Have I gone off path: tail-,marker-,gauge lights not working, blown #9 fuse, brown wire grounded etc

TW97C1500 · February 4, 2019

Tail-, marker, gauge lights didn't come on one night when I started truck. IIRC, I don't think they came on then went out. I'm pretty sure that when I started it, the gauge lights didn't come on and that was the tip off that something was wrong.

Long story: #9 fuse was blown (a 20A), blew 2 replacements after changing 2 cruddy, rusty bulbs, disconnected tail light harness, brown wire to lights was good, but brown wire along frame to cab showed continuity to ground. Disconnected rear light harness plug at bottom of firewall at framerail (PIA), brown wire to rear was good, brown wire up to firewall bulkhead showed continuity to ground. Checked at front marker lights and brown wire showed continuity to ground. Disconnected front light harness plug below master cylinder (PIA), brown wire out to lights was good, brown wire to cab showed continuity to ground.

With light switch removed, brown wire at switch plug showed continuity to ground. Went through EXTREME PIA removing firewall bulkhead connector, then steering column, then lowered dash to get access to the wire bundle. The brown wire from the light switch is crimped to other brown wires. One of them goes to back of fuse block at #24 fuse (a 25A). I cut the wire splice crimp out, so now none of the brown wires to the bulkhead connector, and thus to the lights, show continuity to ground. The brown wire to the gauge lights doesn't either.

Fuse #24 is connected to #6, #12, and #18 by I guess it's a "buss bar" in the fuse block. #12 is the one that has the continuity to ground, the Heater & A/C circuit. I don't remember if I disconned a plug or if I cut another crimp, but the wires to the Heater - A/C control panel no longer show continuity. (They did at first, before I isolated them from the rest of #12 circuit).

Anyway, I got over the to right end of the dash and removed wiring, cut another crimp out, etc. and there are 3 brown wires that still show continuity to ground. These go to the HVAC box, to the "Recirculation Door Motor", "Front Temperature Door Motor, Front", and "Front Door Mode Motor". Whatever motor it is that you can see, on the rear-facing wall of the HVAC box, the wire to it does not have continuity to ground with the plug disconnected. However, with the plug connected to the motor, it does show continuity. Now I'm pissed 'cause I'm thinking "Yeah, the motors might be supposed to show continuity, maybe that's what I've been chasing and not a short in this circuit." I didn't disconn the other two at the motors because they are on the front face and I couldn't get to them. I don't know if I can reach them from the bottom or if the box has to come out (which I really do not want to do).

So, if that's the case, that following continuity to ground is on #12 circuit is not proving anything, where the heck is the short, none of the other brown wires are showing continuity now? I think I've covered them all.

Even if I wasted my time following the brown wire to the HVAC motors, it was only a mild annoyance compared to the major PIA's of getting the wires exposed. Now I'm wondering if I've wasted my time altogether. I mean "#9 fuse blowing means brown wire circuit is shorted to ground", right? One of the brown wire circuits anyway. And they're all connected it seems.

I've found a couple of places where the brown wire's insulation was wearing, but it wasn't all the way through. At least I've averted future problems maybe. And I put the blend door back in that had fallen out, so now I'll have full flow at the windshield when on Defrost, so this lowering the dash project hasn't been a total waste. I said when I got to the point of having to remove the bulkhead connector and lower the dash that it would be much easier to run a new brown wire off the light switch, drill a hole in the firewall (or find an existing one), and splice my new light power wire into the existing brown wires just ahead of the connector plugs. That would at least get me tail-,marker, and license plate lights.

Although, after this thought that maybe the #12 circuit brown wires show continuity due to the motors and not a short, it makes me wonder if the light switch is bad. I removed it and immediately checked the brown wire for continuity to ground and when it showed, I figured it meant that circuit was shorted.

Any thoughts or answers?

97 C1500 extcab, 5.7 auto 2WD

Gooie · February 14, 2019

Are you using a continuity tester or a 12 volt tester? Once a bulb is plugged in the voltage passes through the filimant and goes back to ground.

TW97C1500 · February 14, 2019

Multimeter.

I've disconnected most, and maybe all, of the harness connections on that circuit. I've even cut apart all of the wire splits where one brown wire is spliced to 3 or 4 more. I think all of the lights are disconnected.

Your point is the reason I posted this though. When I found all these splits and common junctions, I struck me that the "continuity to ground" I was chasing was not necessarily the short that blows the fuse. Especially when I found brown wires to the HVAC control motors, it made me wonder if the continuity was through the motor.

I have since found another wire split I missed previously, and that had crossed me up. I was on the wrong brown wire. A brown wire went into that rat's nest wiring bundle and split, with one branch having a lot of extra wire folded up. I didn't remove all of the tape, but instead pulled on the known end of the wire and could only see one other wire moving on the other side of the taped mass. The branch will the extra folded wire wasn't moving, so I missed it.

I later got all the tape of that big knot of wiring and found the split. Now, what I was looking at makes a little more sense with what I've read. Some people have reported that their #9 circuit short was in the door wiring. I was checking the passenger side today, but got too much of a runny nose from the cool air and began to get stiff from leaning over the lowered down dash for a while. Hopefully I can get to the driver's door tomorrow, then I need to go buy some plastic door panel retainers for all the ones that broke (yes, I was using the proper tool).

TW97C1500 · February 15, 2019

I didn't have all the bulbs disconnected. The illumination bulb in the power mirror control switch in the driver's door panel was giving the path to ground.

The tail lights and license plate lights work when hooked up to a battery as a stand-alone circuit, with a 20 amp fuse that didn't blow. The front marker lights do too. I've got too much of a mess right now to try rigging up the gauge lights. I'm going to start re-wrapping the wiring bundle and get a lot of the rat's nest out of the way, then maybe I can try it.

With the brown wiring under the dash all separated into individual circuits and disconnected, none of them are showing continuity to ground, so I guess the short is in a switch, motor, or light. Or it could be wire-to-wire but not in contact now that I have moved them. I haven't yet checked every inch of brown wire for being worn through. Some of it is just too dang hard to get to. People have posted that they had a short where the wires bend between the body and door and I was checking with the doors open, so maybe I broke the short-circuit temporarily. On second thought, I watched the #9 fuse blow 3 or 4 times with the driver's door open of course.