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Old 08-28-2010, 05:00 PM
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4l80E/T-400 parts interchange?

I checked the knowledge base first - honest!

The 4L80E in my '92 GMC 2500HD pickup has suddenly lost reverse gear. Other web sources tell me that either the reverse gear servo piston or the third gear /reverse clutch drum assembly or the reverse band are most likely to be the guilty culprits. The "klutz" drum and band will require a total trans re-do if they are the ones that need it, but the servo can be reached by just dropping the pan.

Since the servo is easy to get at, I thot I'd try that first. The GM part number for the servo piston assembly seems to cross to the same thing in TH400 (Part number TH400.PIS04)

Anyone know if it IS the same? (I have a late model TH-400 I can cannibalize if this will work)

If I gotta go into the whole trans - I won't as I can get a used 4L80E in running condition for $300.00. The whole truck not worth the cost of re-doing the trans


Drives perfectly in all gears, going forwards
2 nites ago, my son took it to the store and when he was backing out of the parking spot, it backed up a couple of feet, went "bang" and then the RPM came up while the truck stopped moving.

Now we can't even park in the driveway as we could not get back out again!


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Old 08-29-2010, 10:10 AM
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If you use a t-400 servo as replacement, make sure you have the matching cover.

4l80e servo piston can crack around the groove for the square cut rubber seal

your 1992 trans should have the early design and will match up to the t-400 parts

There is the possibility that 'pop' noise was the rear band breaking
Dont worry how old you are... it is the mileage on your body that adds up.
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