GM 1227749 Mods for Low Z Injectors
This page describes the modifications to a GM 1227749 ECU that are necessary to run 8 low impedance (peak and hold) fuel injectors. Mine has been running for a couple of month now without isssues, but YMMV!
I replaced the 2 stock injector drivers with Fairchild Semiconductor HUFA76429P3 hexfets. The sense resistors also need to be replaced by .05 ohm 1% 2 watt resistors to allow the additional current to flow.
Both of these items are available from Mouser Electronics.
Note that if you're modifying multiple 749s you can use the resistors you remove from one in parallel with the existing resistors in another instead of replacing them. That's how I modified my first 749, by scavenging the resistors from another.
This shot shows the heat sink (top plastic piece), injector drivers already removed, and the replacement components (2 resistors and 2 injector drivers). Use care when removing the heat sink, it's glued in by the conformal coating. The 2 ferrite beads need to be placed on the replacement drivers' left terminal (where they came from) before they're soldered back in.
The areas marked in red show where the 2 injector drivers go (Q1 and Q2, top right, 3 holes each) and the 2 sense resistors R76 and R93.
This pic show the location of the injector drivers and the 2 sense resistors that need to be changed. Note the ferrite beads on the left legs of the hexfets, and the resistors positioned up in the air to help dissipate heat.
This shot shows the locations of the 2 injector drivers and the 2 sense resistors from the bottom of the board. Note that I have also added a jumper wire to the connector. The 749 has 3 injector outputs, 2 on one injector driver and one on the second injector driver. I've taken an unused pin (C10) and used it as a second injector output for the second injector driver to reduce the amount of current passed through the connector pin.
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Last updated: May 4, 2004