1990 f-150. how to test door lock solenoid?

 

Use ohmmeter to measure coil resistance.

If coil resistance measures very low, say, less than 10 ohms, it is short-circuited. If coil resistance measures very high, thousands of ohms, it is open-circuited. Either condition calls for replacement. Sorry, I do not have my manual available listing expected normal coil resistance. imp

 

Remove the door panel and check for voltage going to the solenoid. If there is current when you toggle the switch, the solenoid is bad. One side will current going to it when the switch is activated.