DIAGNOSTIC DIVERSION

"NEW" DOES NOT ALWAYS MEAN "GOOD," AS THIS SHOP LEARNED I was called to a shop with a complaint of a check engine light on a 2004 Jeep Wrangler with a 4.0L having about 61,000 miles on it (Figure 1). The sensor could be inoperative, the signal could be corrupt by noise, or the crankshaft to camshaft timing could be out of synchronization. The shop did not have a scope, but the mechanic was willing to look over my shoulder to get a crash course on how to set up a scope and put together a game plan of what to view. Vehicles were getting more complex and engine management systems were moving to multiple ignition coils and multiple cam sensors. Time to test The cam sensor on this Jeep is a three-wire Hall Effect sensor that has a power reference, a ground reference, and a 5-volt signal line that's toggled to ground. The configuration and speed of the trigger wheel will determine the pattern and frequency of the signal you view on the screen. The signal patterns were now running true, the cam sensor ^TECHNICAL motor age garage was toggling between 0 and 5 volts, the check engine light was off and there was no pending code P0340 in memory. S JOHN ANELLO // Contributing Editor JOHN ANELLO owns Auto Tech on Wheels in northern New Jersey, which is a mobile diagnostic service for 1,700 shops, providing technical assistance and remote programming.