1998 K2500 4L80E Torque Converter locking up in 1st and Reverse enough to stall engine.

[fivezero]

The truck is a plow truck and has been all its life. I only use it to plow, and it never leaves my property so it lives in 1st and Reverse almost all the time. Lately, the torque converter has been locking up while in 1st and Reverse both. I can feel it slip into engagement and if I try to stop I have to hit the brakes extremely hard. This then stalls the engine. I have taken to having to shift into gear to move and putting it in Neutral when I want to stop. I have disconnected the TCC lockup input line from the PCM so my problem appears to not be electrical. I presently have the valve body off and am replacing the TCC solenoid and the TCC regulator valve with a Sonnax replacement. I am really at a loss as to why the converter is locking up when it shouldn't. When I pulled the valve body one of the gaskets ripped so I am waiting on a replacement to see if that fixes it. The only other thing I see I can replace is the torque converter clutch valve and spring. Anyone have any suggestions? This is a pos so I am nearing the point of parting it out if this can't be fixed without replacing the whole trans. As it is if I take out the trans the cab will have nothing to support it. Any help is appreciated.

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[Rock Hard Concrete]

I have a 1994 Isuzu NPR Box Truck with a 4l80 that started doing the same thing when cold a couple of years ago. From what I can tell, there is a seal that fails when the fluid is cold, allowing pressure to the lockup clutch. I noticed that if I let the truck warm up for a long time, it would work. Reverse was also going out on that truck.

I was able to use that truck for another year and a half like that, but the whole transmission needs replacing now. Second gear and overdrive no longer work. Doesn't help that it's a Jasper trans either so I'm sure it wasn't put together properly from the get go.

 

[fivezero]

I got it all buttoned up yesterday. I can say getting warm did not help and the rest of the trans is in good shape. No slipping. When I pulled it into the shop it did the lockup thing right away. When I pulled it out, I drove it around a bit, and it seems my replacements fixed the problem. Again, I replaced the TCC solenoid and the TCC regulator valve and spring. The new Sonnax replacement has a little seal on the valve as a usual problem is a worn bore that bleeds pressure. This causes a lack of lockup or weak lockup. Looking at the hydraulic schematic, the regulator spring holds the valve shut. This is the off condition for the lockup so if the spring was weak or broken it could allow the lockup when not commanded by the computer. Also, the TCC solenoid could also have failed in the open allowing lockup. The solenoid applies pressure to the regulator valve which in turn compresses the spring and allows lockup. Then the solenoid closes the spring pushes the valve back into the closed position. My old regulator spring was not broken and did not feel weak, so I think the solenoid was defective and causing my problem. I document for anyone else having a similar problem in the future. Problem seems to have been fixed.

 

[tayto]

on a 4L80E there is no fluid supplied to lockup in 1st or reverse, only 2nd,3rd & 4th. i suspect if the unit is indeed fixed it was more likely from the new valve body gaskets vs the solenoid or valve you replaced.

 

[tayto]

bleh, let's get into this. for starters in 1st gear w/ shifter in OD position DRIVE oil is fed from the manual valve to the 1-2 shift valve. if actually in first gear the 1-2 shift solenoid is ON therefore not allowing DRIVE oil to turn into 2-3 DRIVE oil which turns into FILTERED 2-3 DRIVE oil which feeds the TCC solenoid valve. Let's say the 1-2 solenoid was OFF: you wouldn't be in 1st gear anymore, so fairly apparent... Let's say the bore was severely worn or valve had been modified to allowed DRIVE oil to goto the TCC solenoid valve and 1-2 solenoid was ON. Then let's say the TCC solenoid was indeed stuck open. You could potentially have lockup in 1st gear but I bet highly unlikely as there would be other issues such as the front band trying to come on (2-3 shift valve is OFF in 1st gear, 2-3 DRIVE oil feeds front band). If there were crossleaks in the valve body they'd have to be into both the TCC SIGNAL and REGULATED APPLY at the SAME TIME. TCC SIGNAL is needed to move TCC ENABLE VALVE which cut off TCC ENABLE oil to the backside of the converter clutch shift valve and then allows the same TCC SIGNAL oil to engage the converter clutch shift valve (this is called an interlock) which would then then allow the crossleaked REGULATED APPLY oil to feed the torque converter. If only one of these circuits was getting fed by a crossleak it would NOT engage the TCC, it HAS to be both. OR TCC solenoid stuck OPEN and crossleak into TCC SIGNAL oil. Again possible but very unlikely. I suspect there is a crossleak/worn valve in your pump or problem in the converter.