Jeep Repair Problems? Ask a Mechanic for Answers ASAP
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When this happens, the engine controller (pcm) is down. It could be the controller itself or an outside electrical problem that is taking it down. A lost power or ground feed to the module, a sensor ground circuit shorted to voltage, or a reference voltage circuit shorted to ground will all cause the engine controller to go down. You will have no spark, no fuel pressure, and the fuel gauge and voltmeter on the dash won't come up with the key on. There won't be any communication with a scan tool because the pcm is dead to the world. A common cause for this is an internally shorted crank position sensor. This is mounted at about the 11:00 position on the bellhousing as viewed from behind, and the electrical connector comes up to the back of the intake. Start by unplugging the crank sensor and see if the fuel gauge and voltmeter come up. If they do, replace the crank sensor. If this doesn't make the gauges come alive you'll need to do some deeper electrical testing to see why the controller is down. I would start with the powers and grounds, then move on to the reference voltage and sensor ground circuits. I can help you with that if you've got a voltmeter and want to get into some testing while it's acting up.
Ok. We'll have to break out a voltmeter and do some electrical tests. Do you have a meter?
Sounds good. Let me know when you're ready to go!
I'm tied up at work this afternoon. I'll get back with you just as soon as I can!
Ok. Let's start by checking a couple voltages at the TPS or MAP sensor on the throttle body. TUrn the key on and unplug either sensor. Check the voltage on all three wires and let me know what you get.
Ok great. We know the pcm is down for sure.
Let's check the pcm powers and grounds. Unplug the black connector, look at the inside and you'll see that the terminals are numbered. With the key on check for power on pin 2 and 22, and ground on 31 and 32. 32 isn't numbered making it look like there are only 31 wires.
That's excellent. That made or nights better!
Pin 22 gets direct battery voltage from fuse 20 in the power distribution center, the fuse box under the hood. Double check all fuses over there with a test light on both terminals to make sure it lights on both. If you for sure don't have any blown, take note if fuse 20 actually has power or not. If it doesn't have power to it from the start we'll have to track it down. If the fuse does have power but not at the pcm you've got a broken wire in between the fuse box and the pcm.