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Bucky1320 · 05-31-2015, 07:23 AM

1998 Harney on Magnum chassis with 300 CAT 3126B
When I turn the key, it sweeps full, then settles back to about 5 mph. Anyone run into this?

BFlinn181 · 05-31-2015, 09:14 AM

Does it read accurately from 5 mph up? If so, ignore it. Your needle could just be hung up. If it's accurate at above 5 mph, you'll have to decide it it's worth replacing to start at '0'.

Bucky1320 · 05-31-2015, 09:16 AM

No. It seems 5 mph off. But then this morning I moved it over to the house, and it starts off reading under 0.

wa8yxm · 05-31-2015, 09:33 AM

Compare it to the SPEED display on a GPS or Smart Phone with the speed sensor app running. (I have multiple speed sensors on my smart phone).

IF it's just not returning to zero.. You do not have a problem

if it's always 5MPH off.. You can fix the problem by making the adjustment mentally.

If its scatter plot you have a problem (Sometims high,sometimes low)
Go to a speedometr shop or... USE GPS/PHONE speedometer.Those are very very highly accurate plus/minus not enough to matter.

Bucky1320 · 06-01-2015, 04:35 AM

Quote:

Originally Posted by BFlinn181

Does it read accurately from 5 mph up? If so, ignore it. Your needle could just be hung up. If it's accurate at above 5 mph, you'll have to decide it it's worth replacing to start at '0'.

No. The inaccuracy carries.

BFlinn181 · 06-01-2015, 09:18 AM

A speedometer shop should be able to check and adjust the gauge for accuracy. Or, you could buy a new gauge and exchange it for the old one. Being electrical, a gauge going bad is not a difficult issue to solve.

Bucky1320 · 06-01-2015, 11:09 AM

It could be the speedo for sure. But I suspect something electrical going on here. maybe in the speedo. I will keep an eye on it a bit longer and see if I learn anything else along the way.
Thanks for the suggestions all!

BFlinn181 · 06-01-2015, 11:28 AM

It's an electrical gauge because you describe that it sweeps full range then settles back. Perhaps there's an electrical current feeding into the wires, but I'm not sure how you'd measure that or what it should be. You could try turning the ignition on and move wires about, seeing if it changes the needle to zero.

dpinvidic · 06-02-2015, 11:52 AM

These type of gauges are actually controlled by stepper motors. They are not like the old analog meters that moved the pointer base on some small amount of current.

The most common problem is that the stepper motor goes bad. It can be caused by binding.
Do a google search for "speedometer stepper motor" to see what they lok like.

They usually have 4 wires to control them and a special driver module that is computer controlled.

Regards,

Dan

Bucky1320 · 06-02-2015, 01:32 PM

Quote:

Originally Posted by dpinvidic

These type of gauges are actually controlled by stepper motors. They are not like the old analog meters that moved the pointer base on some small amount of current.

The most common problem is that the stepper motor goes bad. It can be caused by binding.
Do a google search for "speedometer stepper motor" to see what they lok like.

They usually have 4 wires to control them and a special driver module that is computer controlled.

Regards,

Dan

You know that does make sense. Of course the thing only has 25,000 miles on it. So it ought not be worn out! But I could see that being the issue.

BFlinn181 · 06-02-2015, 01:42 PM

The initial sweep from maximum to minimum is to calibrate the stepper motor. Yours is not going to zero, so it is not accurately calibrating. If it were me, I'd pull the gauge and dismantle it to see if a piece of crud is preventing it from moving properly. If that didn't do it, well I'd have the gauge out and could replace it.

Bucky1320 · 06-02-2015, 02:52 PM

I think I may do just that. I have a couple of weekends at races, and will address after that.

dpinvidic · 06-03-2015, 10:37 AM

Quote:

Originally Posted by Bucky1320

I think I may do just that. I have a couple of weekends at races, and will address after that.

Plan on a two step process. first, find out what kind of stepper motor is used. Either remove the instrument cluster and see what is marked on the motor, or as the coach builder who makes his dash assembly.

Once you find the right stepper motor to order, you should be able to replace it.

the stepper motor is a combination of the motor and gears inside the same housing.
If you were real lucky, you might be able to take it apart and clean it.
It does sound like it is hung up and does not get calibrated.

The needle mechanics have nothing to do with the odometer, so you can replace just the motor.

Dan

Bucky1320 · 06-03-2015, 02:30 PM

That's really great information. Harney isn't in business anymore, and Monaco doesn't always have all the information for Harney stuff. So taking it out is probably going to be the way to go.
Thank you!