Thomas Built Bus won’t crank. Buzzing.

[kansasheadhuntr]

I’m converting a 2000 Thomas Built Safety Liner school bus into an RV and I’m currently having a no-crank issue.

When I turn the key, I hear a steady buzzing sound, similar to the safety/interlock buzzer for the side door, rear exit, etc. The engine will not crank.

Previously, I bypassed the factory safety interlocks (rear door, side door, emergency exits) and the bus did run and drive after that. However, the bus has been sitting for a while.

Since it last ran, I’ve done the following:

• Removed the heater
• Removed the front door air pressure actuator and related air hardware

Now I’m wondering:

• Are the front door air system or any air pressure switches tied into the starter permissive / neutral safety circuit on these buses?
• Could removing the front door air actuator or leaving an air circuit open cause a no-start / no-crank condition?
• Where are the common starter interlock relays or switches on Thomas buses of this era?
• Has anyone permanently bypassed these interlocks during a bus-to-RV conversion, and if so, where did you do it cleanly?

Battery are brand new, but the buzzing sounds like a relay or interlock not being satisfied rather than a dead starter.

Any wiring diagrams, relay locations, or bypass advice would be appreciated.

 

[nikitis]

I know it seems silly, but also do a transmission reset. Put it in Neutral, then drive, then reverse, then neutral again. Sometimes it can think it's in drive and appear in Neutral but it's think it's in drive somewhere and won't start as a result. In ANY no start situation this should be the first thing everyone tries. It resolves a lot of these issues. By doing it in that order it resets the transmission's position physically and in software. Do this and confirm with us it's been done.

Next you can always try to jump start the starter directly to see if the bendix will extend and attempt a crank. If it does then this tells us it's an electrical problem because you bypassed it and the starter works and will crank, it's just not getting the signal. The starter bypass works by sending a 12v signal between the main Red Cable (From battery) going to the starter and power connection on the solenoid on top of the starter which engages the bendix. If this doesn't move when doing this then you have a bad starter.

Doing these three things will tell us more about where the issue lies if it doesn't resolve it.

 

[kansasheadhuntr]

My transmission isn’t even lighting up.

Thanks for the response and good to know.

UPDATE: through a lot of back tracking wires and trying to determine the issue, I think it’s a missing ign run feed to the Allison tcm which was apparently routed through the body/heater circuitry which I removed during conversion.

The tcm briefly boosts when I manually bypassed solenoid (mode light flicker) then drops immediately even when continued hold.

The transmission is good, I think it just needs a stable, fused IGN RUN supply.

Does that sound right? I just need guidance on how exactly to do this

 

[ewo1]

My transmission isn’t even lighting up.

Thanks for the response and good to know.

UPDATE: through a lot of back tracking wires and trying to determine the issue, I think it’s a missing ign run feed to the Allison tcm which was apparently routed through the body/heater circuitry which I removed during conversion.

The tcm briefly boosts when I manually bypassed solenoid (mode light flicker) then drops immediately even when continued hold.

The transmission is good, I think it just needs a stable, fused IGN RUN supply.

Does that sound right? I just need guidance on how exactly to do this

What engine tranny combo you got?

Here's an early xmas present...
Hope it helps you out!

 

[kansasheadhuntr]

CAT 3126 and Allison Transmission

 

[ewo1]

CAT 3126 and Allison Transmission

I don't know cat....
Maybe you'll find something in that doc?

 

[kansasheadhuntr]

Update / Fix (2001 Thomas Saf-T-Liner, CAT 3126, Allison 2000)

Posting a follow-up for anyone converting a Thomas bus and chasing weird electrical issues.

On these buses, IGN/RUN power for the Allison TCM, CAT ECM, starter enable, and rear body lights is originally routed through Thomas body/interlock wiring. When that body wiring is removed during a skoolie conversion, you can end up with “ghost voltage” that shows up on a meter (2–4V) but won’t actually run anything. This causes TCM flickering, buzzing relays, no-crank, crank-no-start, and rear lights not working even though the front lights do.

In my case, nothing was actually broken. The fix was to restore clean RUN power using relays:

• One relay for the Allison TCM
• One relay for the CAT ECM (fed through the engine fuse block)

Both relays are triggered by the ignition switch RUN/START signal and powered directly from battery (properly fused). Once the ECM had real RUN voltage, the engine started immediately. Same principle applies to rear lighting circuits, they often lose their RUN feed when body logic is removed.

Key takeaway:
If you’re converting a Thomas bus and see phantom voltage, buzzing relays, or modules that half-wake, don’t assume bad parts. Assume the RUN power path was removed with the body wiring and recreate it cleanly with relays.


Hope this helps the next person.

 

[ewo1]

Update / Fix (2001 Thomas Saf-T-Liner, CAT 3126, Allison 2000)

Posting a follow-up for anyone converting a Thomas bus and chasing weird electrical issues.

On these buses, IGN/RUN power for the Allison TCM, CAT ECM, starter enable, and rear body lights is originally routed through Thomas body/interlock wiring. When that body wiring is removed during a skoolie conversion, you can end up with “ghost voltage” that shows up on a meter (2–4V) but won’t actually run anything. This causes TCM flickering, buzzing relays, no-crank, crank-no-start, and rear lights not working even though the front lights do.

In my case, nothing was actually broken. The fix was to restore clean RUN power using relays:

• One relay for the Allison TCM
• One relay for the CAT ECM (fed through the engine fuse block)

Both relays are triggered by the ignition switch RUN/START signal and powered directly from battery (properly fused). Once the ECM had real RUN voltage, the engine started immediately. Same principle applies to rear lighting circuits, they often lose their RUN feed when body logic is removed.

Key takeaway:
If you’re converting a Thomas bus and see phantom voltage, buzzing relays, or modules that half-wake, don’t assume bad parts. Assume the RUN power path was removed with the body wiring and recreate it cleanly with relays.


Hope this helps the next person.

Nice job figuring it out!
And thanks for posting the update!
It really sucks when folks forget to share how they got it done!