4R70W Rebuild Diary - Part 4 - Servos and Accumulators | Ford Explorer Forums - Serious Explorations

  • Register Today It's free!

4R70W Rebuild Diary - Part 4 - Servos and Accumulators

Not open for further replies.


EF Tranny Guru
Moderator Emeritus
February 8, 2003
Reaction score
City, State
Sacramento, CA 95827
Year, Model & Trim Level
1992 XLT
Well for many of you this is not your first look into an automatic transmission, maybe because you read the A4LD Diary or because you have been there on your own. ALL of them (ok maybe with the exception of the new CVT models) operate in more or less the same way... and part of that is band apply servos and accumulators.

The 4R70W has an OD band apply servo, and a low reverse servo as well as two accumulator pistons. (1-2 and 2-3). These are in the case, under the valve body.

The low reverse servo is a HUGE thing under a snap ring retained cover. All it needs is a new one with soft rubber on the seals. I'll show you what I mean

The OD servo on the 4R70W is a curious thing. The AOD begat the AODE which begat the 4R70W...(sounds almost biblical, eh?) Now, back in the old days of the AOD, you had some serious choices of increasing size of the OD servo in the AOD...but that is not true in the 4R70W. Your choice is very limited.

Stock 4R70W OD has a servo about 2.5 inches dia inside the case and in a sleeve under a snap ring under the valve body. The best I could do aftermarket was an increase it to 2.7 inches diameter, and without a sleeve. I'll show you my quest. Our limit on this is the size of the case bore.

Accumulators have been a problem, with numerous aftermarket and manufacturer fixes. We will look at those too.

This thread will be short, but fun.

Join the Elite Explorers for $20 each year.
Elite Explorer members see no advertisements, no banner ads, no double underlined links,.
Add an avatar, upload photo attachments, and more!

OD Servo

Well the OD servo, stock, is a manufactured rubber edged servo that goes into a sleeve. Here is a look

(pic forthcoming)

Here is a look from above of the old and sleeve and the new piston


Here it is side by side with the upgrade I selected:


The upgrade is .2 inches bigger.... here is the measurement, first, stock


and upgrade


The upgrade is an aluminum piston which is rather thick and which uses a replaceable teflon sealing ring. Stock is one piece and is replaced as a unit.

I took the upgrade one step further. I added a Sonnax servo pin with O ring seals on it. Here is the naked pin with the O rings in place (Stock has no seals whatsoever).


One of those O rings seals the pin bore.... the other seals the cap. Neat idea.
You reuse the spring and hardware... assembly required use of pliers to get the shaft retaining ring in place by compressing the spring. Not hard, just a little tricky by ones self. Here is the final result:


So, a modded mod. Two things... notice which way the lip seal faces. And the little washer I left out can be used to increase spring tension for "firmer 3-4 shifts. I declined. Time will tell. Oh, you may have noticed the empty lower ring on the piston. A teflon scarf seal goes there, but it would not stay on for pics, so I left it out. It will be there on assembly. So the piston has 2 seals instead of 1. Nice touch Superior...BUT I have an issue. The scarf seal properly needs a seal compressor/installer to install OR remove that poston without damage. Shame on you for not realizing that. Frankly, in retrospect I think a D ring seal would have been a better choice than a scarf cut teflon one. Lacking a seal compressor from the aftermarket gurus I made my own out of paper... and it worked fine. thought I hate having to out-think the aftermarket engineers. Grrrr.

(pics coming)

Btw. The piston came from a Fairbanks (Superior) "transaction" kit. The O-ringed servo pin was Sonnax. A modified Mod. Who'd a thunk it?

Low Reverse Servo

Sorry guys, no upgrades here. But the stock servo is massive! It fits in the case under a cover held in place by a snap ring, and situated under the Valve Body.

Here one is next to the upgraded 2.7 inch OD servo:


Here are some pics of the servo and the cap and snap ring:



All we really need to do here is buy a new servo. It comes as a unit. The main thing we need to worry about is that the old one will undoubtedly have hardened. The new rubber should be supple.... witness:




We will need to install this once we get the band in place.... it is required to hold the band.

Accumulator Pistons - Choices, choices.

This is Part 4 of a series. Part 3 is here: http://www.explorerforum.com/forums/showthread.php?t=157376

Part 5 is here: http://www.explorerforum.com/forums/showthread.php?t=157440

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

[Note, this section is under major construction... I am exploring some things I am not sure of so have not posted. Stayed tuned for possible major updates!]

Accumulators. Places in the automatic transmission where shift points are "attenuated" or softened.... so the next gear apply is not a sudden jarring grab.

In electronics these would be akin to capacitors.... for those of you with that bent or understanding.

The 4R70 uses two BIG ones. one for the 1-2 shift, and one for the 2-3.

FORD has changed the stock pistons on the accumulators during the metamorphisis of the AOD to the 4R70W. The stock offerrings now are actually pretty good. In fact the 1-2 FORD one now is so good I did not find an aftermarket offering. Here is their 1-2 accumulator piston:


It is FORD part No. F7AZ-7F251-AA and lists for $13.58.

As for the 2-3.... here is the stock model....

Pic forthcoming

But compare it to the Superior aftermarket offering:


(no seals added yet to this piston)

Here they are side by side. (Superior one came from the TransAction kit, and I have not put seals on it yet.) FORD's offerring comes as their part no. F7AZ-7H292-AB for $7.02. .com[/url] since I got most of my parts on E-bay.

Pic forthcoming

The 2-3 piston goes into this bore in the case:


The 1-2 goes here (I should post a disassembled pic, and will later)


So, for accumulator upgrades, that's it. I have no issues with just staying current FORD stock. The aluminum one from Superior is for high performance usage, and i am not thinking nowthat is where this tranny is headed. I may have it for sale later <g>.

The next part, Part 5, is here: http://www.explorerforum.com/forums/showthread.php?t=157440

If you have questions, comments, ideas or just feel an overwhelming need to post about this thread, please go here ...


I am trying to make it easy for folks just wanting basic information. The questions etc can have their own thread. Thanks for understanding.

Not open for further replies.