wicked_sten

i rebuilt a friend's 4l60E a few months back because of a 3-4 clutch failure, ended up rebuilding a completely different core trans so we could swap the new one in right away.

He brought it over last weekend because it was doing the same thing the first trans did, free revving when shifting to 3rd, then bump past to 4th (2-4 band only engaging)

we put it up on the hoist and tested the solenoids under load by back probing the harness while the trans shifted through the gears. they all checked out good along with the force motor and pressure manifold. Next dropped the pan, the fluid was slightly tinted darker than normal, with minimal debris in the pan.
(which leads me to believe the 3-4 clutch is ok). pulled the valve body to find the 1-2 accumulator piston cracked, so we ordered the sonnax pinless billet upgraded one along with their forward accumulator piston to eliminate all the plastic pistons. We also installed the sonnax servo release check valve. the 2-4 servo assembly and valve body was disassembled and inspected. no hung valves or check *****, and the seperator plate and gaskets looked great.

We put it all back together and took it for a road test and it shifted perfectly through the gears leaving the shop, then again a mile up the road at the stop sign. we went about 3 miles down the highway and turned around to head back to the shop, again shifting through the gears flawlessly. when we turned down the last stetch of road the problem came back, first to second fine then free revving in 3rd and the trans sees that 3rd isn't working and bumps it to 4th (band only)

what else could this possibly be aside from a bad apply piston?

mrvedit

First I would put a pressure gauge on the trans, preferably while you are driving it.
It could be erratic line pressure that is causing the intermittent 3rd. The pressure control solenoid (PCS or EPC) are known to stick and cause intermittent pressure problems. A good build replaces it with each rebuild, just to be sure.

I know you used a new core, but the 3rd accumulator check valve inside the servo area are know to also stick and/or leak, which results in 3/4 clutch slippage and failure.

wicked_sten

i forgot to mention i did check line pressure when we were running it up on the hoist, at idle it was around 110 psi. and on the throttle in drive it was about 150 psi. through 1st and 2nd. when it tried to shift to 3rd the needle snapped back down to 100-110 psi. idling in reverse was about 175psi. all the line pressure charts i found were pretty vague, they said:

drive, park or neutral 55-189psi
reverse 64-324psi

mantycarguy

I agree with what mrvedit has said in that a leaking 3rd accumulator check ball capsule could be the cause of your erratic 3rd gear issue. If it was a stuck pcs it would likely cause other problems to surface. Looking at the pressure readings you posted it appears to have a leak in that circuit as your pressure drops back to 100/110 lbs which is way to low to apply the 3/4 clutch pack with any kind of force. I do replace the capsule on all of my builds as these are a known problem.You will need to remove the valve body and the servo to remove the capsule.

mrvedit

I'm glad a pro builder like mantycarguy has chimed in. While I defer to his greater experience, I will point out that replacing the PCS is much simpler that replacing the 3rd accumulator check ball capsule and you might get lucky.
In case it wasn't clear from his post, you DO NOT have to remove the trans to replace that capsule.
If you need, I can probably find my previous posts on directions for removing and installing the valve body. There are also tricks for removing the capsule and each builder seems to have their own method.

wicked_sten

thank you for the replies fellas! I've replaced the capsule in the past on other units. i will try the PCS and 3rd accumulator check first before I commit to removing and tearing the trans down. I wanted to make sure I've ruled out every other possibility before doing so. thanks again

wicked_sten

well that didnt do it, pulled the trans today and will report back with my findings

mrvedit

Sorry to hear of your troubles.
For future reference and other readers, several ebay sellers have a "4l60E test plate" (typically about $70) which can be bolted in place of the valve body for a thorough air test of all the clutches/band without having to remove the trans. Of course, most of the time, it will reveal a leak and you will have to remove/open the trans.

Also, a member I have been helping via email rebuild his trans reported that he lost 3rd gear after just 100 miles. He then disconnected the trans connector and confirmed the car was in 3rd gear. As expected, he then also measured line pressure of 250 psi (with Sonnax or Transgo boost valve). He then reconnected the connector and the trans again worked perfectly. I took a wild guess that a valve was stuck in the valve body and the high pressure freed it; I do not know if he replaced the shift solenoids.

mantycarguy

As long as you have it apart look for a torn apply piston seal on the 3/4 clutch or a leak between the input shaft and the input drum.Its possible to air check the input shaft by putting a slight amount of trans fluid around the area where the two meet and applying air through the feed hole in the input shaft.