Multimeter.

Herd Thinners · November 8, 2015

This may not be the correct spot for this question but here goes. I have a multimeter and yes I'm pretty green soooo my question is.

I'm testing the 7 pin tow connections on the back of my 2015 Sierra. When I check the positive and negative the meter says approx 12 volts. Then I hit the back up prong and negative I get some crazy 200-400 reading. Any ideas.

I'm trouble shooting why my back up light harness where I tap into the back up wire isn't working and I'm having trouble with the trouble shooting hahaha.

kevin_nj · November 8, 2015

Digital VM? Set to auto range? If yes to both and the 200-400 you referenced bounces around and is mV, then that means it's an open circuit, meaning no power to the supply pin.

The above is based on the assumption that you have a good ground, which it seems like you do, since you get 12V on the other pin.

Locking the DVM into a range can limit the confusion. Or just pick up a simple test light.

Herd Thinners · November 9, 2015

It is a digital VM. Was set to volts. I tested a few things making sure the reading I was getting we're correct. It's a Klein tool from Home Depot. Quick question. Power would run to it when the truck is set to reverse correct.

So the range means open circuit.

My harness is hooked up to the battery. It has a relay. The switch works which means the ground is solid. The lights don't come on when I place it in reverse. They sent me a new relay still the same issue. Now I want to test the wire used when placing it in reverse to see if it is bad. Not sure how to do that. I know to place the red into the relay where the reverse wire is but the black either on the reverse pin which will cause a zero I think or ground it and see. I'm talking out of my behind on this. It's above my pay grade lol.

kevin_nj · November 9, 2015

Yes, reverse pin on the 7 pin should have 12V when transmission is placed in reverse.

So you've got a relay installed. 12V constant, ground, trigger (reverse signal) and load. Are you sure they are plugged into the correct pins on the relay?

I don't see what year your truck is, perhaps there's an issue with the 7 pin tow connector? If you can't get a reading there, then you do, unless it is an error on your part.

Herd Thinners · November 9, 2015

Truck is a 2015 GMC Sierra. When I test the pins. Positive and negative on the back of the truck I get 12 volts.

Not the case when testing the reverse.

The one question do I have to put the truck in reverse to test the reverse pin. I would guess yes.

To answer your question yes on the relay. I sent pics back to the place I bought it to confirm.

I'm sure the error is me lol. Maybe.

What I want to do is test the wire from the relay to make sure the connection is being made from within. My only theory is the wire from the relay is bad or not connected proper. Any ideas. Thanks for helping me by the way.

kevin_nj · November 9, 2015

Yes, transmission must be in reverse for that pin to be hot, otherwise reverse lights on trailer would always be on.

Not sure which wire you're after.

If you mean testing a wire itself, that's called checking continuity. You would need to access both ends of the wire, set the DVM to resistance setting and you're basically creating a loop. Resistance should be 0 (or very low), most DVM will have an audio option, beeps when good.

Another option is to put one end of the wire on a known good 12v supply and then check on the other end. Just make sure neither end of the wire touches ground while you're doing this.

You mentioned a switch, did you put a switch in the cab so you can have the lights manually go on or auto go on with reverse lights, or are you looking to have them only work with the reverse lights? If the later, depending on the lights, a relay is probably overkill. If the former, there is a bit more involved.

Herd Thinners · November 9, 2015

Not sure which wire you're after.

The wire I'm after is the wire off the relay.

If you mean testing a wire itself, that's called checking continuity. You would need to access both ends of the wire, set the DVM to resistance setting and you're basically creating a loop. Resistance should be 0 (or very low), most DVM will have an audio option, beeps when good.

For this I can touch the on end of the pig tail wire meant to connect to the reverse lights inside the relay and the other on the end of the wire correct. Question does the truck need to be placed in reverse making the relay switch to the pig tail wire. I think I answered my own question.

You mentioned a switch, did you put a switch in the cab so you can have the lights manually go on or auto go on with reverse lights.

Switch meaning on and off in the cab all the time or on when in reverse hence the relay.

kevin_nj · November 9, 2015

So you want to be able to turn the lights on at anytime, or have them automatically turn on when truck is in reverse? To do so, you need a 3 way switch ON-OFF-ON. There is a little bit more involved in the wiring for that also.

Herd Thinners · November 9, 2015

The relay and harness I have is set up for this. It runs power and ground to the switch. Then this wires run to the relay. From the relay there is one wire that connects to the reverse. When the truck is put into reverse it should activate the relay and send power to the lights. When the switch is flipped it should open the relay to activate the lights. Both senerios the other circuit would be closed within the relay. I think that is how it works.

kevin_nj · November 9, 2015

That doesn't sound quite right to me, but I'd have to see it first hand. Sorry.

Eddie 70 · December 13, 2015

I found a blown fuse on my 7 pin connector for the reverse lights. The fuse at the under hood fuse block and is a 10 amp fuse.

I don't know if it holds true for the 7 pin connector, but in order for the trucks stock backup lights to come on, my truck has to be running. Just putting the truck in reverse with the emergency brake on did not allow the lights to come on. Truck running, the back up lights come on.