ENGINE CRANKS BUT WILL NOT START
When the engine cranks normally but won't start, you need to check ignition, fuel and compression. Ignition is easy enough to check with a spark tester or by positioning a plug wire near a good ground. No spark? The most likely causes would be a failed ignition module, distributor pickup or crankshaft position (CKP) sensor.
A tool such as an Ignition System Simulator can speed the diagnosis by quickly telling you if the ignition module and coil are capable of producing a spark with a simulated timing input signal. If the simulated signal generates a spark, the problem is a bad distributor pickup or crankshaft position sensor. No spark would point to a bad module or coil. Measuring ignition coil primary and secondary resistance can rule out that component as the culprit.
Module problems as well as pickup problems are often caused by loose, broken or corroded wiring terminals and connectors. Older GM HEI ignition modules are notorious for this. If you are working on a distributorless ignition system with a Hall effect crankshaft position sensor, check the sensor's reference voltage (VRef) and ground. The sensor must have 5 volts or it will remain permanently off and not generate a crank signal (which should set a fault code). Measure VRef between the sensor power supply wire and ground (use the engine block for a ground, not the sensor ground circuit wire). Don't see 5 volts? Then check the sensor wiring harness for loose or corroded connectors. A poor ground connection will have the same effect on the sensor operation as a bad VRef supply. Measure the voltage drop between the sensor ground wire and the engine block. More than a 0.1 voltage drop indicates a bad ground connection. Check the sensor mounting and wiring harness.
If a Hall effect crank sensor has power and ground, the next thing to check would be its output. With nothing in the sensor window, the sensor should be "on" and read 5 volts (VRef). Measure the sensor D.C. Output voltage between the sensor signal output wire and ground (use the engine block again, not the ground wire). When the engine is cranked, the sensor output should drop to zero every time the shutter blade, notch, magnetic button or gear tooth passes through the sensor. No change in voltage would indicate a bad sensor that needs to be replaced.
If the primary side of the ignition system seems to be producing a trigger signal for the coil but the voltage is not reaching the plugs, a visual inspection of the coil tower, distributor cap, rotor and plug wires should be made to identify any defects that might be preventing the spark from reaching its intended destination.
Tuesday, June 2nd, 2009 AT 7:50 PM