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Discussion Starter · #1 ·
Please tell me about the 4L80Etransmission of my 1994y GMC SUBURBAN 6.5 DIESEL.

My SUBURBAN lost reverse gear and rebuilt the transmission.

I put that transmission in my car, and I drove it, and all the gears switched normally.

However, manually switch to the 2nd gear and pull out the throttle, the engine brake will not work.

The engine speed returned to idle, as if the clutch pedal had been pressed while driving.

I used PA CARGO 34325A for the front band during the rebuild and I suspect this is related.

Please feel free to share your thoughts with me.
 

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so in manual 2nd you have no engine brake, but it drives normally?
 

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Discussion Starter · #3 ·
so in manual 2nd you have no engine brake, but it drives normally?
Thank you for your reply.

Manual 2nd does not have an engine brake, but it drives normally.

Release the throttle, the engine speed will return to idle, but insert the throttle, power will be connected.
 

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Discussion Starter · #4 ·
Today, I removed the oil pan and the valve body to find the cause.

First, when I pushed the front servo piston, it seemed that the piston had bottomed before the pin pushed the band.

I replaced it with the ACDELCO 24223081 that I had prepared, and when I pushed the piston again with my hand, I felt like the pin was hitting the band.

During this process, I remembered that the separator plate was worn during the previous rebuild, so I changed the separator plate to TRANSGO 48 PLT01.

I refilled the fluid after installing everything and did a test drive

The engine brake of the 2nd gear became good, but a new problem occurred this time.

When I drive at manual low, I feel a deceleration different from an engine brake.

When I step on the throttle, it accelerates, but I feel the car is heavier than usual.

It's like somebody's on the brakes, it doesn't creep.

The symptom disappears when I change the shift lever to 2nd.

All gears operate normally in both D range and OD range, and there is no problem with reverse.

What more should I check?
 

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I know you stated reverse is okay but Did you measure reverse piston travel with a digital dial indicator?I set a customers travel to .161” using a modified servo cover that I had on standby.That’s what sonnax recommends for piston travel.You may have to build up the servo pin with welding and grind it down,etc to get the depth you need.Also do you have engine braking in manual 1?And if so can you feel the truck pull back when you release the throttle while on the road when driving at low speeds?
 

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Discussion Starter · #6 ·
I solved everything by myself.

First, the front servo piston was OK.

The cause of the problem was that the gasket slipped and caused an internal leak.

I was checking only the transmission side of the separator plate, but the cause was on the valve body side.

The gasket was caught when installing the separator plate and it did not fit in the correct position.

Thank you to everyone who replied.
 

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I solved everything by myself.

First, the front servo piston was OK.

The cause of the problem was that the gasket slipped and caused an internal leak.

I was checking only the transmission side of the separator plate, but the cause was on the valve body side.

The gasket was caught when installing the separator plate and it did not fit in the correct position.

Thank you to everyone who replied.
Glad you found the issue.
Thanks for giving us an update!
(y)
 
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