Chasing down electrical issues

[C15_Sierra]

Bear with me, I've got some explaining to do...

Before I got my battery hooked up in my truck, I'd been advised to test the circuits with a smaller hobby battery (still 12v, just less amperage) so that I don't set fire to anything when I go to hook the big battery up.

Did that the first time, everything seemed to work except the headlights, etc, and the flasher didn't work. I chalked it up to the fact that I didn't have the 12v input to the lights hooked up (b/c they run independent of the ign. switch), and I couldn't really explain why the flasher wasn't working (the lights lit up, just didn't flash, same with hazards).

Second time I hooked up the small battery, the dash indicators for the turn signals lit up irregardless of whether they were supposed to be on or off. This is with the ign switch ON by the way. Off, everything functions as normal. That's when I started getting the 'uh oh' feeling that I have a short. Additionally, when I had the headlights on, flicking on the turn signals would switch on the seatbelt warning light and buzzer. Weird. Still, I didn't find anything grounds that were suspect

Fast forward to today, I get the big battery charged and hooked up. Now, there is no power going to the cab when I switch on the ignition. I have headlights/taillights/parklights, but no brake lights. Also, when the headlights are switched on, the pass. side turn indicator on the dash goes on and is dim. Drivers side does not come on. All ext. lights are on, except for the R front marker light. Coincidence, maybe the bulb went out, but I remember it working before. Anyway, still no power to the cab with the IGN on. I checked the power splits, grounds under the hood, etc., everything has good voltage. The wire going to the ign. switch sees the same voltage as the one running the headlights, so its beyond me what could be the cause. I don't see why my ign switch would work last week and not work this week...

So, for anyone who's still reading, any suggestions where to start? I'm out of ideas...

[C15_Sierra]

Cliff notes:

1) Tested circuits with small battery, first run was success, second run produced some weird behavior with dash lights
2) Hooked up big battery, no power to cab/dash lights/switched +12v power to run engine.
3) Headlights/taillights all good, no brake lights or hazards, dash illumination comes on along with dim pass. side turn indicator light
4) Signs point to bad grounds, but I don't know where to start/all grounds I can find are good
5) Need help!

Thanks!

[rs72z]

Do you have both of the grounds for the front light harness hooked up?
Cab ground to engine?

[travis401]

definatly got some bad/disconnected grounds. the flasher not working could just be a bad flasher unit though.

[rs72z]

If the front light harness grounds aren't hooked up it will do all of these things.

[hatzie]

Light grounds on the radiator support.

[C15_Sierra]

I have all 3 light grounds on the radiator support hooked up.

No cab to engine ground hooked up, where does the ground wire come from the cab? Long story short, I bought the truck 9 mos. ago, then got relocated to a new job and took the truck with me, and I'm now getting to finishing up the engine swap, so excuse the stupid questions...

Should I just run a new ground from the engine to the cab?

[john_p]

sorta works on low current.. may not have enough push to charge the flasher 100 percent... hit the same wires with full current and symptoms change... you are looking for a bundle of pinched wires,, either a spot on the firewall or if you pulled the bed, could be an old trailer hook up.. pressure cross, enough resistance to almost work low power,, not enough going to ground to smoke or even blow a fuse.. a bad switch can cause that... i have seen a wet fuse box do that.......

[C15_Sierra]

^^^ Ah, good call on the old trailer wiring. I DID pull the bed months ago, but I was able to drive the truck around with the bed off, so I dunno. I will check on that...

[john_p]

trouble shooting? disconnect the clock/radio if you have one.. do a current test leaving the battery,, remember that is a series test... if your drawing any current, start pulling fuses until that stops.. pull one. put it back in, pull the next.. might be able to at least isolate.

if you have zero current. much harder. pull all the fuses then read resistance between them..

remember most problems can be SEEN, no magic tools, no sheepskin, almost no experience.. my grand dad used to say, put your hands in your pockets, shut your mouth, open your eyes... 60 years later that still works...

[C15_Sierra]

You're right about that...I initially missed the light grounds, and that's how I found them

One peculiarity...I rewired the engine fuse/relay block according to LT1Swap, and noticed once I installed it that I have continuity between +12v switched and ground (12 and 6 o clock positions). I don't know much about wiring (but, always ready to learn), this seems counterintuitive to me, and might be the source of my problems. But, if for some reason this is how its supposed to be, then maybe that'll point me in the right direction

http://lt1swap.com/fuseblock_obd2port.html

[hatzie]

Originally Posted by C15_Sierra

I have all 3 light grounds on the radiator support hooked up.

No cab to engine ground hooked up, where does the ground wire come from the cab? Long story short, I bought the truck 9 mos. ago, then got relocated to a new job and took the truck with me, and I'm now getting to finishing up the engine swap, so excuse the stupid questions...

Should I just run a new ground from the engine to the cab?

Bellhousing bolt to cab is where I ran mine. I'd add a frame to block strap too.

I had a 79 c20 do what you're describing. It looked like the rad support grounds were OK. They were actually green powder inside a PVC jacket that was still attached to the ring terminals strain relief. I could be all wet but it's worth a look.

[C15_Sierra]

I checked for continuity between the cab-engine block, as well as cab-battery negative post. All good Continued, looking for loose grounds, found nothing.

I then checked the fuse block, I have +12v at the fuse block, so that's powered up. What I discovered I DON'T have, is +12v to either of the red wires going into the ignition switch. Now, looking at the wiring diagrams, I am now confused because those wires go straight from the cab to the battery, and I have them hooked up to the stock junction block, which I re-located to be next to my engine fuse block under the hood on the drivers inner fender. I checked those before, I have +12v on either side of the terminal, so I am confused as to why I'm not getting +12v at EITHER of the red wires leading to the ign. switch (one on the black connector, one on the white connector). Any ideas??

[john_p]

does that wire have a fuse-able link? it does not look like a fuse, a lump in the wire. your right that wire needs to stay hot. did you pull fuses and test for crosses? those are hard to find and can cause strange problems? has the bed been off? check that harness that runs to the back tail lights..

[rs72z]

Originally Posted by john_p

does that wire have a fuse-able link? it does not look like a fuse, a lump in the wire. your right that wire needs to stay hot. did you pull fuses and test for crosses? those are hard to find and can cause strange problems? has the bed been off? check that harness that runs to the back tail lights..

You beat me to it.
The fusible link looks just like regular wire but will have a connection on either end. If you have a short it will melt the wire inside the jacket and sometimes you can't see the problem. Start checking for voltage from the junction toward the fuse block. I bet you will find the problem. Then look for the cause of the burnt fusiblelink.

[C15_Sierra]

Stupid me for not thinking before posting.

The reason why I had 12v at the junction block and not in the cab is because....the crimp came apart! The butt connector I used to hook up the cab power came off and it was loomed, I didn't notice until I undid the loom. Hooked it up, voila, everything powered up (had to shut it off quickly b/c the fuel pump is running w/ no fuel)

I swear, whenever I post problems up on the interwebs, I almost always find the answer 5 mins later myself

THINK!

Thanks for bearing with an electrical idiot

[john_p]

best thing about the keyboard.. may not be the answer, but it gives you a reason to stop for a minute.. glad you found it.. that one will not burn you again