You plug a code reader into diagnostic port. Auto repair shops have them or you can buy one for around 40.00 some auto parts stores(make specific) The good ones can cost a lot of money & work on all cars. It will read error codes & then you look them up in a manual or online or a repair shop can look up for you.
You cannot do this as there are no OBD ports on vehicles preceeding about 1993. You'll have to diagnose the problem using trial and error... or take it to a garage.
The EEC TEST connection should be in the engine compartment by the Power Distribution Box in a 1995 Ford Ranger ( EEC - IV , OBD I )
On a 1994 Ford Ranger ( which is OBD I , EEC IV ) should have an EEC TEST connection in the engine compartment ( P.S. I'm not a mechanic / technician )
Run a compression test to verify the condition of the engine.
Swap the relay with another like relay.
The transmission fluid filter could be clogged. There could also be an electronic issue, run the test codes on the vehicle computer to see if a problem is shown.
Keep alive memory test failed
pounds of compression per cylinder
The temperature sending unit can be checked cold, and then hot, for resistance change. The test is performed with an ohm meter.
If by any chance you mean the Data Link Connector where you plug in a code reader on a 1996 Ford Ranger : It is located in the lower part of the dash , beneath the steering wheel and between the emergency / parking brake release and the inside hood release * it is behind a small removable plastic panel ( maybe it also flips up )
P0500 is loss of vehicle speed signal. You need to test the speed sensor.
A 1994 Ford Ranger is OBD I / EEC IV and has an EEC TEST connection in the engine compartment by the power distribution box In 1996 , the Ford Ranger changed from OBD I / EEC IV to OBD II / EEC V and has a Data Link Connector in the dash , below the steering wheel , covered by a small removable plastic panel
Hayne's Manual does not specify a definite amount of time or mileage. It says the engine may run rough for several miles while the computer re-learns itself.