Mitsubishi Magna Gearbox & Transmission Problems

It definitely could be the transmission’s main pump. Without this pump working properly, it can’t create the pressure necessary to select gears via the valve-body or indeed, produce enough drive force to actually get the car to move.

But, you could also be looking at a broken valve body, faulty selector cable from the cabin to the transmission, a broken torque converter or a complete, catastrophic internal failure of the gearbox. Instead of guessing, take it to a transmission specialist who is very likely to have seen this very problem many times before.

Unfortunately the old Magna was known to suffer from idle shake when stationary and in gear, and there's not a lot you can do to get rid of it. Owners often put it into neutral to overcome it instead of leaving it in gear at traffic lights etc. As for the hunting in gear you could check the throttle position sensor and make sure that is adjusted correctly.

It would appear the fluid has been overheated, so I would be doing a full flush of the transmission, not just a fluid change. I wouldn't bother about the coolant.

There's very little engine braking available with an automatic transmission, and with the desire to achieve the best possible fuel economy there have been changes to the transmissions, gearing, torque converters etc. that have further reduced the effect.