1997 Dodge Truck ignition

  • 1 POST
  • 1997 DODGE TRUCK

Electrical problem
1997 Dodge Truck V8 Two Wheel Drive Automatic

truck won't fire crank sensor replaced voltmeter shows 2.7 volts at red and green terminals at sensor when cranking

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Saturday, February 6th, 2010 AT 4:10 PM

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Where is this sensor? The crankshaft position sensor has three wires. None of them are red or green. The violet / white wire is the 5.0 volt supply. The black / light blue is the ground wire and should have about 0.2 volts. The gray / black wire is the signal to the engine computer. It produces a square wave signal that switches between near zero volts to near 5 volts. You can not measure that with a digital voltmeter because it takes a reading, analyzes it, then displays that value while it analyzes the next reading. This display will bounce around depending on what the voltage is when it takes each reading. On the DC volts scale, the meter does not average the readings.

The AC voltage scale will respond to a square wave, but it is only accurate for a 60 cycles per second sine wave, not a square wave pulse. Most of the time, the sensor's signal voltage will be 5 volts. It will drop to 0 volts only for the very low percentage of each engine revolution when a notch in the flywheel passes by. If you could average the signal voltage, it would be around 4.8 volts. The pointer of an old fashioned DC voltmeter changes direction so slowly, that it will tend to average the signal voltage, but it still isn't an effective diagnostic tool.

The first thing to do is to determine if the Automatic Shutdown (ASD) relay is turning on. One of the easiest ways to do that on this engine is to back probe the dark green / orange wire at the ignition coil. You should find 12 volts there for only one to two seconds when you turn on the ignition switch, then again during engine cranking.

If there is no voltage for that first second, try swapping in a different relay. The AC compressor relay should be the same part number. If that doesn't work, suspect the engine computer, but that would be rare. Most likely you will have 12 volts for a second or two. If it doesn't reappear during cranking, the most likely suspect is the crankshaft position sensor. Since you already replaced it, suspect the camshaft position sensor next. Also be aware the crankshaft sensor needs a very specific air gap. If your new one had a raised plastic rib on the end, that is to set the air gap. If you are installing a used sensor, the remaining part of that rib must be removed and a paper spacer applied.

If you have 12 volts at the ignition coil during cranking, the crank and cam sensors are working. Check for spark and fuel pressure. If only fuel pressure is missing, try banging on the fuel tank while a helper cranks the engine. If the pump starts running and the engine starts, the brushes in the pump motor are worn. It will stay running until you stop the engine. It may or may not run the next time you crank the engine.

If the problem just started in real cold weather, and only the fuel pump is dead, there were some early pump bodies that warped when cold and caused it to bind. If the engine starts after the truck sits in a heated garage overnight, suspect the pump.


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Thursday, February 11th, 2010 AT 11:54 PM

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