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Analog VDO speedometer failure.

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Mike And Mary Engen:
Recently fueled up, reset the analog trip odometer as we were driving out of the fuel station. The needle and odometer both have been sitting at zero ever since. I have the 12 vdc to the unit, I have reading to ground at the quick disconnect. There is a plug that has 12 vdc going into it, then I believe out to the sending unit, so I would like to figure out where the sending unit is located, if there is something I can check there, is the speedometer bad?

Even though I have a reading to ground at the quick disconnect, my instrument bulbs at the speedometer and air pressure gauges quit working, so I may still have a grounding issue which I will check further in the next couple of days.

This is a 95 Beaver 300 hp Cummins, Allison 6 speed automatic with an analog VDO speedometer. I see they have a facility in Cedar Rapids, Iowa. Has anybody else used this option or is the normal semi truck repair facility the place to bring this if all else fails?

Mike Engen

Gerald Farris:
I would have to see your coach to be sure, but the speed sensor is probably an electric generator. The speed sensor should be located in the transmisson near the drive shaft so that it can read the speed of the drive shaft. Most cars use a speed sensor that create a square wave current so that the computer that controls the fuel system can tell the exact speed the vehicle is traveling by the speed the pulses that are coming from the sensor.  It is very possible that your speedo only reads the amount of current (amps) created by the sensor and therefore the higher the current the higher the speed indicated. The sending unit (speed sensor) should not require a 12v power source.

Gerald

Mike And Mary Engen:
Thanks for the quick reply, I was thinking maybe there was a prox switch that counted revolutions so I thought I would need to send out 12 volts, then return with 12 volt pulses back. I had not realized there were pulse generators on rvs'. I used to be an industrial electrician, so still learning rv side.
I will try crawling underneath this next week to see if I can see anything that does not look correct.

Gerald Farris:
Mike, a fellow Beaver owner called me today and said that he thought that the speed sensor for your speedometer was at the left front wheel. If it is at the left front wheel I do not know if there are two sensors or if the speedometer is able to use the left front ABS sensor.

Gerald

Mike And Mary Engen:
Thanks Gerry,

I will be doing some things on the coach tomorrow and will look in both places for the sending unit or pulse generator to see if there is anything obvious. Thinking it's probably the gauge as I had just reset the odometer when it quit working, but want to check everything first.

In the meantime the GPS is fairly close.

Once I get it repaired I will update my findings in order to help others.

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