4L60E Hard shifting after cruising in 4th

Gabe1879

Newbie
Original poster
Jan 10, 2022
3
Huntertown Indiana
I have had my 1999 Silverado for about 6 months and about a month into having it I noticed hard shifting when I got done cruising in 4th gear for a while. after I get off the highway and stop it goes into first normally, but then it slams hard enough into 2nd that I get scared I'm bout to lose my tranny. then it will hit 3rd and feel like 50 pounds got dropped into the bed out of nowhere. but shifting into 4th is perfect and so is down shifting. Any ideas on what this could be, or if I'm just gonna need a rebuild or entire new transmission?
 

Reprise

Lifetime VIP Donor
Jul 22, 2015
2,449
It's likely that your line pressure isn't being reduced as it normally would be when coming out of lockup / 4th. Normally, pressure is lower when the trans is in lower gear, and rises as it progresses through the gears. But if it's prevented from dropping, and the line pressure remains too high, you get the harsh shifting in 1-2-3, as you have now.

The question will be finding out *why* the pressure is too high. There are a few causes, both inside / outside the transmission, so I won't detail all of them here, so that you don't go on a wild goose chase.
The good news is that if it's found / corrected, you probably won't need to fully rebuild / replace the transmission. If the issue is with the torque converter or the valve body, then you'll need to disconnect the trans from the engine, or at least drop the pan & remove the valve body.

As a temporary workaround, you can try the following...

After exiting the highway, find a place to pull over. Put the truck in N or P and shut it off, then restart it. You may need to experiment with how many seconds to wait (start with 10), but if you get it right, the pressure will return to normal when you restart the truck and engage the gears again, just as like when you normally get to your destination, shut the truck down, and restart it later. Cycle repeats when you get back up to lockup and stay there for a period of time at higher speed.

If you're just in full lockup for a few seconds, you probably won't experience the issue.

If that helps, you can continue to use that until you find a shop / mechanic, or you diagnose the cause, depending on your own proficiency as a mechanic.

You do have tow/haul mode in your transmission programming (button on the gear selector). That raises the line pressure by itself, so it won't solve your problem. But you could experiment with driving in tow/haul on the highway, and seeing if the line pressure drops afterward (or disengaging when you come to a stop, etc.) I will bet that it won't help. But it may be useful info for diagnosing the issue further (and / or relaying to your shop).
 
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MRRSM

Lifetime VIP Donor
Oct 22, 2015
5,710
Tampa Bay Area
This Link leads to the ATSG "Bible" for GM Automatic Transmission Problem Diagnostics and Repairs:

 

Mooseman

Master Blaster
Moderator
Dec 4, 2011
21,822
Ottawa, ON
If you can find someone with a Tech 2 or other advanced scanner, try to reset the trans. pressure adapts to get the PCM to relearn the pressures. It's literally a 1 minute process.

What's the fluid condition? Is it a nice red or brown and/or burnt smell? In any case, it would be worth it to drop the pan, look for any bits of metal or excessive junk and replace the filter and fluid as a first attempt to fix this.
 

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