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Old 03-17-2021, 07:11 PM   #1
Mini-Skoolie
 
Join Date: Feb 2016
Location: Tennessee
Posts: 22
Year: 89
Chassis: Chev
Engine: Dt 8.2
Rated Cap: 78, 72
89 Thomas Built

I have an 89 Thomas Built on a Chevy Chassis that is having electrical issues. Problem is everything is a jungled mess and Iím having a hard time figuring out what is what.

If anyone has any advice it would be greatly appreciated.

I have tried to get a manual but so far no luck.

Iím attaching the control panel.

Thanks!
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Old 03-25-2021, 10:36 AM   #2
Almost There
 
Join Date: Jan 2017
Location: USA
Posts: 95
Year: 1995
Coachwork: Thomas
Engine: Cat 3116
Rated Cap: 84
If your Thomas is anything like mine, a 40ft RE, you will have three “main” harnesses.
1. Body wiring
2. Engine wiring
3. School wiring

This makes it a bit easier to sort everything out.
After years of monkeying around by well intentioned School district maintenance, they can get a bit jumbled.

You will absolutely need a digital voltmeter.

If you can go on Google and look at vehicle wiring harness from the era of your bus, you’ll start to see how things are typically wired.

School bus wiring is a bit different with all the interlocks, alarms, flashers, etc... but still straight forward enough to sort out.
The body harness and engine harness are extremely similar in function and layout to GM stuff from back in the day.

Seeing as this is a GM chassis, this shouldn’t be too hard at all.

Best way to do this without a schematic is to carefully trace each wire, visually as much as possible, and then confirm with the DMM.

Thomas had not been all that great about making stuff available to the used market. I was lucky and the main body harness was printed out and put on the electrical access door. Still had to figure everything else out, but I have a couple of decades of messing with GM wiring.
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Old 03-25-2021, 11:22 AM   #3
Bus Crazy
 
Join Date: Oct 2020
Location: Near Flagstaff AZ
Posts: 1,127
Hi BBQ...I may sound like a bit of a broken record...or people will think I own stock in the company... ...but I think every bus owner should have a Power Probe. It would absolutely help you figure out what's what in that mess of wiring. It's essentially the digital voltmeter Klingon mentioned, but with handy functionality that is super helpful.

Touch the probe tip to a terminal and see if you've got power...like a regular voltmeter...but then you can push a button to apply either ground or power (both circuit breaker protected) and see what happens. It's the best thing I've found for testing circuits and lights and wiring.
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Old 03-25-2021, 11:39 AM   #4
Almost There
 
Join Date: Jan 2017
Location: USA
Posts: 95
Year: 1995
Coachwork: Thomas
Engine: Cat 3116
Rated Cap: 84
Hi.
I do like the Power Probe.
With that said, I absolutely do Not recommend one for a novice.
It’s a great way to burn out a circuit or engine sensor if you are not very careful.


No offense to you rossvtayler !!!


Anything with a computer in it will have multiple circuits that are 5v or less running through them.
A Power Probe will make an ECM into a very expensive paperweight!

Only use a DMM on anything that might have a computer.
You probably do not want to use a power probe on an all mechanical engine as a newbie either.

Stick with a DMM for safety’s sake!


I do like a good test light too, but even those can fry an ECM if you aren’t very cautious of what you are probing.

Just use a DMM!

It’s not that hard at all to use a jumper wire to provide temporary power to something, if it’s needed.
A few minutes of reading and some experimenting with the DMM will test/verify/ inform you of anything you might want to know in your bus.

You can get a simple voltage/continuity meter from almost anywhere for less than $30 and while not 100% accurate, they are more than accurate enough for what you’ll be using it for.
Once you learn how it works, you can get a more capable DMM if you think you’ll need it.
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Old 03-25-2021, 12:13 PM   #5
Bus Crazy
 
Join Date: Oct 2020
Location: Near Flagstaff AZ
Posts: 1,127
No offense taken, Klingon! And you're right...I was thinking more for tracing out the lighting and relays and buzzers and interlocks and switches and such. One would definitely not want to push 12V into the terminals of a 16-pin ECM or TCM connector.
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Old 03-25-2021, 02:22 PM   #6
Almost There
 
Join Date: Jan 2017
Location: USA
Posts: 95
Year: 1995
Coachwork: Thomas
Engine: Cat 3116
Rated Cap: 84
Quote:
Originally Posted by rossvtaylor View Post
No offense taken, Klingon! And you're right...I was thinking more for tracing out the lighting and relays and buzzers and interlocks and switches and such. One would definitely not want to push 12V into the terminals of a 16-pin ECM or TCM connector.
A Power Probe does make testing certain circuits much easier, thatís no lie!

Just until you are fairly confident you know what youíre doing, stick with a DMM! LOL
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Old 03-26-2021, 12:15 AM   #7
Mini-Skoolie
 
Join Date: Feb 2016
Location: Tennessee
Posts: 22
Year: 89
Chassis: Chev
Engine: Dt 8.2
Rated Cap: 78, 72
Thanks so much. Your coaching is invaluable.
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