Code 81 – Secondary Air Injection Diverter Solenoid failure AM2. The secondary air diverter valve isn’t working on the Thermactor Air System (smog pump). Look for broken or missing vacuum lines on the solenoid valve to the diverter valve. Look for +12 volts at the Red wire and look for the Lt Green/Black wire to switch from +12 volts to 1 volt or less. The computer controls the valve by providing a ground path on the LT Green/Black wire for the solenoid valve.
Code 84 - EGR Vacuum Regulator - Solenoids. The EGR vacuum regulator solenoid is inop, missing vacuum, broken vacuum lines, vacuum leaks, or bad electrical wiring from the computer or no power to the solenoid.
Code 87 – fuel pump primary circuit failure. check inertia switch, wiring to/from inertia switch, red wire going to inertia switch for +12volts. Check other side of inertia switch for +12 volts.
To help troubleshoot the 87 code, follow this link for a wiring diagram
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
Codes 94 & 44 - Check vacuum lines for leaks, & cracks. Disconnect the big hose from smog pump: with the engine running you should feel air output. Reconnect the smog pump hose & apply vacuum to the first vacuum controlled valve: Its purpose is to either dump the pump's output to the atmosphere or pass it to the next valve. The next vacuum controlled valve directs the air to either the cylinder heads when the engine is cold or to the catalytic converter when the engine is warm. Disconnect the big hoses from the back side of the vacuum controlled valve and start the engine. Apply vacuum to the valve and see if the airflow changes from one hose to the next.
The two electrical controlled vacuum valves mounted on the rear of the passenger side wheelwell turn the vacuum on & off under computer control. Check to see that both valves have +12 volts on the red wire. Then ground the white/red wire and the first solenoid should open and pass vacuum. Do the same thing to the light green/black wire on the second solenoid and it should open and pass vacuum.
Remember that the computer does not source any power, but provides the ground necessary to complete the circuit. That means one side of the circuit will always be hot, and the other side will go to ground or below 1 volt as the computer switches on that circuit.
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
http://www.veryuseful.com/mustang/tech/engine/images/88-91eecPinout.gif
Be sure that there are no vacuum leaks/cracked or damaged vacuum hoses. The EGR and Thermator air System (smog pump) systems have usage of vacuum as a common failure point.