Andy,
'common wisdom' is that the S5 AFM is better, but has anyone verified that it makes more HP? Mazda made changes S4 to S5 in some cases just to save $$, like the one piece LCA's.
The wiring conversion from S4 harness to S5 meter is not too burdensome. I'll cross my fingers and hope this link works - here's the schematic:




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Curtis
'86 RX-7