I remove the connector's cover, then stick a pin through the rubber seal next to the wire. That pin provides a solid test point that I can connect a small clip to that can be connected to the meter's probe.
Be aware that when no current is trying to flow in a circuit, there can be high resistance in it, as in corrosion between two mating connector terminals, or all but one strand of wire broken in a wire. That high resistance is similar to standing on a garden hose. You'll still have full pressure at the nozzle as long as it's closed, but if you open it and try to get water to flow, your foot adds resistance, and the pressure at the nozzle will drop.
Voltage is electrical pressure. If there is high resistance in a 12 volt feed wire, you'll still measure 12 volts at the computer as long as the computer is turned off and no current is trying to flow. For your voltage readings to have the most validity, they should be taken when the computer is operating.
If you find 0 volts where there should be 12 volts, that obviously is a problem, but when you find 12 volts where you expect it to be, THAT'S where you want to be sure. One way to do that is to add a 12 volt light bulb to the circuit while you're taking that voltage reading. The bulb will insure there is current flowing in the circuit, and that will make any high resistance show up as the voltage goes lower.
There typically are up to four 12 volt supplies to the Engine Computer. One will be there all the time to keep the fuel trim, minimum throttle, and diagnostic fault codes in memory. One comes right from the ignition switch to tell the computer to wake up and do its thing when the ignition switch is turned on. When the automatic shutdown, (ASD) relay is turned on by the computer, it sends 12 volts to the ignition coil(s), injectors, alternator field, oxygen sensor heaters, fuel pump or pump relay, AND back to another terminal for the Engine Computer so the computer can verify that relay turned on. That 12 volts is the power supply for some circuits run by the computer. There is usually another 12 volt wire coming from a part of the system that needs a constant, steady charging voltage. That 12 volt terminal is the reference voltage for the charging system's voltage regulator. GM taps off the instrument cluster when they have a digital dash. That is to reduce flickering due to unstable charging voltage. If there's a problem with that circuit, charging voltage will usually be too high. It should be between 13.75 and 14.75 volts. By the way, all those places where I said you should find 12 volts, "12" is the generic value in a 12 volt system. In reality, with the engine running, the battery voltage should be between 13.75 and 14.75 volts, and that voltage is also what you should find at all four terminals at the Engine Computer.
Ground wires pose an additional problem because they end at a terminal bolted to the body where rust can develop on the body sheet metal and the wires can corrode where they're crimped in the metal terminal. All of that is exposed to the elements. If you try to measure them for continuity with an ohm meter, the meter's leads can have a few ohms of resistance, and that can mask one or two ohms of resistance in the ground wire. Where the voltage would drop in a 12 volt feed wire due to resistance, the voltage will rise in a ground wire due to resistance. If all but one strand of wire are broken in a ground wire, it will still read okay with an ohm meter, but the high resistance will cause a voltage drop when current is trying to flow through it. Therefore, a voltage reading on a ground wire is much more accurate than a resistance reading.
There is another potential problem the engineers foresaw and addressed. Ignition coils and injectors draw very high current for very brief periods of time when those items fire. Those pulses of high current will cause pulses of high voltage drops if there's excessive resistance in the circuit. Suppose that resulted in a pulse of 0.4 volts on a ground wire. If that is the same ground wire used by sensors, all of their readings will be high too. For example, at exactly half throttle, the throttle position sensor will read 2.5 volts. If there's 0.4 volts on its ground terminal, the signal voltage will go up to 2.7 volts. That is significant to the Engine Computer, but it is much more important to the MAP sensor's readings. 0.2 volts equates to a huge difference in fuel metering calculations.
To avoid this possibility, the engineers use four separate ground wires. Two are called "power" grounds, meaning they are for circuits that use a lot of power. That's the injectors, ignition coils, alternator field, etc. The other two ground wires are "signal" grounds, meaning ground circuits for sensors and anything else that would be adversely affected by slight changes in voltage. There's two of each wire in case one develops high resistance. If a pulse of voltage appears on a power ground wire, it will not have any affect on the signal ground wires. You can't measure those voltage pulses with a voltmeter. You'd need an oscilloscope to see them, but all that's good for is to see visually what you've already found with some other test.
To test the signal ground wires, the engine should be running, then measure the voltages on them. Ideally you should find 0.00 volts, but in reality, you might find a few hundredths of a volt. You can also measure on the ground wires for the throttle position sensor or MAP sensor, but due to the monitoring circuitry inside the computer, you can expect to find 0.2 volts on them.
Thursday, September 3rd, 2015 AT 11:38 AM