Auto Service World
Feature   October 1, 2004   by Jim Anderton

Home Brew Fuel Injection Part 6

Back to the Board

Last month we looked at the problem of adapting the Chevy Astro 4.3L TBI injector unit to the Rover (Buick) 3.7L manifold that originally held dual SU carbs. Now it’s time to continue building the ECU from the MegaSquirt plans. With the power supply circuit and computer interface section built and tested using the MegaStim injector simulator we built in Part 2, we move on to the input section.

Pic 1

Taking it apart to build it. We needed the Motorola processor in place to test the interface section, but now it’s at risk during the assembly. Remove it by prying with a small screwdriver VERY carefully.

Pic 2

Place the chip back on the anti-static foam that it shipped on. Note the eight-pin chip that’s still on the foam. Leave parts in their anti-stat bags and foam until needed.

Pic 3

The plastic shipping tube says it all. Save the tubes once you’ve installed parts like the optoisolator chip pictured for storing replacement I.C.’s.

Pic 4

The small “glass” diode is a 5.1 volt Zener-type. Bowling and Grippo (the brains behind MegaSquirt) claim that it’s not needed for most installations, unless your ignition has a large offset bias. They recommend a shorting wire in its place, but how do know for sure that you won’t need it? It’ll be difficult to add it later when the board is stuffed, so the solution is to install the diode, then solder a jumper wire across it. If it turns out that you need the Zener for your installation, just clip the wire later.

Pic 5

This little lump of plastic is the MAP sensor. The small barbed nipple takes a vacuum line to the intake manifold. Since you can’t mount the MegaSquirt ECU under the hood, you’ll need to pass the hose through a grommet in the firewall. What about sensor lag? Bruce Bowling reports that he’s tested the unit with 30 feet of 1/8-inch tubing and has seen no more than 1 millisecond of lag, so it should be no problem.

Pic 6

This is how the MAP sensor mounts, under the board. Contrary to the instructions, DO NOT solder the unit in place until you’ve completed the output section. Several leads pass under it, making wiring difficult with it in place. It can wait until the end. When mounting, don’t over-torque the nylon screws, since an excessive load will distort the case and flex the load cell inside the unit. That will introduce an offset in its readings.

Pic 7

Another optional component. These resistors, labelled R4 and R7, are bias resistors to interface with stock coolant and air temperature sensors. The 2.49Kohm units pictured are good for standard GM sensors (part number 12146312). If you want to use other brands of sensor, there are a couple of options. In software, you can switch the transfer function files in the ECU processor to remap the system, or do it the easy way and change resistors. Changing resistors also allows you to mix and match brands, like a Ford air and GM coolant sensor.

Pic 8

Another jumper. This one has a complicated history. The ECU takes its signal from the ignition system either from the existing TACH terminal or from the coil negative terminal. The small optoisolator chip buffers the processor from the rough and ready signal from the ignition system by passing it through an LED in the optoisolator chip. The ignition signal can be grounded to the board, or remotely to the vehicle chassis. If you’re tapping the signal directly from the coil, it may reset the processor. In that case clip the jumper and ground with a separate wire, isolating the board ground. If you know you’re using the relatively clean tach signal, put the jumper in anyway, as it lets the MegaStim simulator test your work. Then clip it out later. There are more tricks regarding ignition triggering to come later.

The next step is to test the board and software to see if it tracks up and down the RPM band with good oxygen sensor and TPS performance.

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