Slipping Lockup

[Kevin Sherrell]

He guys. First post, longtime lurker. Just wanting to see if I’m on the tight track here. Truck is a 1998 5.7 4l60e 4wd. I bought the truck fresh out of an engine rebuild @ 265,000 body miles, but I have no idea the age of the transmission or internal parts. My brother in law that rebuilt the engine told me it had a “smaller sized torque converter,” but didn’t know anything else other than that about it (it holds a brake-stall at 2,800). A couple months ago I installed a new second servo because second was wanting to slip a little at fullish throttle. That fixed that.

Anyway, on to my current problem. It has always had this little slide-bump kinda lockup feel, but it’s always locked. A few nights ago I had to stand on my brakes on the interstate to avoid an accident and when I got back up to speed, I noticed the torque converter wasn’t locking and the shifts were crazy hard, so I knew the PCM was commanding higher pressure. After stopping and restarting, the shifting pressure seems to be back to normal, but it’s not locking up. The RPMs will vary by around 500-600RPM when it should be locked up, but it still feels like it’s trying to lock up because I can tap the brakes and can see/feel the RPM’s jump a couple hundred RPM. It’s as if it’s acting like a tight low stall torque converter when it’s supposed to be locked up, then acting like it should unlocked.

I’m thinking a bad TCC valve may be the culprit. I have one of those Teckpak Fitzall replacement valves I never got around to installing.

I’m also thinking it could be the TCC itself because that “smaller size converter” may not have been one that could hold up to PWM style lockup.

Can y’all think of anything else it could be before I pull the pan off?

 

[Kevin Sherrell]

Well, the really hard shifts came back this morning. It doesn’t even try to lock up when the shifts are hard. I’m assuming the computer sees it slipping, so it stops triggering lockup and increases the pressure. Is that right?

 

[Supercharged111]

I'm not sure how the 4L60 detects slip. I know the 4L80 has an input and output speed sensor. Could be that it perceives the higher stall converter as trans slippage.

 

[Kevin Sherrell]

I'm not sure how the 4L60 detects slip. I know the 4L80 has an input and output speed sensor. Could be that it perceives the higher stall converter as trans slippage.

I don't think the PCM looks for the slipping when the converter is unlocked. I think it just looks for slip during lockup. It would still solidly lock up before, even with whatever converter is in there, but it's all over the place in lockup now when it tries to lock, but then it's like it shuts down any attempt at lockup after it sees the slipping. I'm going to try a fluid change and the Fitzall valve this weekend and see where that gets me. I'm going ahead and making the assumption that I will have to replace the torque converter after that doesn't work. My bet is the torque converter wasn't one made for PWM lockup and the clutches are gone in it. The Fitzall valve bypasses the PWM solenoid, so I'll get a torque converter specific to on/off lockup.

 

[Kevin Sherrell]

Update: A friend and I installed a Transgo SK this morning. Fluid looked good. Install went good as the instructions were very thorough. We did find that the TCC valve bore had already been reamed and had a Sonnax valve in it. We installed the TCC valve that came with the SK into the sleeve and it fit like a glove. I love the way the trans is shifting now.

The crazy thing is that lockup is going the same thing it was before. It’ll try to lock up, but a little throttle (cruising throttle) and it climbs around 500rpm. When I let off or ease up, it moves back down, but never fully locks... After doing that a few times, it gets smart and disables lockup and I’m running with a “non-lockup” converter.

Is the converter the only think left that could be the problem or am I missing something simple?

 

[Kevin Sherrell]

One thing I have noticed since I installed the shift kit is that it takes longer to go into gear from park or neutral. Before, it was fairly quick, but now it takes a good 2-3 seconds... Any ideas on that?

 

[Biggershaft96]

Kinda sounds to me like the 3-4 clutch pack is getting wore out. Mine did almost the exact same thing starting about 2 months prior to them actually going out. I first noticed it when i was pulling a heavy trailer and had to stick it to about half throttle (the position about right before it would downshift to 3rd). It would also command max line psi just like yours. Im not sure about the lag going into gear though, mine still always goes into gear when i pull the lever.

 

[Kevin Sherrell]

Kinda sounds to me like the 3-4 clutch pack is getting wore out. Mine did almost the exact same thing starting about 2 months prior to them actually going out.

I haven’t thought about that possibility, but it makes sense. A couple months ago, I had some slipping in second and put a corvette servo in to correct it.

A couple days ago I checked the fluid and saw I was a little low, so I decided to put a quart of Type F in it to top it off. Almost immediately the lockup mostly came back. I got to thinking back and realized I put in 2 quarts of synthetic Dex 6 right before I lost lockup. I’m starting to wonder if this aftermarket converter requires Type F and I’m the one that caused the problem by using synthetic Dex 6.

It’s a long shot, but I’m going to drain my pan and refill it with Type F. If that doesn’t fix it, I’m replacing the converter.

 

[Biggershaft96]

Probably wont hurt to try that.