I need to know how to check the transmission.

TRANSMISSION FLUID CHECK
NOTE:
The transmission fluid temperature is measured with scan tool MB991958 (MUT-III sub assembly).
NOTE:
If it takes some amount of time until the transmission fluid reaches its normal operating temperature [70 - 80°C (158 - 176 °F)], check the transmission fluid level by referring to the diagram.

1. Drive the vehicle until the transmission fluid temperature rises to the normal operating temperature [70 - 80°C (158 - 176°F)].
2. Park the vehicle on a level surface.
3. Move the selector lever through all positions to fill the torque converter and the hydraulic circuits with fluid, and then move the selector lever to the "N" position.
4. After wiping off any dirt around the dipstick, remove the dipstick and check the condition of the transmission fluid.
NOTE:
If the transmission fluid smells as if it is burnt, it means that the transmission fluid has been contaminated by fine particles from the bushings and friction materials. Transaxle overhaul and cooler line flushing may be necessary.

5. Check transmission fluid level is at the "HOT" mark on the dipstick. If the transmission fluid level is less than this, add DIAMOND ATF SP III
transmission fluid until the level reaches the "HOT" mark.
NOTE:
If the transmission fluid level is too low, the oil pump will draw in air along with the transmission fluid, which will cause to form bubbles. If the transmission fluid level is too high, rotating components inside the transaxle will churn the fluid and air into a foamy liquid. Both conditions (level too low or too high) will cause the hydraulic pressure to drop, which will result in late shifting and slipping of the clutches and brakes.
NOTE:
In either case, air bubbles can interfere with normal valve, clutch, and brake operation. Also, foaming can cause transmission fluid to escape from the transaxle vent where it may be mistaken for a leak.

6. Securely insert the dipstick.

NOTE:
The transmission fluid should always be replaced under the following conditions:
l When troubleshooting the transaxle.
L When overhauling the transaxle.
L When the transmission fluid is noticeably dirty or burnt (driving under severe conditions).