That's really interesting because my 1989 300SE doesn't have an air pump but my 89 420SEL does. Your 1990 300Se has the CIS-E ECU and controlling the air pump but my 420SEL has it operating when the ECU is not funcioning, when the OVP fuse is blown. Here are the conditions, all apply:

1. Coolant temp is above 62 F
2. engine speed is below 2500 (how this is determined I don't know)
3. Throttle full load contact is not engaged (???)
4. CIS-E control unit is not operating.

see the schematics page 110/16 and 110/17 (PDF pages 40 and 41)

To answer the previous question, the air pump injects air into the exhaust port to promote complete combustion under certain conditions, like when the engine is just started.

__________________
Regards

Warren

Currently 1965 220Sb, 2002 FORD Crown Vic Police Interceptor

Had 1965 220SEb, 1967 230S, 280SE 4.5, 300SE (W126), 420SEL

ENTER > = (HP RPN)

Not part of the in-crowd since 1952.