I have a 2001 ford f150 4x4 supercrew. The 4wd does not work. Front axle will not engage. Everything behind the front axle is working correctly. Did some troubleshooting and neither one of the front axle vacuum lines to the 4wd actuator have vacuum, in 4wd or 2wd switch mode. Solenoids on firewall appear to be not functioning. How do you test these solenoid vacuum valves?