I purchased a 1998 2500 12 valve high milage flatbed truck for farm use 6 months ago. This truck has high milage and has an Allison 545 transmission (installed last year). The Allison 545 is an automatic with 4 forward speeds. The down side of the Allison is it doesn't have a Park setting and with the oversized rear tires I get a top speed of 62 mph, but ok for my use.

Problem: When slowing down to a stop, the transimission shifts abruptly (hard) from 2nd to 1st. -----Shifts to next higher gear for all speeds 2100 rpm. ----Shifts down from 2nd to 1st at 12mph. fluid is clean, no leaks.

The truck is currently in use. I realize there maybe a number of issues that could cause the problem. Just looking for easy, more common solutions first.

Ok, one more question: Is their an easy fix to get a higher road speed (like change rear end gears)?

 

That tranny is bad for that, it was built for medium duty trucks with a gas engine. I always prepare my self for those violent shifts. Is it the one with the cable modulator or is it electronic? also they are known for weak torque convertors.

 

Thanks Jack for your reply. I don't know if the modulator is mechanical or vaccum, but I will find out and post.

 

Your symptoms sound about right for a medium duty trans in a light duty truck. The light weight of your truck is magnifying the firm shifts. I am not a gasser mechanic so I can't comment on the gas engine application but a lot of these trannies were used in medium duty chassis behind diesels such as Cummins 6B, and 4B, T444, T444e, 3208 n/a. Yes lower numerically diff gears will give you a higher road speed, and to a lesser extent larger diameter tires will too. This trans is about maxed out torque and H/P wise behind a PSD or Cummins 6B, but since your truck is so much lighter then the intended application you could get by with some more h/p and torque. As far as I know all 545AT's were cable modulated and had a lock up t/c, Allison went electric modulation with the New World trannies. None of the AT, MT, or HT series had a park pin due to the GVW of the intended chassis was too heavy and would shear it off easily. That is why the Hyd. braked medium chassis had a drum parking brake mounted on the drive shaft either mechanical or air actuated. And of course the air brake version had the "Maxi" park brake.

 

Thanks Ggg, I can't find any vaccum lines so guess it must be cable. I have some information on the Allison but it's abreviated and doesn't address the modulator.

I was thinking about adjusting the shift point (2 to 1) in an effort to soften the hard shift, hmmm. Sounds like it may not help. stihly

 

You probaly have a cable modulator valve it's easy to tell by looking at the valve at the side of the transmission. It will have a cable or wires coming out. Ggg is right it will shift harder or more positive because of what the trans was built for (heavy gvw). I have had problems with the the 545 if the modulator cable is broken or misadjusted.