When is 12V Not 12V in an Electronic Fuel Injection System?

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Written by wolfems on September 25, 2009 – 3:30 pm

Hi There.

Now, you may remember in a previous post we talked about some of the basic concepts of capacitors, diodes and resistors?

This post goes through what can happen when something that is not meant to be a resistor – becomes a resistor.

“How is that possible?” I hear you ask.

Well, you may have heard about corrosion buildup between pins in a connector, or between the connector and the wire it is crimped to. This corrosion is resistive. That is, it has resistance.

Now, image that you are controlling a relay with your ECU. The relay is switching when you make the output of the ECU go to ground, but the device that you are controlling (we’ll use the example of a fuel pump) does not start running. If you disconnect the fuel pump from the relay, there is 12V there when the relay is turned on. But, when you connect the fuel pump, the pump doesn’t run, and the voltage at the +ve terminal of the pump is almost 0V.

This is occurring because of resistance that has developed between the relay terminal and the terminal on the wire going to the fuel pump.

So, on the face of it, it did initially appear that the 12V from the relay was there and being supplied to the fuel pump, but the resistance in the connection at the relay meant there was only a very small supply of current. There was not enough current to power the fuel pump.

If you just used an LED test light, you will think there is no problem here. But you’d be wrong. An old style test light may give you some more information, as it may glow dimly due to the resistance in the connection.
Sometimes you need to think outside the box if things don’t seem to be adding up.

As older cars running electronic fuel injection continue to age, these types of problems are going to occur more often and diagnosis is going to become more tricky than before.

Keep an open mind.

Until Next Time..

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