To Ken Robinsons original question. It turns out that the manual I have is (a copy first) probably from about 1979 or 80. "Porsche Engines with Oxygen Sensor". Covers the " 924/924 Turbo and 911SC" and honorable mention of the 928 somewhere. The table of conditions I have is only for the 924/924T and is in the yearly supplement for that model for 1980.
The "P Engines w O2 Sensrs" does have a wiring diagram for the SC. Also a trouble shooting logic tree. However the fuse numbers do not match those you all quoted.
I would suggest a test light or voltmeter test at the frequency valve first. It's not buzzing; but why? There should be 12 volts at one of the two connections of the FV, as the power is fed to the FV and the computer switches the ground side. The diagram in the pamphlet shows the frequency valve wired to a 6 pin multiple plug. Pin 4 of that plug should be the 'hot' lead of the frequency valve and connects to 87b or the 'Power Supply Relay'. 87 of the same relay supplies the O2 Control Unit pin 8. 87b and 87 of the PSR are both supplied from 30 of the same relay which connects in turn to "Fuse box in trunk - fuse 1)" If you gots power to all these places then its time to check the (squarewave) output of the brain.
Check that either by the book at the test connector (which connects to pin 17 of the O2 Cntrl Unit) or directly at the FV using the ole dwell meter.
Note that the functioning (or not) of the sensor and other junk is not really important to getting the frequency valve running. In fact, as previsouly mentioned, I would disconnect the sensor and forget about it. With a good cat I bet it would pass emissions without.
Grounds are problematic with these things. I see two to check. The O2 Cntrl Unit pin 16 and Power Supply Relay 85 both show connected to the same engine ground. O2 Ctl U pin 5 goes through the 6 pin multi plug pin 2 to a body ground.
Hope this helps.
Tomorrows a track day. Good night.
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