Tail light voltage drop

[cragenj3]

1992 f250 supercab long bed 2wd 7.3, manual

I haven't compared this number to my front turns but my rear turns only have 6v in the socket or at any point in the wiring, down the frame, up until it reaches the front of the cab, where I stopped following it..because it doesn't seem to matter, because..

6v is weird, right.
My bulbs are running/turn/brake (dual filaments...running is a fillament, and stop/turn is a filament) anyway, 6v is weird, right, but my back driver turn/brake works. My back passenger turn/brake doesn't work. Same bulbs, good bulb won't work in the bad socket. Socket tests as having 6v...but suddenly tests as 0, anywhere along the circuit, when a bulb is connected. Replaced the socket, issue remained.

Tested with known working bulb, still nada...The running light filament of the bulb works; but the brake/turn filament won't work, even a known good bulb won't work, a new socket didn't change anything, it all still reads 6v, which is weird, but the back driver works on 6v just fine.. the back passenger reads 6v but doesn't work, and if you actually put the bulb in then it suddenly reads 0v all the way up the frame rail.

Figured the capacitor in my flasher relay was dying, hence low voltage/no voltage, but 2 different types of new flasher relays didn't have any effect.

I don't have any idea where to go from here; I'm about 10 more minutes away from running 20ft of wire from the working front socket straight back to the rear socket. Someone save me the hour and $20 please..or at least solve the puzzle so I can know it is solveable rather than some sort of opposite-miracle.

 

[IDIBOBS]

Is it the same with stop, hazard and turn? Did you check all your grounds. Bad grounds cause issues. If all else seams ok I’d look at the turn signal cam. Brakes, hazards and turn all go through that switch.

 

[cragenj3]

I did check all the fuses; I had a reverse light fuse out but that's a seperate bulb that doesn't matter.

There was a fuse marked trailer right turn, but it was a good fuse.

The lever assembly on the steering column, you mean? The cam... I mean the front right works and both lefts work...so the turn lever itself id imagine is fine, I don't know how many terminals it's got in it.. but it sends 6v flashing to the bad rear until it's got a bulb in it.. I didn't check brake voltage to the rear, just know it also doesn't work.

Theres no real diagram on the ground location... Brown is running, O/g or maybe it's G/o (one is left one is right) is turn/brake, and black at the pigtail is ground...but it doesn't leave the harness before it reaches the cab, so it's like my turns aren't actually chassis ground, the ground wire goes all the way to the engine bay somewhere; didn't chase it that far because back left works on 6v, weird that back right won't work on 6v.

Don't know why it's 6v, either; could theoretically be a ground issue, just not sure where to find the ground, guess I have to chase wires between the firewall and the engine...probably better off just hot wiring the bad rear turn off the good front turn.

Still be nice to solve the puzzle; I'm not gonna rush to hotwire turn signal, haha.

 

[cragenj3]

Oh, hazard the bad filament doesn't work... Turn/brake/hazard is one filament and running is the other filament.

So, the turn lever on the column shouldn't be the issue since hazards and brake to that bulb also don't work. And it shouldn't be a ground, because running filament uses the same ground but works.

But perhaps brakes and hazard do go through the turn signal switch..since the bulb as 3wire dual filament, with ground, running, and brake/hazard/turn, I suppose brake/hazard/turn, as one wire, might all go through the turn signal on the column...

 

[IDIBOBS]

All brake, turn and hazard go through the turn cam in the colum. It will cause many issues just like this. The front turn/hazard is totally different circuit from the rear due to the brake circuit. I’d bet money if all the grounds and flashers are good it’s the turn cam.

 

[cragenj3]

I'll LYK tomorrow if you were right; new lever assembly was $25 online.

 

[Rdnck84_03]

I have had a similar issue with my 84 f150 for the last couple years, I believe it is a turnsignal cam issue. My right brake light will only work if the turnsignal is on, doesn't matter which direction but it has to be flashing for that brake light to work.

I seldom drive it on the road so have never gotten overly excited about trouble shooting it. The issue started at the same time or shortly after replacing the turnsignal cam so I can only assume that it is connected.

Grounds can also cause a lot of weird issues.

James

 

[jim x 3]

Cragjen,

Don't obsess about the 6V. You were making that measurement without a load (bulb out) and with a high impedance voltmeter (probably a multimeter on DCV). With the load (bulb in) you got 0V, showing you the electrical system somewhere upstream can't provide enuf power to light the bulb.

You should troubleshoot methodically using a proper wiring diagram for the circuit, testing for 12V power at various points in the tail/turn circuit (from 12V at battery thru multiple switches to the rear socket to ground. You can start at obvious and easy to get to measuring locations. Use a low impedance testing means - like a grounded test probe with a 12V lamp.

If you want to shotgun the problem, replacing the turn signal switch on the column is a good choice. There's a lot going on in there.
Make sure you have good grounds. Pay attention to IDIBOBS, he seems to give good technical advice.

Regards,
Jim x 3

 

[franklin2]

In 1992, the main ground for the rear lighting is called G100. It's location G100 (Except 4.9L/6-300): LH Front Of Engine Compartment, On Upper Radiator Support

 

[IDIBRONCO]

In 1992, the main ground for the rear lighting is called G100. It's location G100 (Except 4.9L/6-300): LH Front Of Engine Compartment, On Upper Radiator Support

Why am I not surprised? Of course, you could tap into the ground wire toward the back of the truck and run another ground too.

 

[franklin2]

Why am I not surprised? Of course, you could tap into the ground wire toward the back of the truck and run another ground too.

Nothing wrong with adding grounds. As long as you scrape the paint off. You could add one to the frame in the back, but don't count on the frame being a good ground unless you go up front and put a jumper from the frame to the battery negative.

The factory purposely grounded the frame sometimes, but of course these trucks are getting old and crusty, so sometimes it's easier to just make a new ground path.

 

[DaveBen]

I ran something like this for a ground wire

https://www.amazon.com/QuQuyi-Braided-Expandable-Screening-Shielded/dp/B09TKTR374/ref=pd_lpo_d_sccl_2/134-0044930-3890269?pd_rd_w=hRUQb&content-id=amzn1.sym.4c8c52db-06f8-4e42-8e56-912796f2ea6c&pf_rd_p=4c8c52db-06f8-4e42-8e56-912796f2ea6c&pf_rd_r=8PPDBTS5SEWM44DP5YRP&pd_rd_wg=beNUu&pd_rd_r=1533bb8a-7158-4fbd-9fb4-20c9a84208b5&pd_rd_i=B09TKTR374&th=1

 

[IDIBRONCO]

Nothing wrong with that ground. Those used to used on A LOT of vehicles. I've seen them on old tractors too.

 

[onetonjohn]

Nothing wrong with adding grounds. As long as you scrape the paint off. You could add one to the frame in the back, but don't count on the frame being a good ground unless you go up front and put a jumper from the frame to the battery negative.

The factory purposely grounded the frame sometimes, but of course these trucks are getting old and crusty, so sometimes it's easier to just make a new ground path.

My passenger side ground has a pigtail. The main wire goes to the stud on the front of the engine. The smaller wire to the frame by the fender well. I don't know if that's something PO did, or if that's factory. I like it.

 

[franklin2]

My passenger side ground has a pigtail. The main wire goes to the stud on the front of the engine. The smaller wire to the frame by the fender well. I don't know if that's something PO did, or if that's factory. I like it.

You have to do something like that. They ran the big grounds from the batteries to the engine block, but the engine, transmission, and even the leaf springs are mounted in rubber mounts.