MAF sensor
See
https://www.ford-trucks.com/dcforum/aerostar/326.html
In the pic, the red outlining box is centered on the Idle Speed
Controller or whatever, but the MPS is the 2"x3" black plastic box on the firewall at the top of the red outlining box, above the engine on the firewall.
If this isn't clear, let me know, and I'll get a better pic. They don't seem to fail very often. At least, I've never heard of one failing, but if somebody knows different, please post. The MAF, being directly in the airstream, is subject to getting dirty and changing its electrical characteristics, so they do definitely have to be replaced after a while, but the older-design MPSs don't seem to fail that often.
A MAF is more sensitive to airflow changes than a manifold pressure sensing device, which is one reason why the later design became popular with a lot of different FI systems. The MPS that our '89s have is an older design whose principle of operation dates back to the Bosch K-Jetronic system in '68 VW Type IIIs (and later used on Volvos, Saabs, Triumphs, etc.).
Regards,
Al S.








