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natedlee · 07-20-2017, 02:02 PM

The previous owner of my rig swapped out the stock bus gears for 4.78. So my speedometer is about 15mph off. How do I go about recalibrating, or updating, the speed sensor so it's accurate again?

Skip426 · 07-21-2017, 11:54 AM

Welcome to iRV2.

If your speedo is out by 15 mph, at all speeds ; it's not a calibration problem .
If the speedo is out by 15% at all speeds then , you'll have to contact the nearest Allison repair facility about possible reprogramming.
Sometimes a repair facility for your coach's chassis will have the ability too.

natedlee · 07-22-2017, 11:07 AM

Quote:

Originally Posted by Skip426

Welcome to iRV2.

If your speedo is out by 15 mph, at all speeds ; it's not a calibration problem .
If the speedo is out by 15% at all speeds then , you'll have to contact the nearest Allison repair facility about possible reprogramming.
Sometimes a repair facility for your coach's chassis will have the ability too.

Thanks. Good point also - it is not universally 15mph off. At higher speeds it is ~15 but % would be more accurate. So I calculate it's off by 133%. So when it says ~45 I'm going around ~60, and when it says I'm going 10 I'm really going 13.3 mph.

dbarton291 · 07-28-2017, 06:00 AM

The AT545 is not an electronic transmission.
What needs to be checked is how the speedometer is driven. Most AT545s used a worm type speedo drive on the output shaft and a mechanical cable to the speedo. There are some that used an eight tooth tone wheel for an electronic speedometer.
Look at the back of the transmission and see if you've got a cable or wires back there. If you're got wires, pull out the signal generator and see if there is a tone wheel in the transmission or a worm drive type gear.
If it's a tone wheel, the speedo head itself needs to be recalibrated.
If it's a worm gear, you can change the worm gear, or recal the speedo head if it has that capability. Many of the older speedos had dip switches on the back that set the calibration.
Allison service outlets are not trained by Allison to work on speedos, but if the location also has a truck shop, they probably are familiar with what needs to be done.