For the last two days, my 6 speed Allison has been acting oddly. It seems to downshift very hard. I also have to push the mode button to get it to shift into 6th gear. Otherwise it's ok. I checked the tranny fluid and it was within a pint of full. I will also check my brake buddy to see if it is set too sensitive and is pulling on the Barth when it brakes. I did have the TPS cable fall off earlier in the trip as the cotter pin failed but that got fixed. And the fluid just got changed. Any theories???

I sounds like the TPS is not adjusted correctly, typically if the TPS low is out of range, it will not shift to 6th unless you put it into "economy" mode. It should have a message on the display if the TPS is out of adjustment. You can get the diagnostic code by holding down the shift up and shift down buttons together and then turn on the ignition to run position. Don't start. The diagnostics will run thru various codes. I will scan my chart from Allison and post here later this evening.

If you hold these buttons for 10 seconds or more it will beep and reset the codes.

There is a certain range that the TPS needs to work over from idle to full throttle, It is usually adjusted by monitoring the "TPS count" from the transmission diagnostic port. I have found that if the voltage is about 0.9V at the center port of the TPS it is close enough to be in range.

Hey Ed,

Thanks! Can I run like this or do I need to get the adjusted immediately?

Hey Ed,

I did the test and here's what I got:

first d 1

second 2 1

third 1 2

Richard

Hi Richard

YUP, that is the code for

"Throttle position sensor failed low"

That means that the adjustment needs to be pulled up a bit to raise the voltage.

Chances are, the linkage is very worn and it now, after repair, is just a bit long so the TPS sensor linkage is not tight enough. I hope this is clear.

I drove mine almost 600 miles from Washington State to California, I was told that it should be OK by a Allison mechanic before I left. (they wanted $400 to $600 to rebuild the linkage) It cost me all of 0.30c for a washer to bring the TPS sensor back into cal.

HOWEVER---

That linkage finally failed and I have since rebuilt it again, failed in Houston, this time using a small heim joint. In Houston I was told that it wasn't a good idea to drive it long distances as the error code kinda forces the transmission into limp home mode and supposedly reduces the pressures, I have not verified this but I think it should be attended to fairly quickly.

I am in the process of scanning the booklet concerning the diagnostic codes, Should have this up in a few minutes.

Link to the Allison codes

This booklet is free for the asking at Allison dealers

Allison codes

Hey Ed,

Today the tranny continued to downshift vewry hard until we passed thru a toll booth and it refused to shift out of second. I know it meant the TPS cable had come undone again but this time the stem on brass cylinder it is connected to broke. The mechanic who took the road call actually went back to the shop and made one. My question is what is the real name for the part where the TPS cable connects to the TPS? I need to order one of those.

The part he made is wider than the original so I now shift into all 6 gears.

Thanks for your help.Richard

The TPS linkage has been a major failure point of the early Allison transmissions, Mine had failed before I got the Barth and it was repaired but the repair was not well thought out. I kluged up a repair when it broke the first time and then my kluge broke. Then I repaired it with a small throttle ball joint from NAPA, that was better but the ball joint failed so that didn't last to long. That is when I repaired it with a heim joint. It will not fail again I assure you!

ball joint repair

I believe I have some pictures of the heim joint repair but have not been able to find them. If I can't find the pictures, I will in there and take some, it won't be of the assembly during the fix but it will give you the idea.

Personally I wouldn't go back to the old system, it will fail again as soon as the slotted linkage wears thru the brass cylinder.

Took a picture of the heim joint link tonight, if need be, after my next race event (June 26), I could undo it and take pictures and write up the process, let me know if needed.

Heim joint TPS link

More information:

Here are links to the two pages in the Allison manual describing the TPS installation and electrical diagnostics. The optional slip link is the part that you were looking for. If you still want to replace that at least you have a picture of it.
mechanical installation

adjustment and calibration

Hey Ed,

Thanks for the info. Just got back to Springfield. I will show this to my mechanic to see if we can make this.


Richard