48RE 2-3 shift flare and odd pressures

G

gunslinger

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Friend of mine just rebuild his 48RE transmission. It has electrically controlled TV. After rebuild transmissions works perfectly, except 2 -> 3 shift flares badly. It feels like there's "neutral" between the gears.

On the rebuild he put in Sonnax 2nd Gear Super Hold Servo Kit. Band apply lever is stock 5.0 ratio. He also put in Transgo shift kit and Borg Warner governor pressure solenoid.

Yesterday he measured pressures on the transmission:
  • Governor pressure seems to rise as it should until ~ 35 - 40 mph. From there, around 45 -50 mph it rockets to over 100 psi and and then stays there no matter how high speed rises.
  • Front servo pressures seems OK to me. 75-80 psi at idle, 175-180 psi around 1600rpm and 205 - 210 psi at WOT.
  • Accumulator port shows same pressure at idle, but significantly lower pressures from there on. If I remember correctly, it was around 100 psi at 1600rpm and around 170 psi at WOT.

Should the accumulator pressure follow front servo pressure all the time, and should the governor pressure follow speed all the times? Is it possible that the super servo and stock lever are causing timing issue? Or is there anything else you might suspect wrong with this information?
 
Governor pressure will be 0psi at a stop and increase with MPH during the 1-2 and 2-3 shift, once lockup is called the governor pressure command usually goes to 127psi (more than the gov circuit can make) and will usually only make 95-107psi actual governor pressure.

A 2-3 shift flair is from the 2nd gear band releasing before the direct clutch is applied. Possible causes are: 2nd gear band adjustment too loose, leak in 2nd gear Billet servo seals, 3rd gear Direct clutch clearance too loose or leak in Direct drum piston (which delays 3rd gear clutch apply).

Line pressure (accumulator port) should be same as the 2nd gear port, If there is a pressure drop once 3rd gear comes on, its a sign that there is a leak in the direct drum causing the drop in pressure.

Lavon
 
So the governor pressure sounds good after all.

He already tried to tighten the front band, so that shouldn't be the issue. Also, he was very careful doing the rebuild and he's certain that the clearances are what they should be. That leaves us leak in 2nd gear servo or leak in direct drum piston. If the direct drum piston leaks, does it necessarily mean the leak is in the drum itself? Is possible that that the leak is somewhere in valve body?

Also, I read the BigBlues thread about rebuilding 48RE. He wrote that he uses 4.2 band apply lever and only billet cover on the front servo to get best 2-3 shift timing. Is it possible that the super servo and 5.0 ratio lever are causing wrong timing?
 
The Sonnax servo is the tightest servo on the market, so usually you end up with a bind in the 2-3 instead of a flare, and the 5.0 lever is more likely to bind then flair, so your flare is not likely related to the parts used. You can remove the valve body and hit the 2nd gear servo port with compressed air using a rubber tipped air nozzle and shop air, then you can also air test the direct drum circuit also. You should be able to pinpoint a internal leak that way.

Lavon
 
Thank you for all the info! We have to air-test the tranny and hope that sheds some light on the problem.
 
He may have the inner seal installed backwards on the direct drum piston or the outer seal may have the lip twisted in a spot causing 3rd gear apply delay.
 
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