To be a little more specific TAB stands for Thermactor Air Bypass and TAD is Thermactor Air Divert.
The solenoids control vacuum to the valves in the Thermactor Air Injection system which controls where the air from the pump is directed.
The TAB valve is right behind the pump itself and as the name implies it allows air from the pump to be bypassed, or dumped, to atmosphere. This valve allows air from the pump into the Thermactor system when vaccum is applied and dumps air when vacuum is absent. The soleniod that controls the vacuum is normally closed so it has to be energized for vacuum to get to the TAB valve. Air is bypassed during deceleration mainly, to avoid popping from the exhaust
The TAD valve is the next valve downstream from the TAB valve and is located on the passenger side of the engine just about where cylinders 2 and 3 are. This valve determines whether air from the pump is sent to the cylinder heads (upstream) or to the cats (downstream). This valve is normally in the downstream position so it goes to upstream when vacuum is applied. It then follows that the solenoid is only energized when upstream air is called for. Upstream air is only called for during cold engine operation. All other times the air is downstream.
It should be noted that the solenoids get power from the EEC power relay just like all other EEC items (computer, injectors, etc.) and the computer grounds them to activate. If they are disconected the computer will note that there is no voltage on the ground circuit and set a code. The word is that a 100 ohm resistor across the connector will fake the computer into thinking the solenoids are still there. I haven't tried it myself.
The computer uses the O2's to determine if the Thermactor system is operational. During the KOER test it will command upstream air and watch the O2 signal for a steady lean condition (from all the air on the O2's). If it doesn't see that then you'll get various codes. Sometimes I've noticed O2 sensor codes due to inoperative Thermactor systems.
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