Jeep Cherokee Sport-no Start-no Fuel-no Communication
Fuel Pump ?
The other day a 99 Jeep Grand Cherokee 4.0l was towed into the shop. The car had died on the highway and wouldn’t start anymore; it would crank but would not start. The check engine light was on so the first thing that I did was to check for codes at the pcm (powertrain control module) but there was no communication with the scan tool. At this point my best diagnostic tool was out of commission so I had to go back to the basics and check for spark and fuel pressure, just to find out that I had none of these. I checked the fuel pump relay and noticed that the pcm wasn’t grounding the relay and that was why the fuel pump wasn’t working and in turn no fuel pressure. The reason why the pcm wasn’t grounding the fuel pump relay could be because of a bad input sending the wrong information or no information at all, or a bad pcm itself not being capable of processing the information or not being capable of sending the output signal. So I started to check for powers and grounds at the pcm on the c1 connector. Pins 2 and 22 are power and pins 31 and 32 are ground. They were all good. Now because I had no communication with the scan tool and no spark and no fuel I had the gut feeling that I was possibly looking at some sort of shorted sensor or even a shorted wiring harness and the easiest way to start checking for that is to unplug one sensor at a time and check for scan tool communication. I started with the camshaft position sensor, unplugged it and nothing, still no communication with the scan tool, next was the throttle position sensor, and again nothing, the next one was the crankshaft position sensor and VOILA, the pcm woke up and scan tool communication returned.
All it was, was a shorted crankshaft position sensor. After it was replaced the car started right up and we had one more happy costumer.
If I had had communication with the scan tool to the pcm I would have been able to look at the engine rpm when cranking and called for a crank sensor if no rpm was seen. Since I had no communication the fix become a bit harder and I needed to go around it. Basically the description above was the quickest and easiest solution to find the problem.
P.S. The wiring harness could have also been shorted to ground on one of the 5 volt power supply wires.
Jeep Dealer...yup...make Sure the Sensor Is from the Jeep Dealership