transmission Slamming

[Sandflea]

I pull my 12,000# FW 90 miles to Silverton, Colorado several times each summer. Last weekend, after cresting 2 different passes above 10,000', I manually downshifted the 48RE into 2nd as I started downhill, as always. Usually it shifts smoothly, with the revs coming up, slowing the rig down along with the help of my PacBrake. This time, however, nothing happened for a couple of seconds after putting the gear selector in 2nd. When the transmission finally downshifted, it slammed very hard when the revs came back up. This happened twice coming down off both passes.



transmission temp (at the pan) was around 225 at the top of each pass, which is not unusual when I pull these hills, but drops back to 180 on the flat stretches (always in Tow Haul mode). I left the FW at the campground, and drove home for work Sunday night. I haven't noticed any shifting problems since then. I'm heading back up to Silverton later this week, and will eventually cross both passes in the other direction, pulling the FW home after Labor Day. Any ideas what's causing the very hard shift into 2nd after having no problems before? I've got 117k miles on the truck, and only pull the FW 6-8 times per summer.

 

[LNaugle]

stock trans? aftermarket pan?

 

[fox]

Check the front band adjustment.

 

[BGlidewell]

Check the front band adjustment.

super easy to do too. unlock the lock nut and count the turns until resistance, then back it out a 1/4 turn IIRC. (count the turns in just so you can get back to where it was before you messed with it if need be)

 

[Sandflea]

No mods to the transmission, but the pan is the standard size Mag-Hytech. I'll check the front band adjustment. Thanks for all the help!

 

[cerberusiam]

super easy to do too. unlock the lock nut and count the turns until resistance, then back it out a 1/4 turn IIRC. (count the turns in just so you can get back to where it was before you messed with it if need be)

This is why there are so many disputes about band adjustment, and, why most builders reccommend leaving it alone. Adjust the band incorrectly and the transmission is trashed in a short time.



Correct procedure is tighten the adjustung screw to 75 in lbs then back it out 1 3/4 turns. This also assumes the lock nut is backed off far enough and the threads are clean on the adjuster and case, the band strut has not bent, the ears on the band have not bent or twisted. Changing out internal parts or shift kits can change the backoff as much as a 1/4 turn so its not really an exact science but will definitely cause issues if it is done wrong.



The 225 on the probe is not that bad but it is hard telling what the ECU saw. It was likely enough to trigger it into its fail safe mode and cause the hard shift. If it only does it after long hard pulls it is probably normal. Possibly changing the transducer could change the operation but that is hit or miss.

 

[Sandflea]

Thanks, Cerb. This is the first time over these passes that it slammed so hard into second. I'll see what happens on the way back after Labor Day, and check the band adjustment per procedure.

 

[BGlidewell]

This is why there are so many disputes about band adjustment, and, why most builders reccommend leaving it alone. Adjust the band incorrectly and the transmission is trashed in a short time.

Correct procedure is tighten the adjustung screw to 75 in lbs then back it out 1 3/4 turns. This also assumes the lock nut is backed off far enough and the threads are clean on the adjuster and case, the band strut has not bent, the ears on the band have not bent or twisted. Changing out internal parts or shift kits can change the backoff as much as a 1/4 turn so its not really an exact science but will definitely cause issues if it is done wrong.

The 225 on the probe is not that bad but it is hard telling what the ECU saw. It was likely enough to trigger it into its fail safe mode and cause the hard shift. If it only does it after long hard pulls it is probably normal. Possibly changing the transducer could change the operation but that is hit or miss.

thanks for the correct procedure. I probably should not have posted, but from my personal experience with over 30K on the first (and) last band adjustment my transmission is still shifting good and running cool. I used a calibrated torq wrench (snap-on) and verified at least in my case that the same could be achieved with the method used without the torq wrench. Probably not the best way to do it. But as you state there may be more wrong and a band adjustment would not resolve, might event hurt things.

 

[cerberusiam]

I used a calibrated torq wrench (snap-on) and verified at least in my case that the same could be achieved with the method used without the torq wrench.

There is nothing *wrong* with how you do it, I seldom use a TQ wrench either, but the problem is the definition of "snug". Can and will be different for you, me, and the OP which will affect the outcome of the procedure.

I just pointed out the correct proceudres as a common reference point for clarity. The reality is there is probably a 1/2 turn fudge factor built into the adjustment so your snug plus 1 1/4 turn is likely the same as 75 in\lbs and 1 3/4 turns. Can't rely on that for ever transmission though.