[urq] Interesting MAC-02 Characteristic

urq urq at pacbell.net
Sun Dec 9 23:38:59 PST 2007

... thought I'd post up what I've learned since my initial post ... 

#1) There's no question that the MAC-02 will go into 4k max RPM cut out mode
if the OXS Freq Valve circuit opens up.  I removed the jumper on my car and
the fuel pump shuts off at 4k RPM, replaced the jumper and it revs to red
line.  I don't know how often the OXS Freq Valve opens up, but this is one
for those with MAC-02s to note ... 

#2) [found the remainder of the wire on the other metering plate] I was
concerned that jumpering the wire directly to the fuel pump relay would
damage the OXS Freq Valve driver ... I can report that once I removed the
jumper and reconnected the OXS Freq Valve I heard the good old buzzing
sound, and the engine revs to red line.  WHEW!  I guess the resistance wire
prevents excessive current from flowing.  

Steve B
San José, CA (USA)

-----Original Message-----

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?

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