05-14-2013, 08:34 PM
Join Date: Sep 2005
Total Cats: 1,528
I'm not entirely certain that I'm interpreting everything you have written correctly, but...
The diode shall always be installed across the two wires leading to any solenoid. Schematically, this places the diode between +12 (the supply to the solenoid) and the ECU wire which provides a switched ground for it.
The diode MUST be placed such that the striped side is connected to the wire which supplies +12 to the solenoid. In general, this is a white/red wire in most Miata applications.
The '01 and later engines, oddly, supply +12 directly from the ECU on dedicated wires for the IAC and VVT control (OCV) valves in particular. For these specific applications:
IAC + is the Orange wire at ECU 2P
IAC - is the Violet / Red wire at ECU 2Q
OCV + is the Yellow wire at 4D
OCV - is the Violet / Yellow wire at 4R
NOTE: You should verify that the wiring of your ECU matches the above, by unplugging the solenoid, switching the ECU on, and checking for +12 at the indicated wires (relative to ground) without the diode installed. (eg: the wiring inside the ECU / ECU harness may be reversed relative to the OEM configuration.)
If the diode is installed backwards, it may explode. That's a good indication that you have reversed the polarity. Damage to the ECU is also possible, as the diode is essentially supplying an unlimited amount of current to the ECU driver.
(In theory, the ECU drivers should always be internally current-limited, which would prevent both ECU damage and exploding diodes, however this is not a universal truth. In this context, it would depend upon what specific output circuits you are using.)
The fact that you are measuring +12 on both legs of the VVT wire suggests that you may have have installed the diode backwards on that line. I suggest removing the diode and then re-checking both wires for +12 relative to ground with the solenoid disconnected.