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Old 08-29-2009, 09:18 PM   #1
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Thomas Speedo problem

Anyone have to deal with an intermittant Speedometer recently? I took my bus down the road to put some diesel in it as I'm getting ready for another trip to Cape Hatteras next weekend. On the way back to the shop the speedometer was cutting on & off. Once I got back I popped the dash and poked around a little in there, the speedo is electronic, I jiggled the plugs etc, but have no idea what I'm dealing with here... don't know if it's the speedo or perhaps a speed sensor somewhere...
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Old 08-30-2009, 01:37 PM   #2
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Re: Thomas Speedo problem

Cliff,
A lot of these buses use electronic sensors which are no more than magnetic switches or proximity switches mounted in or on the transmission. They pick up pulses, usually from the final drive shaft or output gear, count those pulses or convert them to a variable analog output that drives your speedometer readout. Moisture, looseness, or misadjustment of the sensors can cause loss of signal to the speedometer circuit. So the first thing I always check is, is the sensor tight, are the wires attached, and does it appear that the insulation on the wires is intact. There can also be a "black box" which is the amplifier board, either behind the dash, or in the area of the power distribution box (above and behind the left headlight assembly on my bus). Check to make sure this is firmly mounted (the mounting screws are normally the circuit ground), and that the power and sensor plugins are clean and firmly inserted. Last, but not least, check to be sure that the breaker or fuse feeding this circuit is connected well and is not corroded or overheating. I hope this helps and you find and fix your problem.
Here's a link that also might help. http://schoolbusmechanic.blogspot.co...on-wiring.html

Best wishes,
Lee
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Old 08-30-2009, 09:58 PM   #3
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Re: Thomas Speedo problem

Thanks for the info, I'll have to get under it and take a look around. Seems like loose wire, because as I hit bumps it would go on or off...
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*Cliff*

You just might be a Redneck if...
...your motor home used to be a school bus!
...Your living room has a steering wheel!
...Your home has brake lights

1994 Jeep Grand Cherokee
1989 Thomas Diesel Pusher (Cat 3208/Freightliner)
Chesapeake, Virginia
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Old 08-31-2009, 10:54 PM   #4
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Speedo problem

Most electric speedo units are getting pulses from the ring gear in the differential. WD-40 will remove any moisture and improve the connections. Sometimes the back of the seedo can have a poor connection. Look around your area and call the local Blue Bird dealer or school district and ask about any speedo problems and solutions. Frank
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Old 09-01-2009, 03:31 PM   #5
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Re: Thomas Speedo problem

when all else fails, you can always use a cheap GPS as a speedo.
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Old 09-01-2009, 10:15 PM   #6
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Re: Thomas Speedo problem

I'm not terribly concerned, the bus won't go over 60mph anyway, so as long as I don't go faster that traffic in lower speed zones I'll be fine. But I still would like to fix this, hasn't done it before now either... I suspect a loose connection somewhere.
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*Cliff*

You just might be a Redneck if...
...your motor home used to be a school bus!
...Your living room has a steering wheel!
...Your home has brake lights

1994 Jeep Grand Cherokee
1989 Thomas Diesel Pusher (Cat 3208/Freightliner)
Chesapeake, Virginia
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