1994 Allison 3060 hangs in 4th gear

[Piker]

Originally Posted by dbarton291

In brief, when the ignition cycles the counts range of the TPS is basically shrunk a little bit and the high and low limits reset as the throttle moves.

I think I read this in the service manual... and it's always perplexed me. In theory I think I understand it - by shrinking the count a little bit, the TCM can adjust itself to any wear in the linkage or other small changes in the sensor travel.

But... if the TCM shrinks the count by a little bit each time, and you NEVER go to full throttle, won't your TPS eventually be out of calibration as it keeps shrinking the count for WOT down below where WOT actually is? I must be missing something or misinterpreting how it actually works.

I find myself having to reset the TPS with the ignition cycling trick an awful lot. The shifts always seem to go south after a lot of stop and go driving in the mountains where I'm doing a lot of manual shifting. It doesn't seem like poor shifting afterward would be the normal result? I know I've had a lot of issues with oxidized connections on just about every electrical device on the rig... maybe that's all there is to it. I've cleaned everything I could find... What do you think?

-cheers

[dbarton291]

Originally Posted by Piker

I think I read this in the service manual... and it's always perplexed me. In theory I think I understand it - by shrinking the count a little bit, the TCM can adjust itself to any wear in the linkage or other small changes in the sensor travel.

But... if the TCM shrinks the count by a little bit each time, and you NEVER go to full throttle, won't your TPS eventually be out of calibration as it keeps shrinking the count for WOT down below where WOT actually is? I must be missing something or misinterpreting how it actually works.

I find myself having to reset the TPS with the ignition cycling trick an awful lot. The shifts always seem to go south after a lot of stop and go driving in the mountains where I'm doing a lot of manual shifting. It doesn't seem like poor shifting afterward would be the normal result? I know I've had a lot of issues with oxidized connections on just about every electrical device on the rig... maybe that's all there is to it. I've cleaned everything I could find... What do you think?

-cheers

Yes. If you never go to full throttle physically on the pedal, the autocal function of the TPS will eventually be giving you full throttle shift points at whatever the maximum travel the linkage has actually achieved.
You can either do the ignition cycling thing or just simply move the throttle to wide open while driving.


Manual shifting doesn't have anything to do with the autocal on the TPS. It's all about moving the throttle pedal linkage.

[deandec]

I guess I am thick headed on this topic.

If I go WOT while in sixth gear, what is the result for the Allison shift points.

I have the same question for the other 5 gears.

Obviously, once the driver's perfect shift pattern was achieved, going WOT would not be on his to do list?

[dbarton291]

Originally Posted by deandec

I guess I am thick headed on this topic.

If I go WOT while in sixth gear, what is the result for the Allison shift points.

I have the same question for the other 5 gears.

Obviously, once the driver's perfect shift pattern was achieved, going WOT would not be on his to do list?

I think you are over thinking this. The discussion on vehicles with a mechanical TPS only has to do with the autocal of the TPS.
Drive your vehicle as you normally would.

The discussion we are having over autocal relates to a situation where the operator of a light vehicle may never use wide open throttle. Eventually the TCM will interpret the maximum position the TPS sees as wide open throttle and utilize wide open throttle shift points (which are higher than closed and partial throttle shift points) which may cause a delay in shifting if the operator doesn't step on the gas. The remedy? If it doesn't shift, step on the gas! Then the engine increases RPM, hits the shift point and autocal moves the wide open throttle setting back out to true wide open throttle.
If you've got a later model rig with an electronic engine that communicates directly with the trans TCM, this is not an issue at all.

[Piker]

Originally Posted by dbarton291

Yes. If you never go to full throttle physically on the pedal, the autocal function of the TPS will eventually be giving you full throttle shift points at whatever the maximum travel the linkage has actually achieved.
You can either do the ignition cycling thing or just simply move the throttle to wide open while driving.


Manual shifting doesn't have anything to do with the autocal on the TPS. It's all about moving the throttle pedal linkage.

Thank you for the insight!

It makes sense of what I'm experiencing... when we take a long drive through the mountains at a leisurely pace, I'm pretty conservative with the throttle, especially on the downside of the pass, where the throttle might not get used hardly at all. This would probably not be a big deal, but we stop a lot... which would keep shrinking the WOT throttle setting every time we shut the rig off and turn it back on. The manual shifting is just another variable that effects the use of the throttle.

It almost seems like it would then be a good practice, every time I start the rig, to first turn the key to the on position so the TCM lights up, cycle the throttle pedal all the way to the floor and back, and then start the engine. Does that make sense?

-cheers

[dbarton291]

Originally Posted by Piker

Thank you for the insight!

It makes sense of what I'm experiencing... when we take a long drive through the mountains at a leisurely pace, I'm pretty conservative with the throttle, especially on the downside of the pass, where the throttle might not get used hardly at all. This would probably not be a big deal, but we stop a lot... which would keep shrinking the WOT throttle setting every time we shut the rig off and turn it back on. The manual shifting is just another variable that effects the use of the throttle.

It almost seems like it would then be a good practice, every time I start the rig, to first turn the key to the on position so the TCM lights up, cycle the throttle pedal all the way to the floor and back, and then start the engine. Does that make sense?

-cheers

Hey. Not a bad idea. It does make sense to try that.

[deandec]

Originally Posted by dbarton291

I think you are over thinking this. The discussion on vehicles with a mechanical TPS only has to do with the autocal of the TPS.
Drive your vehicle as you normally would.

The discussion we are having over autocal relates to a situation where the operator of a light vehicle may never use wide open throttle. Eventually the TCM will interpret the maximum position the TPS sees as wide open throttle and utilize wide open throttle shift points (which are higher than closed and partial throttle shift points) which may cause a delay in shifting if the operator doesn't step on the gas. The remedy? If it doesn't shift, step on the gas! Then the engine increases RPM, hits the shift point and autocal moves the wide open throttle setting back out to true wide open throttle.
If you've got a later model rig with an electronic engine that communicates directly with the trans TCM, this is not an issue at all.

I had not thought of 31,000 lbs being a light vehicle!

I am the OP.

When the shift did not occur even taking the RPMs to 2300 in 3rd gear I did not get a shift result.

I think I got my needed answer from Piker so will leave it at that.

[deandec]

After a couple of short road trips, the procedure of turning on the ignition and pressing the throttle pedal to the floor with a slow release seems to solve my shift sequence problem.

Thanks to all who assisted with this issue.

[Piker]

Excellent!

-cheers