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Old 12-20-2018, 04:27 PM   #1
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Bouncing speedometer

I have a 2004 Thomas front-engine (Flat nose) bus with a Cat 3126 engine. The ammeter doesn't work (volt meter works fine), and when I put the ignition to the "run" position, like when waiting for the engine pre-heat to come up to temperature, the speedometer gently bounces between 5 and about 15 mph. When I start the engine the speedometer behaves normally.

Does anyone have any idea what is causing this?
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Old 12-20-2018, 04:38 PM   #2
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Grid heater or glow plugs in that motor?


Seems the speedometer is getting an electrical pulse from the engine heater going on and off. Is that preheat light close to the speedometer, might be some dust/moisture connecting them on the back of the board. Then when the heater is off the issue goes away, which is a good thing to watch. Wouldn't worry about it till it works otherwise.


My shot in the dark, lol


John
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Old 12-20-2018, 05:18 PM   #3
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I had a similar problem with the tachometer on my boat. I agree with with BlackJohn, keep an eye on it, but don;t panic.
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Old 12-20-2018, 05:37 PM   #4
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Check with your local Freightshaker dealer.
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Old 12-20-2018, 08:23 PM   #5
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Quote:
Originally Posted by MadCompSci View Post
I have a 2004 Thomas front-engine (Flat nose) bus with a Cat 3126 engine. The ammeter doesn't work (volt meter works fine), and when I put the ignition to the "run" position, like when waiting for the engine pre-heat to come up to temperature, the speedometer gently bounces between 5 and about 15 mph. When I start the engine the speedometer behaves normally.

Does anyone have any idea what is causing this?
I would guees a poor ground between engine- frame - instrument panel enough of a difference in potential of the grounds that sends some back through the instrument. Try the tried and true loosen and tighten connections pull apart multi pin connectors and squirt some contact cleaner in them and reseat.
Click your heels together and say There is no place like Ohm...
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Old 12-20-2018, 08:46 PM   #6
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Click your heels together and say There is no place like Ohm...


No place like Ohm.
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Old 12-21-2018, 11:21 AM   #7
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What kind of gauges do you have? My VDO speedo and tach bounce a little and then do a full sweep during startup. I think it is part of their startup test.

Ted
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