terminal of the battery to the ground at the lights, still only 9 volts, we then ran a wire directly from positive side of battery to the lights and got the full 12.5 volts, and the lights got a lot brighter. This means somewhere between the battery and my tail lights, I'm loosing voltage.

Corroded terminals and/or wire somewhere between battery and lights. Could be bad contacts in a relay. Could be a bad breaker. Could be the switch.

You'll need to figure out where the feed wire is connected to the battery, and where it goes. Start at the beginning and check the voltage at every
junction along the way. Could be the main battery cable is corroded right at the battery terminal. Not uncommon. Could be corrosion in a wiring panel.
Also not uncommon as many were installed in the engine compartment where there's persistent road spray from the tires and radiator fans.

[Bryan]

We have traced the wire back to the main junction box above the engine and have found a little voltage drop. We ran a jump wire from the switch all the way back to the junction box and got some increased voltage. Is this am ok thing to do? I didn't know if there were relays or anything I'm bypassing that could cause an issue.

[gumpy]

We ran a jump wire from the switch all the way back to the junction box and got some increased voltage. Is this am ok thing to do? I didn't know if there were relays or anything I'm bypassing that could cause an issue.

Are you saying you're now running the tail and marker lights through the light switch?  Maybe ok, depending on the rating of the switch, but personally, for the number of lights you have
going, and the potential current draw, I'd probably run that wire to a 30 amp relay or two, fed directly from the battery with 10 gauge wire.

I don't know whether your bus had relays or not. I don't think so, but I'm not a GM guy.

[Lin]

What is the difference in the voltage between the two?  Did this solve the trailer light issue?

[Bryan]

About 2 volts. We haven't hooked the trailer back up yet. We are concentrating on getting the voltage right first then going install trailer wiring harness again. Working on this today so hope to let you know how it goes. We have located the factory wiring and adding 12 gauge to it for markers, breaks, and signals.