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Downshift "clunk" at closed throttle, with a 440T4 transmission, when approaching a stop

If you have a 440T4 transmission (also called the 4T60), commonly found in many '84-'92 GM front wheel drive vehicles, (four speed overdrive transmission), and experience a 2-1 downshift "clunk" at closed throttle, you may be able to cure the problem a couple of different ways, of course, depending upon the cause of the problem. Unfortunately, this does require some knowledge about how the 440 operates, to understand the proper coarse of action.

The 440T4 transmission is fairly unique in the way that it deals with getting information about throttle position, and engine load, to determine shift points and main-line control pressure. Most transmissions traditionally use a Vacuum Modulator, or a Throttle Valve Cable (TV cable), but not usually both, to sense engine load and throttle position.

How the 440 differs from "most transmissions" in the way shift points and feel are determined

Most Transmissions controlled by a Vacuum Modulator:: In most transmissions, controlled by a Vacuum Modulator, the vacuum modulator, and modulator valve, are responsible for determining engine load, to help determine transmission shift points (by raising shift point speeds under hard acceleration, and commanding "passing gear", for instance) and shift "feel" (by adjusting main line control pressure; higher pressure means harder shifts, and lower pressure, obviously means softer shifts). Raising the modulator valve pressure (by adjusting the screw in an adjustable modulator), causes most transmissions to shift harder, and shift "later", at a given throttle setting, while, lowering modulator valve pressure causes most transmissions to shift softer and shift "sooner", at a given throttle setting. A problem arises, with this system, in that if you have the perfect shift timing, but you would like to firm up the shift feel, chances are that when you adjust the modulator to raise modulator pressure, your shift feel may firm up, but the shift points may then be too late, and the converse is true.
Most Transmissions controlled by a TV cable:: In most transmissions, controlled by a TV cable, the TV cable, and TV valve, are responsible for determining engine load, to help determine transmission shift points (by raising shift point speeds under hard acceleration, and commanding "passing gear", for instance) and shift "feel" (by adjusting main line control pressure; higher pressure means harder shifts, and lower pressure, obviously means softer shifts). Raising the TV pressure, causes most transmissions to shift harder, and shift "later", at a given throttle setting, while, lowering TV pressure causes most transmissions to shift softer and shift "sooner", at a given throttle setting. Like the "modulated" system, the TV system is limited in that if you have the perfect shift timing, but would like to firm up the shift feel, raising the TV pressure may get you the shift feel you want, but may also cause the transmission to shift too late.
The 440 does not work like "most transmissions": The 440 uses both, the Vacuum Modulator and the TV system, making things more confusing for a "newbie", but making far more precise adjustments possible, to the savvy technician. The 440 uses a vacuum modulator, solely for the control of main line control pressure, to control shift feel; raise modulator valve pressure, by adjusting the modulator, and the 440 will shift firmer, with no change in shift timing, and, obviously, lower modulator valve pressure, and the 440 will shift softer, again, with not change in shift timing. The TV cable, and valve, control shift timing, but not main line control pressure; raise TV pressure, and the 440 will shift later, with no difference in shift "feel", while lowering TV pressure will make the transmission shift sooner, with no difference in shift "feel".

So what in the heck does this have to do with my clunk?

Ohhhh yeah.... The clunk. Sometimes, if the modulator pressure is too high, or, if the TV cable is not adjusted properly, a downshift clunk can be felt. If, after you have lowered the modulator pressure, without severly softening the shift feel, and the TV pressure is set properly (the transmission is shifting at the correct speed for a given throttle setting), I have been able to cure a downshift clunk by installing a 1/2 servo apply spring, in the 1/2 servo, located at the rear of the transmission (as installed in the vehicle), and accessed from underneath the vehicle. This servo apply spring is a GM updated spring, specifically designed to get rid of the clunk, and it usually works. The GM part# is 8668123.

Installing the servo apply spring

Here is where it can be easy....or difficult. Some vehicles are harder than others. Some vehicles have the subframe in the way of removing the servo cover, and some do not. Some vehicles require removing mount bolts to lower the transmission down, in relationship to the subframe, to gain access, and some are wide open. However yours is, you must remove the servo cover snap ring, servo cover, and servo assembly. Then, simply remove the e-clip, remove the old apply spring, and replace with updated spring, and then reinstall the e-clip. A new o-ring must be installed on the servo cover, and the o-ring and case bore greased, to aid the cover installation without cutting the o-ring. Then, simply reinstall the snapring. On a hoist, with no frame in the way, this is a five-minute job; if the frame is in the way, it could add considerable time to the job. How long does it take on the ground, without a hoist? I don't know...and I even have a hoist at home so I won't have to find out!!<Grin>