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Old 04-01-2024, 08:29 AM   #1
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Year: 2005
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Engine: Cat C7
2005 Thomas cranks - but no dash lights

My 2005 Thomas MVP-EF CatC7 has been great. Super reliable. I had an intermittent starting issue. Basically, I would turn the key to fire the glow plugs and the dash would be dead, i.e., no lights, no air buzzer, nothing. If I turned the key the rest of the way to start, then the engine would crank (crank no start). I would cycle the key a couple times and it would finally light up the instrument cluster and everything worked fine. I thought it was the ignition itself, so I ordered a new one. Installed and all was great for a few weeks. Now the same thing is happening. I have a generic electrical schematic, trying to figure out where to start. Ignition relay? Fuse? Any direction on a starting point would be super helpful?!

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Old 04-01-2024, 10:15 AM   #2
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Originally Posted by Thomas Driver View Post
My 2005 Thomas MVP-EF CatC7 has been great. Super reliable. I had an intermittent starting issue. Basically, I would turn the key to fire the glow plugs and the dash would be dead, i.e., no lights, no air buzzer, nothing. If I turned the key the rest of the way to start, then the engine would crank (crank no start). I would cycle the key a couple times and it would finally light up the instrument cluster and everything worked fine. I thought it was the ignition itself, so I ordered a new one. Installed and all was great for a few weeks. Now the same thing is happening. I have a generic electrical schematic, trying to figure out where to start. Ignition relay? Fuse? Any direction on a starting point would be super helpful?!
First things first, contact a Thomas bus dealer, give them the vin and ask for the wiring diagram for tge starting cicuit.

Second thing, identification only for now, does your bus have a body control module ?

Intermittent issues usually means something is loose somewhere, usually but not alwys .

Also what yoy can do in the meantime, turn key on, if dash lights are iff then start wiggling/ jiggling wires to see if the dash light come on.

Sorry for the typis, using my cell and my fingers are too fat !
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Old 04-01-2024, 11:06 AM   #3
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Heading to Thomas dealer tomorrow. Pulled dash apart - key on - lots a wiggling. Nothing yet. Can't find a body control module. Will ask tomorrow. Thanks
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Old 04-02-2024, 04:53 PM   #4
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Went to Thomas dealer. Super hard to get exact wiring schematics for 20 year old bus. I did get a 694 page wiring manual of "generic" 2004 MVP-EF overall wiring ... but was specifically for Cat3126 (only a few pages on Cat-C7). So far, no body control module listed. My current plan is to swap out fuses and relays to everything related to cluster and ignition. Then find someone that can test CAN BUS system.
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Old 04-02-2024, 07:33 PM   #5
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Originally Posted by Thomas Driver View Post
Went to Thomas dealer. Super hard to get exact wiring schematics for 20 year old bus. I did get a 694 page wiring manual of "generic" 2004 MVP-EF overall wiring ... but was specifically for Cat3126 (only a few pages on Cat-C7). So far, no body control module listed. My current plan is to swap out fuses and relays to everything related to cluster and ignition. Then find someone that can test CAN BUS system.
There is a 2 page wiring diagram in post #1 of this thread

Take a look at it and see if it is close to your bus.

On the first page, locate the key switch and identify the "IGN' terminal on the key switch.
Turn the key swicth ON, got voltage here?

One of those 2 wires goes to the C1 connector and then to the Ignition Power relay.

Is the relay working properly when you turn on the key switch?

One of the wires from that relay appears to be going to your gauges via a 10 amp breaker. via wire 3/14RD, thru D1 of the 42 way connector, to the oil pressure gauge. You will notice that the power wires eventually goes to all of the gauges.

My initial thought would be that if that power relay was not working properly it might not be sending a start voltage (trigger) to the ECU.

You will notice from the diagram that the "ST" terminal on the key switch takes a different route thru connector E1 and then travels to the starter relay which is probably why the starter/engine cranks but does not start/run.

Not being there at your bus and going only off this diagram I would say that it would be helpful in checking to see if that relay of circuit breaker is seeing power when you turn on the key.

If no voltage then you got to find out why.
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Old 04-02-2024, 08:17 PM   #6
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Page 2 of the diagram

Check breakers and check for voltages at...

Front panel #34 (engine ECM power feed)

and also side panel #10 (engine ECM key switch)

None of this is a guarantee but it is what I would do If I was troubleshooting
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Old 04-03-2024, 07:33 AM   #7
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The two page schematics on the link were for a Cat3126. Mine's a CatC7. The schematics don't align, i.e., I traced the wires to the relays but they go to different relays then on the prints. However, your instructional steps are still good ... regardless of where the final location of the relays are in the panel. Hoping to do your step by step on Friday. Thanks
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Old 04-03-2024, 07:44 AM   #8
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2004 was pivotal year for thomas.. many of the HDX and MVP series still had 2003 chassis in 04 which would have a 3126, 05 should all be C7 if CAT equipped..



I dont believe a BCM came into play in the transit series into the second gen C7 (08 model year?)... the C2 got the Multiplex system when it debuted in 06.
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Old 04-06-2024, 07:58 PM   #9
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Originally Posted by cadillackid View Post
2004 was pivotal year for thomas.. many of the HDX and MVP series still had 2003 chassis in 04 which would have a 3126, 05 should all be C7 if CAT equipped..



I dont believe a BCM came into play in the transit series into the second gen C7 (08 model year?)... the C2 got the Multiplex system when it debuted in 06.
Traced to WABCO ABS module. Either harness or module itself. If I wiggle the connector enough it lights the dash and everything works fine. Found a used module on-line for $150. Will try that first before attempting to rewire the module plug - lots of wires!
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