Automatic transmission shudders

This is due to the wrong transmission fluid. What is in there now doesn't have the right additives to prevent that shudder. The biggest clues are the speeds at which this happens and the noticeable drop in engine speed. Those were dandy observations.

Chrysler developed the lock-up torque converter and first used it with some engines in 1977. Those were strictly on or off according to road speed. Today every manufacturer uses a similar design but they're electronically controlled. The solenoid that controls the flow of the fluid that applies the clutch can be cycled on and off dozens of times per second to make the clutch engage gradually in the 30 - 40 mph range, for comfort, so you don't feel the harsh engagement. Cycling the control valve causes the pressure to that clutch to increase gradually, and with the right additives in the fluid, that clutch will slowly grab harder and harder until it is fully locked up. When those additives are missing, the clutch will grab too hard, then engine torque will cause it to break free, then it starts all over. You feel that as that shudder you described.

Another clue is the torque converter clutch will not engage until engine coolant has reached a certain temperature. I've found with my vehicles they don't engage until I'm about a mile down the road in summer, and as much as seven miles on a cold winter day. There won't be any shudder at those times.

Another feature with most of these systems that are electronically-controlled is the clutch will unlock when you tap the brake pedal, in preparation for coming to a stop. When the shudder occurs, hold the accelerator pedal and road speed perfectly steady, then lightly tap the brake pedal with your other foot. You'll see the shudder cease for about two or three seconds, then it will start up again when the clutch starts to engage.

There are other clutch packs in the transmission for all the other gears, but those do not cause that same kind of shudder. Those apply more quickly to give a nice crisp, solid shift. I've had this same shudder in my '94 Grand Voyager daily driver for the three years I've owned it. The previous owner seriously neglected this van. It is in such bad shape, it doubles in value every time I fill the gas tank! Rather than spend the time and money on replacing the transmission fluid, I found if I press the switch to lock it out of overdrive / 4th gear, the shudder occurs much less and at a much lower speed, as in around 25 - 30 mph. In the city, most of the roads I drive on are posted "35 mph". At that speed in third gear, the lock-up clutch is fully engaged, so it doesn't shudder.

My bigger concern is that shudder is causing a hammering action on the soft metal bushing the hub of the torque converter rides on. I had a torque converter hub crack on me back in the mid '80s. I was able to make it home 80 miles, but as soon as I stopped the engine, it plopped a couple of quarts of transmission fluid on the ground. If the hub were to crack enough for the rubber lip seal around it to leak, that is something I can't fix on the side of the road.

I never expected this van to keep going so long due to its bad condition. Had I known it would last over three years, I would have put the right transmission fluid in it. For a vehicle as new as yours, I would recommend a common fluid and filter change, which is typical of what most manufacturers recommend roughly every 36,000 miles. That gets about half of the old fluid out. When it is refilled with the right fluid, there should be enough of the needed additives in it to stop the shudder. About half of professionals are not sold on the value of a transmission flush. Besides being much more expensive, a flush will not solve problems caused by mechanical defects that have already occurred, and they have little value for preventative maintenance. The standard drain and fill should be sufficient. The fluids are designed with additives that will provide the needed qualities for the duration specified by the manufacturer.

You can also check out this article:

to see what is involved with servicing the transmission.