Here are a few random scope captures from some of the cars we worked on the last few weeks.

In this one, the Grand Am would intermittently crank but not start. This was after taking care of a bad crank sensor, and fixing a wire that had been cut to the ignition switch Passlock sensor.

This was one of the events where the car cranked and started, but it did not start right away.
The tan trace is power to the coil pack, the green is the electronic spark timing command from the PCM to the module,
The red is the reference signal from the module to the PCM, and the blue is the crankshaft position sensor signal.

Here you see a close up of the failure, during key on you can see the tan at 12v, but it falls to 0v when the engine is being cranked. The blue trace shows the crank sensor signal, but with no 12v to the module assembly, you don't see the reference signal to the PCM to inform it that the engine is indeed being cranked.

Here is a Chevrolet Prizm electrical system test. The customer had a battery installed but wanted to be assured if everything else was OK or not. This first capture is the starter draw and batter voltage during cranking.
 The blue trace is the current and this is a classic drawing of a good starter, battery, and cable connections. The initial draw is over 600 amps for a brief moment and the battery is only pulled down to 9.2v. This occurs just before the starter starts to spin. Once the engine is cranking a normal cranking amperage is seen in the 150 amp range.

Here in the next capture we tested the alternator at 2000 rpm and it is putting out 70 amps and the alternator ripple  waveform shows that all of the diodes are good. A bad diode would have a repeating pattern  of distortion instead of all of the peaks being uniform.

The last thing to check is for a drain on the battery. This test  passed with no parasitic drain .

By the way, remember this from a few weeks ago? This is just a bad starter from a Honda and it makes for a good comparison for the capture on the Prizm.

#GrandAm, #alternator, #drain, #Chevrolet, #Passlock