So I’ve got a 12v relay wired up to run my inside lights mounted under my seats and also have a set of F-150led lights wired up under the cab. Everything was working fine. But the undercab lights stopped working. I checked the relay, it’s fine. Puts out 12v as it should. When I test the wire coming off the relay, I get 12v, but when I hook up the under cab lights, the voltage drops to 6-7V. I don’t get the same drop when I test the lights inside the truck, only when I test the lights underneath. I also tried hooking up some other 12v lights, and they work just fine and no voltage drop either. But... when I hook the under cab lights up directly to a 12v battery, they work just fine. Anyone have any insight as to what may be going on?

 

Classical electrical theory would say voltage drop is due to high line resistance.......wiring is too small for load, or relay is too, pinched wire, poor connector/connection??

But led's dont pull much power. So its a bit surprising.

Follow wiring back to source power checking voltage. Add up all the wiring distances, determine wire gauge, and add up total amps. Then look up the expected voltage drop on of the many websites out there. See if wiring is too small.

 

Wire is good. Voltage out at relay is same as voltage at end of wire. I did some rechecking and it looks like whatever I hookup is now causing a voltage drop. Voltage start at 12 and drops to 7.5 when I hook something up. Tried switching out relays and get the same thing.

 

Are the relay contacts rated for the amperage you're pulling through it?

 

Yes. They’re 30/40A

 

Depending on the relay being used, how old it is, if it's been abused, etc., I have seen times when the relay has arced over internally (usually due to overloading) which leads to a high-resistance connection during normal operation... and thus substantial voltage drop. What's the nominal current load through the relay with everything on? If you don't know, hook them up straight to the battery then measure the current with an amp clamp.

If you have a spare, new, never used relay, I'd swap it.

 

How about giving us the details on your relay connections.....

If the lights are working fine wired directly to a 12v source, about the only thing is can be is the relay, or how it is wired.

--Bo

 

I would not hook anything up directly to battery with out a inline fuse. Put a amp clamp on circuit to see if it is pulling to many amps. That may be why the circuit is dropping voltage

 

Check voltage before and after relay amps is constant on a DC circuit but voltage is not.

 

Sorry for the lack of response. I got tired of chasing my tail. The relay had been installed by me a while ago and everything was working fine. I tapped the bed lights in the 3rd brake light for my trigger. Foot well and running board lights were all functioning fine as they should. I did try swapping relays. I had a set of 5. Each relay yielded the same results. Then I realized the only thing that’s happened to the truck since I installed the relay: I got it lifted. Now... I don’t know what that’s has to do with anything. They didn’t do any electrical at all just suspension. I talked to the shop and they’re going to track it down for me and also wire up some rock lights and fix what’s not working. The only thing I didn’t do was re-wire the harness the relay plugs into. Anyways, I appreciate all the help. I got tired of trying and realized I’d rather spend time with the fam instead of checking for some lights that the dumb truck should’ve come with in the first place . What’s funny is a came from a fully loaded pathfinder. That thing had so many more options than my xlt, but the xlt has a much much higher msrp. Oh well. Still love the truck. Thanks again guys.