Montana 4t65e hd swap info

12-05-2011, 06:37 PM
I did this swap last weekend, and had to piece together info from several places, so I decided to put it all in one place. Hope this helps any other DIY-ers.

Reason for swap:
Found the HD trans for $100 when our 2nd trans went out. The HD had 150k on it, only needed seals, a 4th input shaft (known issue), and the 2nd clutch pack. It was cheaper than replacing the broken planetary gear in our 4t65e.

General info:
The W-body (97-00) [Grand Prix, Regal, etc.] uses the same sub-frame as the U-platform (97-00) [Montana, Venture, Silhouette].
The 4T65e is nearly identical to the HD trans. Biggest difference is the HD differential and HD CV axle on the passenger side, and the torque convertor is larger that the one for the 3400.
The 4T65e is a reliable transmission (450,000 miles between two). The biggest problem we had was with the seals leaking, causing max line pressure and 4th shift suspend. Out 4th input shafts never failed, but had significant wear on the splines

Difficulty: 6/10

Parts needed:
4T65e HD trans
Passenger side HD axle from the W-body
Original dipstick tube
Drill new holes in flywheel to match the larger diameter TCC from the 3.8, the 3400 TCC may work, but unknown at this time.
Sprockets and chain from the original transmission
Case cover gaskets
Side pan gasket is re-usable

Get the chassis service manuals for your specific vehicle
4t65e DIY shift kit - $10
Hardened 4th input shaft
Check and repair valves in valve body, especially TCC valve

The sprocket and chain in the HD trans is too aggressive for the 3400 and it will kick out of OD far too often.

12-12-2011, 01:48 PM
Got a question for you... Did you swap out the drive and driven gears in that HD trans? The van was a 3.29 Overall from the factory, the GTP and or SC 3800's were 2.93 Overall... Both use the 3.29 Diff( SC Unit being HD ) but the unit in the van was a 1:1 drive and driven...

Just curious...

I read it again.... I guess you left the drive and driven alone? Making it a 2.93?

12-12-2011, 10:41 PM
The drive and driven gears = sprockets & chain. You would need the drive and driven sprockets (gears) from the vans 4t65e to maintain the correct final ratio. Also note that there is a plastic fluid scoop that is specific to the driver sprocket, and is color coded.

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