A Code 21 will set (if signal voltage high):

TPS reading is above 2.5 volts.

Engine speed is less than 1800 RPM.

MAP reading is below 60 kPa.

All of the above conditions are present for 2 seconds.

A Code 22 will set if(signal voltage low):

Engine is running.

TPS voltage is below. 2 volts (200 mv).

Both of these codes are the same for 2.5 or 2.8 engines.

A Scan tool or Lab Scope is required to perform these tests.

On 2.5 engines ('84 to '86) there should be a heat shield by the TPS or the EGR valve.

On 2.5 engines the TPS is located on the TBI. On 2.8 engines the TPS is located on the engine side of the air cleaner hose on the throttle valve.