Emissions: Air Bypass Valve and Air Diverter Valves
There are a majority of us who haven't completely gutted our Mustangs engine bay and still run with the
emissions system in-tact. The smog pump, air bypass valve, air diverter valve, and assorted solenoids play
a major roll in the proper function of the system. This page should give you a basic idea of the components
function and how the system is designed.
The heat shield wrap on the lower hose is not a standard feature. I found that this hose is rather close to
aftermarket headers and I kept burning through that hose. The above set-up is for my supercharged vehicle,
so the hose lengths may be slightly different than what you see on your car.
Air Bypass Valve
Air flows down the hose from the smog pump to the Air Bypass Valve.
The air is directed to the atmosphere under deceleration.
- With no vacuum at the valve, air is diverter out the bottom of the valve to the atmosphere.
- With vacuum at the valve, air continues down the hose to the Air Diverter Valve.
Air Diverter Valve
As the air enters the Air Diverter Valve it has two choices, it either flows to the back of the heads or
to the catalytic converters.
- With no vacuum at the valve, air flows downward in the valve toward the catalytic converters.
- With vacuum at the valve, air continues to the back of the heads.
At the entrance to the pipes leading to the heads and catalytic converters there are one
way check valves. The one mounted on the pipe leading to the heads is replaceable
(Ford: E9VE-9A487-AB). The one mounted on the pipe to the converters is crimped on and
not easily replaced. You will probably need a new assembly.
TAB (Thermactor Air Bypass) Solenoid
TAD (Thermactor Air Diverter) Solenoid
The Air Bypass and Air Diverter valves are computer controlled
solenoids mounted between the passenger side firewall
and strut tower. These solenoids are referred to as the
TAB and TAD solenoids. In the picture to the left they are
the two black solenoids mounted side by side. They receive
vacuum from the upper intake and distribute vacuum by way
of the small square vacuum junction located between the
strut tower and valve cover / throttle body.
The four vacuum lines that feed off the vacuum junction box into the engine bay are as follows:
- Vacuum source from the upper intake
- Vacuum feed to the EGR at the rear of the throttle body
- Vacuum feed to the Air Bypass Valve
- Vacuum feed to the Air Diverter Valve
- There is a 5th vacuum port but it is not used.