Chassis 12v Short

[supra_rulz]

alright, where to begin...

car's been driving fine for years now and then i let it sit for a few weeks, install an oil cooler, took it out for a spin, noticed i was only getting about 12.3v while driving. thought the alternator was going out so i took it to baxter and o'reilly to have it tested. one said bad, one said good. he tested it 5 times so i'm gonna go with the alternator is fine. re-installed it and noticed the alternator fuse was blown....but the car still ran. (i know, no idea how the hell that happened) . replaced the fuse, started, 13.8~14.2v yaaaay its fixed. next day driving down 405 and BOOM everything shuts down. pulled off and replaced the fuse on the side of the road, when i go to connect the ground (after positive) sparks galore! towed it home and diagnostics.

i've checked the ground from the battery to the chassis and block, nothing touching or rubbing. i've checked the wire that comes off the alternator, no breaks or rub-through. i know the culprit is in the fuse box now. i traced the lines and so far discovered the short is either through the RTR fuse, alternator junction, or dome light. the test light lights up the brightest when the dome fuse is inserted then dimmer for alternator fuse and barely lit for the rtr fuse. i've gone in the fuse box inside the driver side and pulled each connector thing out (the ones that weren't fused to the case) and the test light didn't turn off so my guess is that the short lies between the fuse box inside fuse box (or if i'm really unlucky, one of the ones i couldn't pull out). Inspecting under the fender and as much as i can see elsewhere nothings rubbing, rubbed through or otherwise.

Am i missing something here? or do i just have to rip out the whole body harness :'(

 

[Nick M]

pulled off and replaced the fuse on the side of the road, when i go to connect the ground (after positive) sparks galore! towed it home and diagnostics....i've checked the ground from the battery to the chassis and block, nothing touching or rubbing.

You said one place told you the alternator was bad. What checks did he use, and why did you go with the person that gave you the answer you wanted to hear?

http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?S=CH&P=10

This is just an example of a couple of checks I would do. Not that it helps you on the side of the road. If you have big sparks when you hook up the ground, current is flowing.

 

[supra_rulz]

Right, Baxter said it was bad but he told me that he did not know how to test it properly because something-something. Sounded fishy, so i took it to the o'reilly that i'm more failure with and he said it failed the first time but he cleaned the terminal of heavy corrosion and ran it four more times and found it was producing all the right stuff. I'm assuming they both used whatever tester is available to them.

Current flowing through the ground is not good...?

 

[jetjock]

Unfortunately that comment along with others reveal a distinct lack of electrical knowledge that will likely prevent you from resolving this problem yourself. Seek qualified assistance...

 

[supra_rulz]

ok, so that was worded wrong. yes there is current in the lines that complete a circuit, i'm just saying the ground wire shouldn't spark like the way it did. The chassis has 12v going through it which im pretty sure isnt supposed to happen.

if i could afford qualified assistance i would have. No need to rub it in that i dont know how to fix the problem. Thats why i'm here.

 

[Nick M]

yes there is current in the lines that complete a circuit, i'm just saying the ground wire shouldn't spark like the way it did. The chassis has 12v going through it which im pretty sure isnt supposed to happen.

If you have 12v going directly to ground, you have current flowing. The resistance is just very small, which is why current is very high (ohms law 101) and it sparked. You also said it wasn't rubbing. Recheck all of it.

 

[suprarx7nut]

In my wiring problems on an mr2 I had I found the best diagnostic tool was a digital multimeter. You can buy one cheap at Home Depot or get a used fluke or ex tech on craigslist.

Id measure current with the dmm at the battery. Then pull one fuse at a time. Record the current (likely above 200ma if you have a bad short). That will get you the circuit with more certainty than a test light.

Then trace those circuits and unplug stuff that is in said circuit.

It'll be painful but you should find something helpful from that exercise.

 

[supra_rulz]

Progress Report!

1. I'm torn whether to thank you for making me do this myself and learn something, or call you all bastards for shaking your head and watching me sink. lol
2. I hate working on cars. Most of you have been doing this longer than i've been alive....how?

So far i've traced the short back to multiple locations, its a miracle this thing even ran to begin with. I had a 2v leak coming from my abs computer even though i dont have working abs. Had a 12v leak from the door control unit in the driver door (which explains the weird unpredictable nature of locking) and have two more that i'm tracking down now. One of them is in the dome light circuit and the other i have yet to look up.

 

[suprarx7nut]

Progress Report!

1. I'm torn whether to thank you for making me do this myself and learn something, or call you all bastards for shaking your head and watching me sink. lol
2. I hate working on cars. Most of you have been doing this longer than i've been alive....how?

So far i've traced the short back to multiple locations, its a miracle this thing even ran to begin with. I had a 2v leak coming from my abs computer even though i dont have working abs. Had a 12v leak from the door control unit in the driver door (which explains the weird unpredictable nature of locking) and have two more that i'm tracking down now. One of them is in the dome light circuit and the other i have yet to look up.

1. A couple veterans on this site will offer little to no help at times and then at other times they'll give you better assistance than you could pay for. Just gotta roll the dice and see what you get. The better your involvement and troubleshooting, the better help you will receive.
2. I'm only 12 years into "working on cars", but I can say the better you get and the less time stressed you are, the more enjoyable it becomes. Some people get pissed trying to solve hard problems and then see little relief after they fix it; seeing only wasted time and effort in their wake. Others, like me, may get frustrated from time to time, but get a great sense of accomplishment and reward when a difficult project is done. If you're person A in this example you may find a mk3 Supra is not your best car to own.

If you want to own a MK3 you better learn to like tinkering with it. It's a great car, but it's 22-27 years old and likely to have been abused at some point. It'll take some work to enjoy.

If you unplug the items above, do your current draw (aka short) problems go away (or lessen)?

 

[supra_rulz]

well either way, what has to get fixed has to get done. and i just get pissed all the time, but youre right; after the fix, it feels soooo good.

yes. went away completely. so i'm really excited. have to buy new stuff but oh well. gotta pay to play.

i do just want to clarify one fix though.

http://www.cygnusx1.net/Supra/Library/TEWD/MK3/manual.aspx?S=Main&P=68

is the circuit i'm working on now and i've unplugged the integration relay, 1C, 1L, 1F, 1D plugs. the short still exists, so that means the junction box #1 has an internal short?