The smog pump or AIR (air injection reaction or some such) now called secondary air on later vehicles is an after the fact emissions device.
First used in around 1970 the air was pumped into the very rich (by current standards) exhaust and partial combustion of the waste was accomplished. The first improvement was to add reaction chambers right off the exhaust port. This improved the burning (oxidation of CO to CO2 and HC's to H2o and CO2).
The process was greatly improved with the first catalytic convertor which were oxidation only. The addition of the catalyst to this firebox was a tremendous improvement except that it often oxidized N2 (over 70% of air) into NOx (various oxides of Nitrogen).
This brought about the new rules as of 1980 to regulate NOx. This brought about the lambda (closed loop) fuel system and the oxidation reduction catalytic convertor. The process used in the redux cat is the reduce (opposite of oxidize) NOx back into N2 and O2 in a first stage and use this O2 for the oxidation section. This process needed an exact amount of O2 to be effective and it actually needed a lean-rich-lean control rather than perfect stoiciometry to release the necessary O2.
The weakness in this system was the warm-up period when closed loop control couldn't be effected and where the cat wasn't hot enough to do the reduction reaction. Sooo they added your pump only for cold running (up to oil temp 130-140F (I think - switch next to oil filter housing) at which time the electric clutch (similar to A/C clutch) disengaged and no more pumping takes place. This is very important as the lambda control is ran of the O2 sensor and even small exhaust leaks ahead of it can aspirate in enough air to make the car think its lean and cause full lambda rich correction.
The secondary affect of the air injection is to rapidly heat the cat so that closed loop can start working.
The newest pumps 97 and up (I think) are all electric (no belt) and there function is monitored every drive cycle for OBDII.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician