What causes transmission to shudder? - 1989 Mercury Grand Marquis

Is it the diesel or the transmission doing this shuddering. Could be the transmission, meaning clutch wear and slippage. If in fact it IS the diesel, you could most likely have a dead or plugged injector on one of the cylinders.

might be transmission mount, pretty easy to replace in most vehicles

Torke converter needs to be replaced. Before you do that check the mounts.

Have someone check your transmission fluid level. Probably low, so the shifting up and down is compromised.

If the problem is due to warped brake discs, you will typically feel a mild pulsing in the peddle when braking firmly. Also, when you are experiencing this shudder, taking your foot off the brake (when safe to do so) should cause it to stop. If that is not the case, I would consider again the transmission. I am on my second Caravan, so I speak from experience. The Caravan transmission should have the fluid changed every 30,000 miles to keep it in top performance, especially if you are carrying any heavy loads or pulling a trailer. When you are slowing to a stop, the transmission must downshift through each of the gears. If it stays in the higher gears at too slow a speed, you will feel a shudder. Sometimes this can be corrected by changing the transmission fluid. Be sure to use the proper Chrysler fluid for your transmission. Typically it is the highest (overdrive) gear that causes the shudder. You can determine this by moving the gear selector from "D" to "3" when you are experiencing the shudder. Other possibilities are alignment, worn stabilizer bar bushings, worn ball joints, and out-of-round tires. Most of these also involve some accompanying noises though. Good Luck.

P0430 Catalyst System Efficiency Below Threshold (Bank 2)
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You need to find out which system is causing it first. Lift it and run it like that, get some help and find the source of the shudder

buy shudder fix might need to get it from a transmission shop its the torque converter lockup causeing the shudder

Torque converter clutch not engaging properly.
I recommend taking the vehicle to a transmission shop This way they can determine whats causing the converter clutch to shudder and sometimes if the repair is electricalits not so bad ($). But other times its a rebuild($$$$). Good luck

You can check your motor mounts. They can cause your engine to shudder. With the hood open, have someone shift into drive and watch the motor. If it rocks severely more than likely your motor and/or transmission mounts are broken or ate bad.