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Accumulator Function in an Automatic Transmission

Accumulator Function in an Automatic Transmission

Accumulator Function in an Automatic Transmission

I reciently had to open up a MOPAR 727 to replace the reverse band which had been damaged while backing a horse trailer up an incline with low fluid, (my wife did it but it was my fault). Upon removing the valve body I found the accumulator spring missing. Now at first I thought that it might have fallen out and rolled under something but a check on the position of the accumulator piston said otherwise, it was in the fully extended position right at the top of the casing. My question is; how does this affect operation as the only unusual operation noted has been a harsh manual 2-3 shift at light loads, (with a driveline clank). My guess is that it should even out pressure spikes and dips.
Could this be a "Hot Rod Shift Kit Trick"?
The prior owner had the transmission professionaly rebuilt.

RE: Accumulator Function in an Automatic Transmission

I think you are correct, in that, the accumulator spring's purpose is to absorb pressure spikes. From my days of Hot Rodding in high school, the stock accumulator spring was replaced with a stiffer spring. The stiffer spring then required the transmission to build a higher pressure to move the piston before shifting the transmission. The heavier spring resulted in harder shifts and I believe that the shift point was also delayed (primarily a function of the modulator). I would at least install the factory spring, but since you pull a horse trailer or do some towing, I would consider a "shift kit" from a local performance shop or auto parts store to help you get up and down hills with a trailer in tow. A manual modulator may also help as well to allow the optimal performance when trailering. I have been away from the Hot Rodding for nearly ten years, but my knowledge from my present job with hydraulics tells me that the missing spring allows the piston to be bottomed in the bore and no spike absorption would occurr.


RE: Accumulator Function in an Automatic Transmission

Many, Many, Many, years ago, when I overhauled transmissions for a living, we used to take that same accumulator and turn it upside down with no spring.  The only purpose was to give a crisper shift and to help reduce band slippage as pressure began to build (accumulate).  Since the 727-B transmission has no modulator and relies on throttle pressure for shift harshness and the governor for shift speed, it was not uncommon for some part throttle clank.

Some of the performance shift kits rely on spacers, plugged passageways, drilled spacer plates, reversed pistons, milled out valve bodies, and so on.

The automatic transmission is either one of the simplest or most complicated devices today, and I take my hat off to the first engineers who designed a good production model in the late 40's (I know there were ones earlier, but really good models didn't come out until the late 40's).


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