Tranny SLAMS into gear! (4L85E)

K2500_Sub496 · July 15, 2019

Okay guys, I've never even though about touching a tranny before, but it looks possible. I'm giving it everything I have. Help me out!

Just bought this 2002 Chevy Suburban K2500 with the 8.1L. Here's the problem. When it's time to shift a few things happen, basically it THUDS it hard into gear.

It goes into 2nd gear around 23mph on the speedometer. If I'm over like 2K RPM, it won't go in, it'll just rev until I let off the gas and then it goes into gear. If I do it right and let the RPMs get to idle at the right time, it'll shift PERFECTLY. But if those RPMs aren't very low, it'll SLAM it into gear. From 1st to 2nd is the worst and it gets less severe as the gears go, but it's there all the way up.

Technically, I can let off the gas at the perfect time and it shifts perfectly and you'd never know there's a problem.

Next, CEL is on, whatever the code was for Solenoid A being slow (but it's on both shift solenoids, so no help there. No other codes).

I CAN HEAR SOMETHING! ONLY when it's time to leave 1st gear (like 21 and 22mph) and ONLY when I'm giving it like... 50% throttle.... It sounds like a high pressure suction that is getting muffled. Like trying to suck too little fluid through a tiny straw. But I can ONLY hear it at that speed in that gear, and I'm not even positive it's not like a belt or injector or something. But I THINK it's tranny.

Tranny fluid looks fine, reddish. Not bad. But the dipstick while running shows VERY far over the hot line. I just bought it. Not sure what's up.

So if I drive like grandma, never giving it a lot of gas, I can go from 0 to 100 without any symptoms. But if I drive it like normal, you'd think I just dropped the tranny when it slams it into 2nd. It feels like I could drive it like grandma forever without it getting worse, but who knows. It hasn't really chnaged or the noise really gotten louder in the last few hundred miles.

DOWNSHIFTING doesn't seem to be effected. So if I'm doing 65 and want to pass someone and double pump the pedal, it downshifts perfectly and I go pass the car. So the problem is not here when downshifting. (I haven't tried from 2nd down to 1st, because it feels like the tranny will drop, haha.

When parked and I start the car and then put it in reverse, I think there's a good tranny DING as it goes into reverse. Not sure if that's normal or related. Just seems to be a little more than it should. Feels more like my 1995 4L80E and not this generation. But could be unrelated.

No slipping. No studdering. No shaking.

Manually moving the shifter (to see if it's the PCM being stupid) does the same thing. The slams it into gear whether the computer does it or if I do it with the gear selector. I just put in a new TPS, no change.

Any experts here know what it is? Anyone think it'll be complete tranny swap time? What about the pump? If it's something that requires dropping the tranny, I won't be doing it.

Here's the plan... I'm not a mechanic, but I'm catching on pretty well. Going to watch more youtube videos and then do the following...

Drop the pan, get all the fluid out. See how much fluid was in it. Look at the fluid and bottom of pan. Look at filter. Then swap out these parts...

Shift Solenoids A and B
EPC Solenoid (Pressure Control)
TCC Solenoid (Lookup)
Manifold Pressure Switch
Input and Output speed sensors
Accumulator Piston (pinless ones?)
Wire harness
Filter

Not 100% that I can do all of that, will see after watching videos. Have a great parts store here with all variants of Delco parts. But I'm starting to think that this won't fix it.

Thank you in advance guys!

Formulabruce · July 15, 2019

Mileage on this transmission?

Things to keep in mind.

1. Changing a solenoid that relates to the code may not actually fix the problem because these solenoids slide into a "bore", and if the bore is worn, you can get fluid bypass. When you have bypass, it can cause a late shift and a slamming effect. The wiring harness does get brittle.

What you propose doing might be a good place to start, but keep in mind other hard parts could be worn.

USE the ORIGINAL PAN GASKET. Its rubber and has a Metal plate in it. The aftermarket are thin paper and are not nearly as good.

If it doesnt have an original gasket, I would get one from GM. No need for RTV either.

The bolts under the cross member make taking the pan down a bit tricky.

2. Could be the related clutches are worn.

3. Could be the previous owner had the problem and did a Transmission service before selling it.

4. There was some updates done on the 4L series in 2004 in the Valve body.

5. IF you end up getting a rebuild, I would buy a NEW and updated valve body ( so you are SURE the bores for the solenoids are perfect. Cheap rebuilds don't usually mess with the solenoid bores.

6. Get a New SONAX BOOST valve for the Pump ( front of transmission, not underneath). The pump can have wear in this area as well and you can get uneven pressure with a worn boost valve and or housing for it.

7. On your rig, behind the grille, there is plenty of room for a much larger transmission cooler. For under 60 bucks and 1.5 hours , you can double the size of the cooling capacity of the transmission.

Good luck !