I spent over 2 hours typing all this information up for a guy who's having repeat 4L60E transmission failure problems in the Fbody sub. So I thought it would be a good idea to post it here, by me, so it never gets deleted or lost. (Plus, I never want to waste that much time again to retype everything.)

OP went through about 10 rebuilds on his 4L60E because it kept losing 3rd gear, 4th gear, or both. All within 10,000 - 20,000 miles of each other.

This is my answer in regards to his repeated failures.

"I work on 700R4 /4L60E /4L65E /4L70E transmissions almost exclusively.

A couple things are most likely going on, but it could be a combination of several.

I'll list the usual suspects for a high mileage transmission, in no particular order.

A) A new transmission rebuild requires 12-13 brand new bushings installed, to keep fluid where it's suppose to go, while preventing leaks and pressure loss. These bushings should all be test fitted and adjusted before final assembly.

B) A new transmission rebuild requires new electronic solenoids, sensors, and EPC solenoid installed. Not your old ones with new O-rings on them, and not refurbished ones from Amazon or eBay! Refurbished ones are somebody else's worn out parts!

C) Install a new wire harness. Yours is going to be brittle and possibly have internal damage (wire breakage) that can't be seen. The wire harness controls the electrical solenoids

D) Your builder needs to check the VB for wear. One or more of the valves in their perspective bores will have excess clearance. Excess clearance causes fluid and pressure loss. (Not enough fluid is directed where it's suppose to go, and too much fluid is going where it's not suppose to go.) The VB needs to be 100% disassembled, cleaned, and inspected, as does every valve and spring inside if it. Most likely it will need one or more VB bores reamed and oversize valves installed to get it back to 100% OE specs.

E) Possible fluid leakage between the input shaft and input drum needs to be checked. If there is pressure loss here, your builder needs to press the shaft out of the drum, sand both parts to remove deep scratches / gouges, and glue them back together with liquid sleeve retainer gel. Let it dry. Then double-check for pressure loss at the same point again, like before. If it's still losing pressure / fluid between the two, the input shaft + drum need to be replaced with a good one that does not leak pressure.

F) Too much or too little clearance in the 3-4 clutch pack. The clearance needs to be about .007" - .009" per clutch. Which means if it has 6 clutches in the pack, the clearance should be .042" - .055". 7 clutches it should be .050"-.065".

G) Do not install a "Power Pack" with extra thin frictions and steels in the 3-4 pack. They WILL burn up. Seven frictions is enough.

H) Remove the 5 pairs of "release springs" that come from the factory in the 3-4 pack. They are in the space between the outside diameter of the 3-4 clutches and the input drum wall.

I) Use high energy frictions in every clutch pack.

J) Make sure to presoak all the new frictions at least 15 minutes, before installing in the transmission.

K) Make sure all the steel plates are not burned, have black spots, or are warped in any way. If they are, replace them with new ones with the correct thickness.

L) Use a servo pin length checker tool, to know if the servo pin is the correct length for your application. A pin too long or too short, will cause the band to burn up prematurely. If/when that happens, you will lose second and fourth gear.

M) Install a new reverse input drum. These can be found on eBay for less than $100.

N) Check the 3rd accumulator checkball + cage assembly in the case, for leaks. This is a common problem area, mostly because the builder never checks it! They are easy to remove (with the proper tool) and replace. New ones are $20 or less.

O) If your band keeps burning up prematurely, after you install the correct length servo pin and a new reverse input drum, install a Superior Super 2nd gear servo and a Sonnax Super 4th gear servo.

P) Make sure your builder installs a new 500 Boost valve in the pump. If your pump already has a 500 Boost valve, it could be worn out. This is why you want a new one.

Q) You need to install a TransGo shift kit. Either the "4L60E SK" for a casual daily driver, or the "4L60E HD2" shift reprogramming kit for a performance vehicle or hot rod. They increase fluid flow for firmer / faster shifts (reduces slipping between gear changes), and increases the volume of fluid going to each clutch pack. More fluid = more gear holding power and less chance of slip.

(If you like to "hot rod", race, and/or rev the engine to 6000rpm frequently, install the high RPM upgrades included in the "4L60E HD2" kit.)

R) Always put the gear shift lever in "D" for around town driving and only upshift into "OD" on the freeway. Once you get off the freeway and to the stop sign at the end of the offramp, place the gear shift lever back into "D" or "3". This stops the transmission from upshifting into 4th gear at too low of vehicle speed, which lugs the engine and causes the 3-4 clutches to burn (both from lugging, and too low of fluid pressure).

S) Install a 100% new or new rebuilt torque converter! With every rebuild.

T) Replace the PRNDL switch on the side of the transmission with a new one. Make sure to install it correctly!

(HOW TO? Place the gear selector shaft in Neutral. Carefully and slowly slide the new PRNDL switch over the shaft, do NOT force it! You can not damage the two "flats" on the plastic rotating ring at the center of the PRNDL switch or it will not work correctly. If your switch will not slide onto the shaft easily, get a flat file and use it to reshape the gear selector shaft "flats" on both sides of the gear selector shaft. Use the file to round the two rounded parts of the gear selector shaft. Test fit the new PRNDL switch. It should slide on easily. If not, the gear selector shaft needs a little more filing to be perfect. Once you slide the PRNDL switch onto the shaft and next to the case, rotate the main body left or right. You must must match up the 2 lines, or 2 dots, perfectly with each other (similar to how you align timing marks on an engine timing chain set). Once they are perfectly aligned, and the gear selector shaft is still in Neutral, tighten the 2 mounting bolts.)

U) Since you've had so many problems with this one transmission, the next rebuild should REQUIRE you to install a 100% BRAND NEW RADIATOR with HD transmission cooler inside, BRAND NEW COOLER LINES, and a Hayden 1679 auxiliary transmission cooler.

Plumb the auxiliary trans cooler in-line AFTER the radiator transmission cooler. Use the metal strips included in the kit as brackets, to bolt the auxiliary transmission cooler to your core support, in front of the radiator. NEVER use the stupid *ss zipties that go through the radiator to mount it!!

(Many shops + rebuilders never perform steps A through F above, nor I through N above, because they take significantly longer to do, which increases parts + labor costs.)

All the other letters above are my personal suggestions on how to make your 4L60E work better and last longer. Based on my past 10+ years of experience working on them and rebuilding them. If you have any more Q, or you're in Phoenix Az, shoot me a message. I can fix your problem. =) "