Trailer Light Problem

He has a 2003 Isuzu Rodeo that he is hooking up to.

Someone hooked up the wiring using one of those adapters that taps into your brake lights and uses a 7 pole plug and it has never worked properly.He does not have trailer brakes or backup lights so only the turn signals , running lights and brake lights are hooked up.

I plugged the trailer into my Chevy truck and everything worked fine but if we hook up to the Isuzu the left brake light and left turn signal do not work . The left tail light does.It is a 2 filament bulb .

He used to have very week left brake light.

I have checked current at the plug and there is juice there when he puts on the brakes so it should light the bulb but it does not. It was a little week though (around 8-10 volts ).Of course , I did not pay any attention to the rest to see what they checked .

Someone suggested using a heavy duty flasher on the car to boost the output but we cannot find one through NAPA or Isuzu.

Any thoughts ?

Thanks !
Ford Man

Left brake/turn signal light socket is not grounded well to the ground wire in the trailer wiring harness. But it is grounded to the steel frame of the trailer possibly through the light fixture mounting bolts.

Chevy truck hitch ball is well grounded to truck body so light works when hooked to Chevy getting it's ground through the hitch. Isuzu hitch ball may not be well grounded to the truck body so light doesn't work.

Charles

• 7 pin plug

We do have a good ground and all of the other lights work fine with the trailer plugged in.

I did turn on the right turn signal and take a jumper wire from the car to the trailer and I could make the right turn signal blink and could also make the running lights blink by jumping over to different posts on the trailer plug. The strange thing is that I could not make the left brake/turn light blink like that.

I will have to check the voltage a little closer and see what I can find out .

Thanks Again !
Ford Man

I used one wire to get a ground and then the other wire was used to connect between the terminals from one plug to the other.

I hooked the wire to the right turn terminal on the vehicle and touched it to the right turn terminal on the trailer and the right tail light started blinking. I the touched it to the tail/running light terminal and the tail lights and running lights started blinking. When I touched it to the left turn/left brake light terminal nothing happened.It should have made the left side blink.

This was a pretty crude test but it showed me that there is a problem with the left side wiring on the trailer. We swapped bulbs from left to right and that made no difference. What really puzzles me is that the thing works perfectly when hooked to my Chevy!

Thanks for the input but unfortunately , I am still stumped !

Thanks Again !
Ford MAn

Yes, the Isuzu has the amber turn signal light and someone installed one of those adapter boxes for the lights. They used scotchlocks to hook into the taillight wiring.

I am not too familiar with those boxes as none of my vehicles has ever needed one.

Thanks for all of the help !

Ford Man

"I did not hook up the trailer ball when testing with the Chevy , just used the plugs."

AHA! Perhaps the wiring for the non-working light is shorted to the frame? Depending on how it's wired, not having the tongue connected to the vehicle may make a difference.