Possible transmission issue

You're describing a torque converter shudder caused by using the wrong type of transmission fluid. To verify this, when the shudder occurs, hold the accelerator pedal and road speed steady, then lightly tap the brake pedal with your left foot. The shudder will stop, then come back in about two or three seconds.

My 1994 Grand Voyager daily driver has had this shudder real bad for the three years I've been driving it. The previous owner severely neglected regular maintenance on the cooling system and transmission, but she did have it in three times for a very intermittent failure to up-shift. Turned out to be caused by nothing related to the transmission, but I'm sure at some point the fluid and filter got replaced as part of the attempt to solve that problem.

When I have to stick to around 35 - 40 mph, I press the "Overdrive off" switch to keep engine speed up a little. At the higher RPM, the torque converter locks up solidly so that shudder doesn't occur. At the lower engine speed, the clutch is in partial lock-up to ease the engagement for comfort. With the missing additives in the fluid, that controlled slippage causes the clutch to lock too aggressively, then get torn loose by engine torque, then it tries to lock up again. This repeated action is what you feel as the shudder.

Another clue is this lock-up never occurs until engine coolant reaches a certain temperature. On really cold winter days I can drive up to six miles before the clutch locks up. On mild summer days I get about a mile before that occurs.

I'm not sold yet on the need to flush transmissions. If that is done in an attempt to solve a problem, that problem is usually caused by something mechanical, and flushing the system isn't going to fix that. If the flush is done for maintenance, there is nothing that's going to be accomplished that a regular drain and fill won't do. The drain and fill replaces half of the fluid which is enough to replenish the additives.

If you can put up with the shudder, wait until the next scheduled service is needed, then have the fluid and filter replaced. If this is too irritating, have that service done now, but be sure to mention the shudder and the need to use the right fluid as the reason you want this done.

Hey TMCINTOSH11,

There looks like there is a TSB for water contamination of the A/T fluid on these vehicles that will cause the torque converter lock up shudder previously described. TSB attached below.

The water leaks in around the transmission oil fill tube.

You would need a transmission flush and replacement seals if this TSB is applicable.

You may want to raise this with the dealership too.

Hope this helps, please keep us updated with your progress.

Regards, Joe

Don't use additives in a transmission. The correct transmission fluid already has the additives determined to be needed by the people who designed the transmission. When you start mixing different products, they can have additives that fight each other. This is equally important with engine oil. You can run into a seal conditioner in one brand of oil that is not compatible with a detergent in another brand. There's also dispersants, corrosion inhibitors, viscosity index improvers, and anti-foaming agents that all have to work together.

I left the dealership after 1999. Since then, a lot of manufacturers have gone to designs that prevent you from doing your own service on their transmissions. One can only wonder what that accomplished or how it benefits the car owner.

I was going to post the procedure for checking the fluid level and for performing the seal repair, but those require very specialized tools and equipment that only the dealer has. I prefer to do all my own work on my vehicles, but this is one time when it probably is beneficial to visit the dealer. In the long run you will be better off. The fluid level has to be calculated from reading its temperature with a scanner, the level on a special probe inserted through the tube, then finding a point on a graph.

Here's the service bulletin JOETECHPRO referred to.

You won't get the torque converter shudder at 60 mph. This only happens at partial lock-up which occurs around 30 - 40 mph. At lower speeds it doesn't try to lock up at all. At higher speeds it locks solidly unless you hit near wide-open-throttle. It's just in that small range of partial lock-up that is does so too aggressively, then engine torque breaks it free, and this repeats very quickly. Partial lock-up is supposed to come in gradually so you don't feel it as a clunk or thump. That's strictly for comfort. To engage gradually requires the right combination of additives to let the clutch plates slip and grab smoothly.