1992 mercedes benz 300 a/c not cooling

Could be the key switch or the neutral safety switch.

replace the coolant temperature sensor.

Switch on column sounds like it is bad.

If you have a auto-zone parts store they can check it for free.

Rust, engine wiring harness upper and lower, tune service, head gasket, Plugs are correct Bosch F8DC4's or equal.

Check and make sure that the transmission isn't over or under filled. If not, the clutches that operate the reverse gear Are probably too worn, and don't operate until they are expanded by heat.

The check engine light is on for a reason. You need to pull the error codes, and resolve all of the issues indicated. Then the light will go off.

If a condition were to happen, and be intermittent, the light would go out....But your description appears to have a fault condition detected. It is better and less expensive to fix the issue(s).

Oil /filter.You can and should order a manual from Mercedes. Call the 1-800 for mercedes number, they will sell you one and ship to you.Having and reading it will make your experience with your Benz much better!

Disconnect negative battery cable for 15 minutes, then replace. This will re-boot the computer. See if light comes back on. I assume all fluids have been checked?

A service department will need to hook this up to a computer to clear the code. You could also try to disconnect the battery for a few minutes and hook back up and may stay off. If it does come back on you will definitly need to take into dealer to have cleared.