[urq] Interesting MAC-02 Characteristic

[urq]

When I picked up my car John told me that he didn't have the connector to
attach the OXS Freq Valve to the wiring harness (my car has had an odd
unprotected 2 pin connector under the hood since I got it).  He said that he
was getting a 4k RPM cut out, which he repaired by connecting the end of the
resistance wire to the switched +12 on the Aux Air Reg.  That was a new one
for me, but I said OK and took the car home.  Disbelieving, I disconnected
the wire ... I figured the problem was somewhere else, like overboost or one
of the other standard issues.  Well, I pulled the wire, and sure enough I
got the 4k cut out.  I looked at the partial MAC-02 schematic that is out
there, and sure enough it shows that the OXS Freq Valve signal goes back to
an input on the ECU.  I suspect the ECU looks for an open circuit on this
line and has a 4k RPM shut down mechanism in place for when that occurs.
This does seem a bit mysterious, why would you need to protect the engine
when the OXS Freq Valve opens?

Has anyone else been aware of this mechanism?  

Steve B
San José, CA (USA)