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Old 09-10-2019, 04:20 PM   #1
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Speedometer issue after AT545 to MT643 conversion

So I'm having the speedometer issue after having my transmission upgraded (on a 5.9 12v) from an AT545 to a MT643. It's been quite an ordeal, but I'm (hopefully) down to the last couple issues....

The shop connected the sending unit of the 643 to my existing speedometer and now my speedometer and odometer are both reading double (x2).

I keep reading that the 545 speedometer output is electric, while the 643 has a mechanical output. Well the sending unit on the my 643 does not use a cable- it's a plug with 4 wires- though my bus's speedometer only uses 2 wires - one is power and one is signal. So 2 of the 4 wires are not connected.



Anyone have any insight into a fix for the issue?
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Old 09-10-2019, 04:41 PM   #2
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Originally Posted by NovaTRON View Post
So I'm having the speedometer issue after having my transmission upgraded (on a 5.9 12v) from an AT545 to a MT643. It's been quite an ordeal, but I'm (hopefully) down to the last couple issues....

The shop connected the sending unit of the 643 to my existing speedometer and now my speedometer and odometer are both reading double (x2).

I keep reading that the 545 speedometer output is electric, while the 643 has a mechanical output. Well the sending unit on the my 643 does not use a cable- it's a plug with 4 wires- though my bus's speedometer only uses 2 wires - one is power and one is signal. So 2 of the 4 wires are not connected.



Anyone have any insight into a fix for the issue?
A speed sensor needs at least 3 wires. Supply voltage, ground, and output signal. The fourth wire may be signal ground.

Call Chris at UJoint Offroad. He uses a signal converter after installing 4WD transfer cases with different speed sensors. The company he gets the converters from (name escaped me) will know how to deal with your issue; it's their business.

Edit: Name is Abbott Enterprises Inc.
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Old 09-10-2019, 05:28 PM   #3
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the MT643 is available 2 ways.. with a speedo GEAR and with a Tone ring.. the 545 was the same way also..



if you are using a gear to electric conversion on one or the other then the number of pulses would be off..



stock the tione ring pushes 16 pulses per rev .. the gear with a converter on it whould be made to do the same..



im assuming your speedometer itself is electric? (wire going to it and not a cable?)//


look at your old trans and see what type of sensor it had in it.. and then the new.. its Possible the speedo pickup is grabbing some sort of signal from the work gear but highly unlikely..



the Tone ring or gear can be extracted from the old AT545 and installed into the new 643.. the shaft sizes, lengths and such are identical on both..


-Christopher
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Old 09-10-2019, 05:29 PM   #4
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Originally Posted by alpine44 View Post
A speed sensor needs at least 3 wires. Supply voltage, ground, and output signal. The fourth wire may be signal ground.

Call Chris at UJoint Offroad. He uses a signal converter after installing 4WD transfer cases with different speed sensors. The company he gets the converters from (name escaped me) will know how to deal with your issue; it's their business.

Edit: Name is Abbott Enterprises Inc.
The speedometer in the bus only uses 2, so that's why only 2 were connected to the 643 sending unit... Are you saying I need to ground one of the remaining 2 wires?

I will give Ujoint Offroad a call (or Abbott)- thanks.
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Old 09-10-2019, 05:31 PM   #5
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Originally Posted by cadillackid View Post
the MT643 is available 2 ways.. with a speedo GEAR and with a Tone ring.. the 545 was the same way also..



if you are using a gear to electric conversion on one or the other then the number of pulses would be off..



stock the tione ring pushes 16 pulses per rev .. the gear with a converter on it whould be made to do the same..



im assuming your speedometer itself is electric? (wire going to it and not a cable?)//


look at your old trans and see what type of sensor it had in it.. and then the new.. its Possible the speedo pickup is grabbing some sort of signal from the work gear but highly unlikely..



the Tone ring or gear can be extracted from the old AT545 and installed into the new 643.. the shaft sizes, lengths and such are identical on both..


-Christopher
No Cable, the speedometer is electric.

Edit:I'll see about extracting the gear from the old 545, the shop is holding it for still,,, but from talking to the mechanic and he said the gears were different and weren't compatible.
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Old 09-10-2019, 06:07 PM   #6
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OK.. yes 2 wires.. what i was asking is if the old trans had a 2 wire sensor in it screwed to a gear drive or was it a 2 wire sensor that goes directly into the trans?


and same with new trans?.. there are devices made to convert the gear drive to an electric signal.. its possible the 643 was shipped with a gear and not a tone ring.. and the shop installed a converter device that takes the rotation of the gear drive and converts to 2 wire electric...





like I say my NT643 I got from LKQ shipped with a gear and not a tone ring.. the allison speed sensor only needs 2 wires as it generates a Millivolt AC voltage and the dashboard speedo reads the frequency of the AC signal to get the speed..



the standard tone ring in an AT545 is a 16 tooth as it is if in an MT643.. its possible the chassis manufacturer used a different one..



to extract it and move it over..
order 2 brand new allison transmission rear seals..(you only need one.. but they are easy to FUBAR..)

obtain or make the tailshaft bearing extractor tool...

(I made my own then an old allison guy gave me the real one)
I can post pics of the tool later


obtain or make a allison rear bearing installer.. may be able to rent this from auto zone.. it should be similar to allison 1000 used in duramaxx 2 wheel drive trucks.



on the 545

1. remove the yoke.
2. remove the rear seal
3. remove the LARGE snap ring that holds the rear bearing in.

4. using tool above, pull the rear bearing carefully..

5. spacer and tone ring just pull off of tailshaft.


on the 643. perform the steps 1-5 to pull the old ring or gear from 643..



now install..
insert spacer and bearing onto 643 shaft.
install the 643 bearing(not 545 bearing)(you will know its in all the way when the snap ring groove is revealed fully).
install the snap ring
install a brand new rear seal
install the yoke
install the sensor..


a bit of a PITA tools wise.. your trans shop if they service allisons should have the tools..



ill post pics if you want of the special tools..


-Christopher
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Old 09-10-2019, 06:36 PM   #7
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Quote:
Originally Posted by cadillackid View Post
OK.. yes 2 wires.. what i was asking is if the old trans had a 2 wire sensor in it screwed to a gear drive or was it a 2 wire sensor that goes directly into the trans?


and same with new trans?.. there are devices made to convert the gear drive to an electric signal.. its possible the 643 was shipped with a gear and not a tone ring.. and the shop installed a converter device that takes the rotation of the gear drive and converts to 2 wire electric...





like I say my NT643 I got from LKQ shipped with a gear and not a tone ring.. the allison speed sensor only needs 2 wires as it generates a Millivolt AC voltage and the dashboard speedo reads the frequency of the AC signal to get the speed..



the standard tone ring in an AT545 is a 16 tooth as it is if in an MT643.. its possible the chassis manufacturer used a different one..



to extract it and move it over..
order 2 brand new allison transmission rear seals..(you only need one.. but they are easy to FUBAR..)

obtain or make the tailshaft bearing extractor tool...

(I made my own then an old allison guy gave me the real one)
I can post pics of the tool later


obtain or make a allison rear bearing installer.. may be able to rent this from auto zone.. it should be similar to allison 1000 used in duramaxx 2 wheel drive trucks.



on the 545

1. remove the yoke.
2. remove the rear seal
3. remove the LARGE snap ring that holds the rear bearing in.

4. using tool above, pull the rear bearing carefully..

5. spacer and tone ring just pull off of tailshaft.


on the 643. perform the steps 1-5 to pull the old ring or gear from 643..



now install..
insert spacer and bearing onto 643 shaft.
install the 643 bearing(not 545 bearing)(you will know its in all the way when the snap ring groove is revealed fully).
install the snap ring
install a brand new rear seal
install the yoke
install the sensor..


a bit of a PITA tools wise.. your trans shop if they service allisons should have the tools..



ill post pics if you want of the special tools..


-Christopher
The way I understand your comments is that the correct sensors for both transmissions have only two wires (i.e. no signal amplifier) but that the number of teeth on the tone ring is different on the MT643 compared to the AT545.

Why does the sensor in the OP's photo of his MT643 have four wires? Wrong sensor?
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Old 09-11-2019, 08:10 AM   #8
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the photo isnt showing up on my end..



the way I understand it is the followingL
the Standard Tone ring for the AT545 and MT643 is always supposed to be 16 teeth...

its Possible an OEM Spec'd a different tone ring for their application.. allison did a lot of custom stuff for OEM's..


BOTH AT545 and MT643 were available as a mechanical or an electronic output..


the difference between electronic and mechanical output is a tone ring vs a Gear


the Same Tone ring or gear fits both AT545 and MT643 as the output shaft size on both is the same..



in my opinion the correct way to fix this is to extract the tone ring and spacer from the original AT545 and install them on the new MT643. this would ensure having the correct number of teeth..


an OEM can specify a sensor with 4 wires even though the standard was 2... the sensor can be different from the standard as long as it threads in and poicks up the signal..



the procedure I described can be done with a transmission in a bus or on the ground..

fluid loss will be minimal so you dont even need to drain the pan.

-Christopher
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Old 09-11-2019, 08:16 AM   #9
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I should also mention it may be possible to "program" the speedometer without replacing any parts. I dont klnow much about this bus but I knoiw that most either had dip switches or if the bus is electronic engine the ECM can be adjusted for different gear ratios and provisions..



im not sure if that year thomas used the ametek speedo or not which has a dip switch on it.. some of the freightliners did..

-Christopher
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Old 09-11-2019, 11:48 AM   #10
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I did link a photo in my original post - but it's not showing on my end either,

This bus has the 12-valve mechanical engine. I'm not sure about the dip switches, if I have them would they be on the back of the speedometer?

I'm making a plan to go back to the shop and see about removing the original Tone Ring from the AT545 -

Thanks for all the insights

nova~
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