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Old 01-26-2021, 06:08 PM   #1
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Join Date: Nov 2020
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Year: 2004
Coachwork: Thomas
Chassis: Saf-T-Liner HDX
Engine: Cat 3126
Rated Cap: 90
Thomas HDX Dead in the Water

So I was mucking about in the wiring and I fear that I have killed my bus. I have been kicking myself for not relabeling wires that the old labels crumbled off of, and not restarting the bus after every thing that I did, but I honestly didn't think anything I did would land me in such hot water. Lesson learned. So as it stands my bus won't do anything. Previously I had to crank it from the rear because I had yet to disable the safety interlock and had removed the emergency exits, but now I can't even do that.

When I probe the prongs at the key I have no power there at all, and no power to the instrument cluster. My understanding is that the three fuse boxes left of the drivers seat are all the "bus stuff" (warning lights, safety interlock, door controls,) and the box by the stairs is essentially the "truck" that the bus sits on. I would be fine getting rid of all the bus stuff, but I think there's a signal of some kind needed to allow power to the instrument cluster and the ignition key.

Can anyone help shed some light on this, or perhaps provide some useful diagrams? The school district we fight the bus from hot wired it all to hell, but diagrams are at least a jumping off point. I have layouts for the drivers side boxes (TBB #1, 2, 3) but not the one by the stairs (TBB#4).

Thank you in advance for any help you can give.

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Old 01-26-2021, 07:18 PM   #2
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My apologies if this is way too basic but have you checked the batteries themselves? If you have no voltage anywhere it could be just be dead batteries. I donít have any Thomas experience but the batteries on these things die quickly if theyíre not run as the school bus light flashers and monitors seem to draw a lot of juice just sitting there.
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Old 01-26-2021, 07:29 PM   #3
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Fair question, but the batteries are good and I have power at all the boards.
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Old 01-26-2021, 07:52 PM   #4
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Hi NOLA_Storyteller (fun name...you'll have to share the origin story sometime). Sorry to hear about the troubles - those gremlins can be hard to chase. Do you have a Power Probe? Not just a multimeter, but an actual Power Probe (or one of the good clones). It will let you check circuits but one of the most useful aspects is that it lets you send power (or a ground) with the touch of a button. It makes chasing these gremlins so much easier, because you can touch a probe to a terminal on the key switch, for example, then apply power and see if your dash lights up...or what happens. And it's circuit breaker protected, so you won't fry anything. As often as I recommend those, you'd think I sell them...I don't but I think every bus owner should have one.
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Old 01-26-2021, 08:01 PM   #5
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Hi rossvtaylor. I do not, Iíd never heard of one, but just looked it up and I know what my next purchase is! Thanks you! IDK if that will be the magic bullet, but I can see that becoming an invaluable tool in my box.

BTW the name comes from my old job. I was a tour guide in New Orleans for 16 years until the pandemic killed the industry. My wife and I moved in with her folks in Ohio to work on the bus full time while she wraps up her PhD online.
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Old 01-26-2021, 08:14 PM   #6
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Right on! I had this image of a New Orleans theme/character guide in my mind when I saw the name. What an interesting job...

Anyway, the Power Probe comes in different flavors (three of them, I think?) and the cheapest one does everything you'd want. I hate for you to have to wait for that, to fix your bus, but that's where my ability to help here ends. I don't know the HDX. I'll be watching, though, with fingers crossed for you.
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Old 01-30-2021, 08:00 AM   #7
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Good name. It made me click. That and the HDX part.

So Iím in NOLA and own a 2001 HDX 3126b. AND I just bought a power probe from Amazon last week and am learning how to use it.

I will send some electrical diagrams for my bus that should be similar to yours. You can call Thomas in High Point, give them your VIN for more accurate drawings.

Two years ago I had the exact same symptoms. I had screwed up my interlock system. While doing the ol screwdriver jump the starter relay thing, my bus went dead like yours. After much head scratching I hired a professional. I had blown the fuse inline that powers the ECU.

If you google 3126 ecu fuse, you will see rv forums full of ppl looking for this fuse. We never found it either.
My guy ran a wire from the ignition switch the back to the proper pin on ECU. He also included a 10 amp inline fuse. Clackaty clack she started right up.

So then I had to run a new ignition wire front to back to power the rear box. If I remember correctly, HDXs need to have the switch on in front before you can hold the engine door button and press start in the rear.

The diagrams Iím sending have pics of the wiring pins, and the box in the back. You could probe the input pin with a power probe and power it up. Have someone look to see if your dash initiates,or just listen for the ABS system to make its funny noises. If so thatís your problem. Either find the fuse or run a new wire.
If you run a new wire, you have bypassed the neutral safety switch on the tranny. So if you find the fuse let me know where it is.

Or......it could be something completely different!

This is where I always say I AM NOT A MECHANIC OR AN ELECTRICIAN. I have seen a few things in the last 2.5 years and I relay what Iíve seen.

Peace
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Old 01-30-2021, 08:09 AM   #8
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https://drive.google.com/drive/folde...SAhlAQy_gyI8_9

I have a printed copy of the ecu diagram in my bus in will be there this am and take a pic.
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Old 01-30-2021, 10:12 AM   #9
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Meathead you are a life saver!!! I managed to solve my dead in the water problem; after two days of questioning everything I learned at the Marine Corps Communications and Electronics School, and my basic understanding of reality, I figured out that when my FIL in an attempt to be healthy had disconnected the battery, the small ignition wire had fallen back into the chassis and I hadnít noticed it was missing. A forehead slapped, but a relieving outcome. Iíve given up on bypassing the interlock, proceeded with a wire delete, and am going to hardwire a push button start to the rear panel. The diagrams you provided will be invaluable for that and future projects. The shops up here in Ohio have been very slow to respond and reluctant to provide schematics. Thank you so much!
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Old 01-30-2021, 11:11 AM   #10
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Great! And you are welcome.

I didnít want to include too much info in my earlier post but here is how we bypass the interlock;

Run a wire from the ďstĒ terminal on your key switch to the button in the back. A bunch of us have done this. All of those start relays are in between checking windows and doors. Then pull out those buzzers. Make to keep the air buzzer.

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Old 01-30-2021, 11:25 AM   #11
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Or a button like you said.
I repurposed two bus switches.
An on/off for ignition and a momentary for the start. They are labeled lift and warning and sit among the other switches. I leave the key switch where it belongs with the key in it.
The bad guys, or kids, or my drunk friends will figure dead battery. This is New Orleans. Things have to be drunk proof!!

Peace
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Old 01-30-2021, 12:15 PM   #12
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I might really be off on this, but on my BlueBird, the ECU fuse is in a hidden fusebox on the frame-rail just behind the driver's side front wheel.


I think I remember someone with a Thomas say they had one there also. Worth a quick peak, anyway.


It's a grey steel box with a screw-on cover-plate. Mine was full of water in the bottom, and corrosion all over the ECU fuse, and the main fuse that is also in there. Box looked perfect outside. Other folks here have had problems there; one guy's box was more of a rust-hole than a rusty box, yet his fuses were clean. Go figure.


Yea, it took me 2 or 3 days of taking the dash apart, taking wiring harnesses apart, and tracing the wire to find the fuse.
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Old 01-30-2021, 08:13 PM   #13
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I will look for it. Thanks!

I would like for my neutral safety switch to work!

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