Automatic gearbox shift problem.

[EatSuru]

A. W126 420 SEC 1991
There is occasionally a "thump" when I shift gears on my W126 automatic gearbox. The problem persists despite my recent efforts to fix it by changing the oil in the gearbox and torque converter. I thought the box had a suction connection for the engine, so maybe that's the problem. Are there any additional connections made while shifting gears?
The mechanics were hesitant to remove a "seized" sump bolt for fear of shearing it off, so they refrained from changing the gearbox's filter. Is it possible that the gear shift issue is due to a clogged filter?

Please tell me if anyone else has experienced issues with the bolts that hold the gearbox sump together. The idea of shearing it, drilling it out, and then re-tapping it is obviously terrifying.
Much obliged.

[guyskin]

🙃

[br1anuk]

Yes, seized sump nuts are common on older vehicles; I had them on my W202 a number of years ago.

Seized bolts are an annoyance, but a good independent specialist or gearbox expert will know better than to avoid them.


The technician who worked on my vehicle definitely didn't; he just removed the bolt by applying a small amount of heat to its remaining portion.

[CyberNinja]

It's part of the job, and it can happen to any automobile, old or new. Just ask any technician about seized, rounded, or stripped bolts. They are typically not a problem if you have the correct equipment and knowledge, provided you have at least fair access.

[Darrellll]

Doesn't every service work include the replacement of sump bolts according to MB manufacturer policy?
Is it a good idea to get new bolts if the old ones seize or peel easily?