I need help with truck trailer wiring/circuit please

[Samckitt]

Today I bought a light that plugs into the big 7 plug connector on my 03 Silverado that has a light in it that comes on when the truck is in reverse. I couldn't get the f'ing thing to work. So I got the meter out & did some checking. Long story short, without the big load on it from the light I have 12+ volts at the connector, as soon as the light is on it & get 0 volts. As soon as the light is removed from the circuit, the meter goes back up to 12+ volts. What the heck is going on? Why no voltage when there is a load?

[MrQuick]

test it with a test light, see it the circuit lights it up. Might have a bad connection that will not pass the amperage.

[Samckitt]

I checked it with a meter & had 12 volts. I had a gismo I used last year that has a trailer plug on it & powers 2 fog lights & I made a bracket to mount it in the reese hitch to come on when in reverse. So I put the meter in place of one of the lights & with it alone there is 12 volts, as soon as I connect one of the lights, volts drops to 0, & light does NOT come on, as soon as I remove the light, voltage goes back up to 12 volts. Tried it with 3 different lights & 2 different trailer plugs get the same results....?

[OHCbird]

Where are you measuring the 12vdc? If you're on the input side to the bulb, it sounds like the filament is shorted. OR- you have a very bad connection somewhere 'upstream' that can't handle the load of even the light.
'
I had a blade-fuse connector that was barely touching, so it would show 12vdc w/o a load, then once anything was plugged in- NADA. Check you connections.

[OHCbird]

Reading the reply you just posted sounds like you have a dead bulb that is dragging down the circuit, but not enough to blow the fuse...

You did fuse the light circuit, didn't you?

[Samckitt]

I tried 3 different bulbs & 2 different trailer wiring connectors. Not fused.

[bucks69]

This might be the problem. If you dont have a dedicated ground to vehicle it wont work. My truck will not work with a Trailer unless I have the dedicated wire to the appropriiate ground. Just a sugg!!!

[MrQuick]

agreed. My trailers the same way for safety issues.

your voltmeter only draws about an amp if that. Not enough to find a faulty connection or volts supply. Sometimes even a test light dosen't draw enough.

[MonzaRacer]

OK so you have reverse lights running to your trailer plug? HMMM dont remember that on diagram from work.
Ill try to look up my old hoppy cardboard from my trailer plug.

[hoosiergta]

scott I agree w69camdoover we had this problem on a 2000 silverado.As you know I'm in the area and would love to help if this doesn't work.

[Samckitt]

The plug has a negative ground terminal in it. HoosierGTA, where do you live? I may take you up on the offer to help.

[GetMore]

A meter doesn't give enough load to test a circuit like that. As mentioned, it draws less than 1A. You lights draw at least 70, perhaps 110A. Make sure you have enough of a ground to take that current and see what size fuse is on that circuit.

A "power probe" is better for testing circuits like this. It is connected to the battery + and - and then you touch the probe to a wire and it'll tell you if you have a connection and when you push the switch you can supply power (+ or -) to the wire. So, if you were to put the probe into the wire powering the lights and apply + power and they don't light up, then you know there is some other issue.

[OHCbird]

When testing voltage cicuits, most multimeters draw milliamps, not amps. That is why it is possible to see the 12vdc w/o a load. As for your reverse lights, they'd probably only draw 7-10 amps at best, not 70-110. You'd be blowing most of your main feed breakers or fuses if that was the case.

Even though it has the ground plug/wire, have you checked it for full continuity? You should have a less-than 2-3 Ohm connection from that wire to the battery ground. Anything more than that, and you have a corrosion or bad connection problem.

[Samckitt]

I haven't looked. Any idea where the ground terminal routes to? Does it go clear to the front of the truck somewhere, or directly to the frame near the trailer plug?
Thanks

[hoosiergta]

The plug has a negative ground terminal in it. HoosierGTA, where do you live? I may take you up on the offer to help.I live between Greenfield& New Palistine. Also I'm up at 75th and sr37 for work m-f .Have you ran a ground wire directly from the frame to the trailer the is what we did on Bro-in-laws truck.I'm in the white pages If you want to catch me quick.Dan Barlow

[OHCbird]

IRT ground wire, all I can say is even if it runs up to the front, I'd ground it to the chassis with a GOOD connection. You can never have enough good grounds!

[hoosiergta]

hey scott did you fix it?

[Samckitt]

I haven't had a chance to look into it yet.

[hoosiergta]

No problem . Hey I was up at 116th in between Keystone and I69 today christmas shopping today . Proably should have given you a holler but this was really my first day shopping.

[Samckitt]

Where were you shopping between I-69 & Keystone on 116? Target? That is not far at all from here.

[hoosiergta]

Well I messed up there. Whole story went to the funeral home friend that used to work at Best lock(where I do ) his dad passed away.they have lived up there long time(Shipman) and that was just north of
116th.We live east ofMount Comfort/ Olio rd. and five miles south of Washington /U.S. 40 .So we went up Olio and accross 116 to Allison to roberts and something funeral home.Than down to 96th to HH Greg bought oldest girl a stereo and than accoss to Walmart .Than down 69 to SR 52 thru New Palistine than home. I know you didn't ask for all that but typing makes my brain work.

[hoosiergta]

I wrote this once hit quick reply saw it there went to the garage to work on the v8 astro project cam e back in and it was gone. So..................went up Mount comfort / Olio to 116th ,over to Allisonville up to funeral home . Roberts and ? .Friend that used to work w/me at Best lock, his dad passed on.(Shipman ) Then came back down Allisonville to Shell at 96th .Then went accross 96th to HH Greg then to good old walmart .Then down 69 to 465 to SR52 thru New palistine then up 200w about a half mile.

[hoosiergta]

now it's back..........oh I see it went to page #2..........

[MonzaRacer]

OK so your all wrong about the multimeter and why it goes to zero.
Now let me set this right IF its a name brand auto/electronics grade DMM (Digital multi Meter) it SHOULD have around 1 to 2 mega ohm(uh thats millions of ohms resistance )and even higher depending on speca nad how you set it.
Now as for it doing anything in the circuit it doesnt it reads the voltage at that terminal(I am assuming your touching the black probe to chassis ground and the red to the circuit your trying to test.
OK so what your actually doing is a variation of a voltage drop test.
This is helpfull to check your battery terminal or connections in a circuits voltage transmission capabilities. and when the light is connected you are completing the circuit as it sounds liek a ground controled circuit. If your truch has seperate light bulbe for turn and brake lights it comes with a 3 way to 2 way trailer converter.
Most trailers have old style integrated turn/stop(ie use same bulb filament)lights but if your truck doesnt it has a box that will send proper power to other circuits, usually by the fact the converter is powered and your truck light signal simply turn on and off certain circuits.
Now if you open the circuit it completes the voltage circuit to through the meter and not the actual circuit. Light on, the converter control grounds the circuit.
I just printed this post out and I will try to dig up some wiring diagrams.