offrding

I have a 93 cherokee I bought that has no spark The cap and rotor looked real bad so I replaced them as well as the coil. It shows power to the coil for a few seconds when you turn the key on and than again for a few seconds when you turn it over. It does not show any power going to the crank sensor at any time or to the distributor at any time. can someone tell me what should show power when the key is on and when it should show power to the coil.

C. Boone+

This is what i found having same problems with my jeep.

This is copied from another forum.

"You have to understand how this system works to know how to diagnose it. It's identical to most other brands, but the part names are different.

The Engine Computer grounds the coil in the automatic shutdown, (ASD) relay for one second when you turn on the ignition switch. That sends 12 volts to the ignition coil(s), injectors, alternator field, oxygen sensor heaters, and fuel pump, or it grounds a separate fuel pump relay when one is used. The fastest test is to listen for the hum of the fuel pump for that one second.

Once the engine is rotating, (cranking or running), the computer needs to see signal pulses from the crankshaft position sensor and the camshaft position sensor to know it's time to turn the ASD relay back on. That is done so if a fuel line is ruptured in a crash, the pump won't dump raw fuel onto the ground where it would be a major fire hazard. With a ruptured line, there can't be any fuel pressure. With no fuel pressure, no fuel will spray from the injectors, the engine will stall, there won't be any sensor signal pulses, and the computer turns off the ASD and fuel pump relays. The fuel pump stops pumping fuel.

It's common for a failed cam or crank sensor to not set a diagnostic fault code just from cranking the engine. They usually only set while a stalled engine is coasting to a stop. Also, if the battery was disconnected or run dead, that will erase any fault codes and that valuable information will be lost. When there is no fault code related to a crank / no-start, you need to connect a scanner to view live data, and see what the Engine Computer is seeing. I have a Chrysler DRB3 for all of my vehicles. That lists both sensors with a "No" or "Present" during cranking to show if those signals are showing up.

Another fast way to determine if the ASD relay is turning on is to measure the voltage at the ignition coil or any injector. Look for the wire that is the same color at every injector and the ignition coil, or you can use either small terminal on the back of the alternator. A test light works best for this test because digital voltmeters don't respond fast enough. Back-probe that wire, then watch when 12 volts shows up. That is a dark green / orange wire on cars and minivans. Not sure about Jeeps. You should see the test light turn on for one second, then turn off, when the ignition switch is turned on. That proves the ASD relay is okay and the computer has control of it. What is important is if that voltage comes back during cranking. If it does not, the problem is with one of those sensors."

EvilJeepxj

Check the wire that goes into the coil pack from the cps for a pulse, check the ground strap that connects to the coil as these are junk from 93, when you check for cps pulse you can diagnose a confused signal or a bad CPS by checking for a pulse at the battery ground and the motor ground, if there is a signal at either you may have a bad ground strap but if no pulse at either it could be a faulty cps or ASD mode, make sure all your major components on the intake and temp sensors are all plugged in and none are missing and be aware that the main fuse can blow to cause this as well. If the main fuse is blown start at the rot spot in the right rear corner behind the wheel well wiring harness because that rot spot causes grit to build up on the wires and wear them out from vibration.