Last month we examined the behavior of a failed TW sensor on a 1991 Honda Civic. This month we’ll add a few more details to the diagnosis, and explain the other components that make this system tick. How do the injectors and main relay work? Which injectors fire when? And what about that alternator control? We’ll give you a complete a rundown on the Dual Point fuel injected engine’s system operation, its quirks and several other things you should become familiar with.

Dual Point Fuel Injection means two fuel injectors. The injector mixer, or throttle body, houses the injectors. This may look like a carbureted model, especially if you remove the lid from the helmet on top of the throttle body and see what appears to be a choke butterfly. This butterfly has a curved cutout on the valve cover side to force air to accelerate through it as it passes in front of the main injector at slow speed or cruise conditions. This provides for better fuel atomization.

At wide open throttle, there is a vacuum diaphragm that uses ported vacuum to open the valve fully to increase airflow. The two injectors are on the front, or valve cover side of the throttle body. The auxiliary fuel injector is at the base of the throttle body, below the throttle blade. This injector uses a green plastic connector. The main injector is above it, right under the helmet, with a brown plastic connector.

Auxiliary Injector

The auxiliary injector sits under the throttle blade and fires all the time. It’s the only injector pulsed at idle. Because it sits below the throttle blade, it is exposed to manifold vacuum at all times.

Unlike most throttle body cars you’ll see, it runs the same higher 36-41 psi fuel pressure as its multiport cousin. The yellow/black wire is power from the PGM-FI main relay. The yellow wire goes to the PCM, where it is connected at two terminals on the white connector.

Main Injector

The main injector is pulsed while cranking, after a cold startup while the PCM keeps the idle speed up via the Electronic Air Control valve and at any throttle opening greater than idle. This usually means any throttle value greater than 0.510 volts on the TPS. The car will run if the two injector connectors accidentally get swapped, although poorly. If you just got done laying sod, you might mix up the injectors’ connectors. Remember, it’s green-down, brown-up on this car (refer to Photo 1, p. 28). The connectors on the harness and the connectors on the injectors match when assembled correctly.

Tandem Valve

This is the part that looks like a choke butterfly (refer to Photo 2, p. 28). Those guys at Keihin just couldn’t let go of those wonderful little carburetors! The purpose of the cutout on the valve cover side is to make any air traveling into the throttle body confined and accelerated as it passes the main fuel injector nozzle. This does wonders for fuel economy during most driving modes. As the throttle is opened further, venturi vacuum acts on the diaphragm on the top rear of the throttle body, which connects to the tandem valve. Opening it allows greater air flow at wide open throttle.

Electronic Load Detector

The ELD is like having an inductive ammeter in the fuse box (Photo 8, p. 30) to tell the PCM when it’s okay to turn off the alternator to conserve fuel or help an engine catch its breath after a cold start. Wires to the ELD include a black/yellow wire from the ignition switch (12 volts key ON), a black ground wire and a green with red stripe wire that reports to the PCM. The PCM sends 5 volts out on this wire and waits for the ELD to do something with it. You may have noticed, particularly if you just installed a new alternator, that the alternator may not start charging right after the engine is started. After about one minute, or after turning on an electrical load (headlights or rear defog), the alternator should start to charge.

Stem To Stern

I’m pretty comfortable about the ‘once over’ treatment I gave the once-dying Civic, even though it took me two articles to describe the symptoms, diagnosis and cure. ‘While I was in there,’ I made sure the car got a new water pump and timing belt, along with the fresh set of plugs, cap, rotor and plug wires and new tires.

This Civic will be around for at least another 90,000 miles. Heck, the ball joints are still tight! Wait, what about those CV boots? They must be starting to crack somewhere, I just know it And as cold as it gets during Minnesota winters, cracked CV boots can’t be overlooked. Cracked in September, open in December. I’m sure I heard that somewhere.

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