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Old 05-13-2015, 05:38 PM   #1
Join Date: Mar 2015
Posts: 231
how to remove the Thomas Vandalock ignition interlock

The ignition interlock system on school buses is to make sure that the bus will not start if the emergency exits are locked. This is accomplished by two separate circuits wired in series, the first is the ignition interlock that prohibits the starter from engaging; the second is a buzzer that notifies the driver at the driver seat and at the door that has triggered the interlock that a door is locked.
This is to allow access for emergency personnel should there be a need to gain entry from the outside.

The easy way to start this process is:
1) Look under your dash at your key ignition switch, there will be a #10 wire it will be black or purple, there will be a smaller wire the same color with a tracer color.

2) open the door to your wiring panel and find those two wires coming in from the front of the bus, this is a large bundle of wires so you may need to remove a couple of ty wraps to find these wires.

3) Trace these wires, the biggest wire will go to a terminal and be fasted with a nut, the smaller wire will go to a relay, either directly or be spliced to a different color wire.

4) Trace the smaller wire to the relay, the relay will have one yellow wire which provides a ground to the relay to allow the starter to work, there will be a jumper wire from one terminal to another and that will have a wire that will lead back to the terminal where the #10 wire is connected, it most likely will be the one above or below the #10 wire

5) Disconnect the wire from the terminal that comes from the relay.

6) Cut the wire going to the relay and crimp a ring terminal to it.
(this is the smaller wire from the key ignition)
7) Connect this new ring connector to the terminal you just removed the wire from the relay.

Test your engine and it will start.

9) Unscrew the relay and disconnect the yellow wire, save the relay in your parts box, these are common use items.


The buzzer works opposite of the interlock, it will activate when it finds a ground, where the interlock looks for a continuous circuit.

1) Find the buzzer, it is right next to the relay you just removed.

2) Remove the power wire from the push on terminal strip just above it.

3) Remove the wires from the other terminal on the buzzer.

4) Unscrew the buzzer

5) Start bus and notice no annoying buzzer


Once you have removed the relay and the buzzer make sure you pull the wires you disconnected from the buzzer and the relay out of the way maybe put a crimp cap on them or tape the ends to keep them from touching something they should not, then ty wrap them and the big bundle of wires you opened up to find your ignition wires.

Below are some pictures, and a picture of the schematic that is on the door to my engine.

This worked just fine for me, and I believe it is better than trying to disable the interlock at each of the door locations and the buzzer trip points.

Attached Images
File Type: jpg Thomas bus Vandlock ignition interlock diagram.jpg (216.1 KB, 254 views)
File Type: jpg ignition switch wiring through relay.jpg (181.0 KB, 174 views)
File Type: jpg Interlock relay.jpg (225.6 KB, 153 views)
File Type: jpg interloack relay removed and ring terminal installed.jpg (107.8 KB, 134 views)
File Type: jpg INterlock relay wiring without relay.jpg (242.9 KB, 117 views)
File Type: jpg interlock buzzer.jpg (195.1 KB, 108 views)
File Type: jpg relay gone buzzr still.jpg (324.0 KB, 121 views)
File Type: jpg relay and buzzer removed.jpg (354.9 KB, 106 views)
File Type: jpg yellow wire b32 relay ground source.jpg (158.9 KB, 96 views)
File Type: jpg relay pin out.jpg (117.8 KB, 107 views)
c_hasbeen is offline   Reply With Quote
Old 05-13-2015, 07:06 PM   #2
Join Date: Mar 2015
Posts: 231
I found a link to a wiring diagram that you can print out if you need
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Old 10-29-2015, 11:23 AM   #3
VeldatheWonderbus's Avatar
Join Date: Oct 2015
Location: Belgrade, MT
Posts: 65
Year: 1999
Coachwork: AmTran
Chassis: International
Engine: DT466E International
Rated Cap: 72
Is the screwdriver pointing at a hard start/cold ignition relay in the third picture? And is the (solenoid contractor) relay just outside images 2 and 4 to the left side, mostly shown in image 5, and shown fully in the bottom left of 7 and 8?

Velda has a very similar-numbered relay, 70-111226-5A, but all of the wires connected to it are white. I still have to pull the panel with the ignition switch to see if that has variously-colored wires or if they kept it monochromatic there too...(I'm going to be using a lot of wire labels).
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Old 10-29-2015, 12:11 PM   #4
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Year: 1946
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I don't even own a Thomas but wanted to thank you for sharing this info. Really handy knowledge...good on you!
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Old 10-30-2015, 06:00 PM   #5
Join Date: Mar 2015
Posts: 231
The first relay is the interlocking relay. The other SoLenoir is one of two, the starting solenoid is the closest to the front it also gives gages and low air buzzer power.
The one in the corner of the second picture is the same part number, it is connected to other electrical body stuff, I believe one of those functions is the fast idle.
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Old 11-03-2015, 08:50 AM   #6
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Join Date: Oct 2015
Location: Belgrade, MT
Posts: 65
Year: 1999
Coachwork: AmTran
Chassis: International
Engine: DT466E International
Rated Cap: 72
To expand this thread a little, I'm going to add my photos and explanation, as Velda (1999 International Genesis, AmTran build) was set up a little differently than the Thomas system c_hasbeen described above. But it's still similar-enough to keep it in the same thread and not start a redundant one.

In contrast to the Thomas system, the interlock relay in Velda is wired directly to the buss panel terminals from each post (the larger, top-and-bottom posts on the relay), essentially in the location of the jumper between those terminals in the Thomas system. The "ground" terminal on the relay (the little one in the middle) is connected to the emergency exit doors, and will only allow the interlock relay to connect if the doors are unlocked.

I followed c_hasbeen's steps in the original post, but used one of the wires that used to connect a buss terminal to create the jumper to the other terminal. It took a few minutes to see what was different about Velda than the system above, but once I understood that the same components were involved, but in a different order, it was easy to amend the steps above to remove the interlock system.

Here's the result - note that I still haven't removed the relay in this picture; the wires I removed from it have their terminals covered in black electrical tape.

Again, all of this was made a little more interesting by the fact that whoever wired the interlock used a single color of wire for the whole system (whitish pink in the images above), so the black/yellow/purple distinction didn't help me. Hopefully this slightly different explanation of removing this system helps others.

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Old 11-04-2015, 08:24 AM   #7
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Thanks for the info, I will be getting around to this very soon.
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Old 11-04-2015, 09:18 AM   #8
VeldatheWonderbus's Avatar
Join Date: Oct 2015
Location: Belgrade, MT
Posts: 65
Year: 1999
Coachwork: AmTran
Chassis: International
Engine: DT466E International
Rated Cap: 72
I meant to include this image in my last post, but it slipped my mind. I used c_hasbeen's steps and then drew them out so I could see what I needed to do, step by step. So the numbers in the diagram correspond to his instructions above, which I highly recommend that you print out and have sitting next to you while you're working.

I also watched this video about how the relay itself works, which was quite helpful in simply understanding what I was trying to accomplish.

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Old 12-04-2015, 07:33 PM   #9
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My bus 95 international t444e 3800
My wires were pink and the terminal was under the dash
OK so what I did was
1. Find the terminal "wires on terminal 2 pink 1 ground."
2. Take 2 pink wires off
3. Install both wires on one side of terminal.
4. Finished bus started right up.
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Old 01-27-2016, 10:42 PM   #10
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Year: 1995
Chassis: Thomas Saf T Line
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Thank you guys, you made my weekend.

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