Repost to restore missing images and add links to conversion articles
Background
At some point in the past it was noted that the injectors on 1995 Z32's receive switched 12v power while the injectors on previous models (1990-1994) receive constant 12v. In other words, the earlier injectors are "hot" at all times, even when the engine is not running. The presence of this constant voltage is considered a prime suspect in the premature failure of fuel injectors. In 1995 Nissan made a change so the injectors are no longer powered when the key is OFF (ie. Switched power). Some theories were proposed on how Nissan accomplished this change which included the addition of a relay or possibly even code changes in the ECU. In fact, the change was accomplished in a way which was much simpler.How the injectors work
Contrary to common belief, the ECU does not provide power to the injector when it needs to fire. Instead, the injectors are powered simultaneously and continuously (via either switched or constant voltage), and the ECU simply grounds each injector when it needs to fire. How Nissan Did It
As mentioned above, Nissan did NOT add a relay, change ECU logic, or any other similarly complex remedy. Instead, they made a simple change to the 1995 wiring harness alone. The 1990-94 harness already contains a Switched +12v lead which connects to ECU Pin #45. Nissan simply moved the powered lead for the injectors from the constant-power lead on Pin #58 to this switched-power lead on Pin #45. That's it. So simple, yet effective. 1990-1994 Wiring Harness
Here is a section of a schematic from the 1994 FSM (1990-1994 are all the same) which shows the Constant +12v feed for each of the injectors (highlighted in Green). Within this same schematic I've highlighted the Switched +12v lead in Blue which is used as the injector power source on the later 1995 harness (the switched power was there all along!).
 1995 Wiring Harness
Here is a section of a schematic from the 1995 FSM which shows the Switched +12v feed for each of the injectors (highlighted in Green). Within this same schematic I've highlighted the Constant +12v lead in Blue which was used previously for the injector power source on the 1990-1994 harnesses.
Again, notice that the power leads themselves (constant +12v to Pin #58 and switched +12v to Pin #45) are identical for both layouts. The only change is where the injector leads connect for their power! What does this mean?
If you're in the market for a new EFI harness you may want to consider the 1995 harness for this "bonus" feature. If you have a 1990-1994 harness, you can conceivably "upgrade" your harness to the switched-power approach by relocating the common injector power lead from the lead on Pin #58 to the lead on Pin #45. This clearly involves some disassembly of the harness, and the exact process to make this change is not covered in this post. Any decision to alter your 1990-1994 harness is "at your own risk". The schematics above simply provide a roadmap for the conversion and nothing more. Perhaps someone will produce a "how to" in the future which covers the details of the coversion (they did, see below).
UPDATE - Conversion Articles
- NobleZ posted a writeup for converting early harnesses to the 1995 design.
- jzack posted an alternative approach for converting early harnesses to switched-power injectors.
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