Need some help with electrical, wonky brake/turn signals/fuel gauge

[willem445]

Hi guys,

Really hoping for some advice or pointers.

So back in 2015 I posted this thread: https://67-72chevytrucks.com/vboard/...d.php?t=679423 in which my last reply was:

"Figured I would post an update to this thread. Went home over Easter break and finally got some time to work on my truck. I went through and wire brushed most all of the obvious grounds I could find. Also pulled the steering wheel and replaced the turn signal switch as well as the brake light switch. some ground wires under the dash were pretty bad so I replaced all of those. I know have working brake lights, hazard lights, right signal and left signal lights. My only issue is when I turn on my right signal the left side lights blink as well but very dim. After thinking, I remembered that I did not polish up the ground for the one side rear taillights so Im assuming that ground is what is causing this. Thanks everyone for the help. One step closer to getting my truck on the road."

I never found the reason for why it was doing that, but one day it just stopped, all lights started working just fine, I did more work on the truck after I made that post but I never connected a particular thing fixing the issue. Now yesterday I discovered the EXACT SAME THING happening!

Symptoms:

While driving the truck home yesterday I noticed

• Pressing the brake would pull my fuel gauge to E, after releasing, it returned to normal
• Turning on headlights resulted in same fuel gauge behavior
• My light bar and interior lights did NOT cause the same behavior

When returning home, with truck turned OFF and ignition ON I noticed:

• Fuel gauge behavior NOT happening
• The exterior lights behavior as described above IS happening

The exact behavior of the lights is as follows:

• Headlights low/bright function NORMAL
• Left turn signals function NORMAL
• Hazards (turn signal lights flashing) function correct but license plate light also flashed (not sure if normal)
• Right turn signal results in passenger parking/turn signal light flashing, passenger rear turn signal flashing, license plate light flashing, driver rear turn signal flashing, driver turn signal/side marker flashing dimly. NOTE: The passenger side marker does NOT flash.

I have so far tried the following:

• Replace passenger side marker light that wasn't lighting. NO CHANGE.
• Clean up main BAT to ALT mount ground & frame ground. NO CHANGE.
• Voltage drop with DMM from BAT NEG to all ground points (frame, engine, cab, radiator). All measured below ~50mV. Ohm to all grounds is also nearly 0 ohms.
• Unplug and replug gauge cluster (I did notice one gnd trace was lifted and bent up not making good contact). NO CHANGE.
• Unplug gauge cluster completely. NO CHANGE.
• Jumper the parking brake assembly ground distribution to a known good ground. NO CHANGE.
• Checked lighting harness wiring & interior dash wiring, no obvious issues.

From what I understand, these symptoms generally indicate a bad ground somewhere (ie. the current is instead flowing through the lighting harness instead of flowing back to BAT NEG due to resistance).

I'm flustered because I've basically rewired and addressed every ground point on this truck earlier this year and I never noticed this behavior. The fuel gauge issue is pretty obvious so I'm sure I would have noticed it prior to yesterday.

Unfortunately I've worked on a bunch of stuff in the last couple months so it's really difficult to pinpoint something I would have changed that reintroduced this issue. Couple potential things I changed that might be suspicious:

• Rebuilt gauge cluster, cleaned up PCB contacts and replaced all bulbs. (Note, I left some bulbs depopulated as I've replaced those gauges with mechanical gauges)
• Installed interior firewall insulation and carpet. Working under the dash is difficult so it's possible I bumped some wiring.
• Installed inner fender on passenger side. It's possible the inner fender would have pulled on some of the passenger side lighting harness. (unlikely, I also have all wiring in wire loom and zip tied up so it's not hanging down)
• Repainted my parking brake assembly. This one is my biggest bet. I ground down to bare metal where the ground distribution bolts on but I forgot to grind some bare metal for the assembly to ground to the cab so it's likely paint on paint. I did try to jumper a ground to somewhere else but that didn't change anything.

Any tips on diagnosis or things I could isolate to reduce the problem would be greatly appreciated!

[willem445]

I ended up figuring this out. It was the passenger side rear dual filament bulb. The turn signal voltage was somehow bleeding into the night running lights (brown wire). Replacing bumb fixed it. Of course ripped apart most of my dash before tracing it back there.

I'll make a separate post with some of the lighting circuit technical details I learned.