I have a 2001 Grand Cherokee Laredo with the 4 liter 6 cylinder. I have no idea what this sensor is (or if it is even a sensor). It only has one wire going to it. I am not sure if it is mounted in the right location, but it is on the side of the head near the dipstick tube & cam position sensor. I would like to know what it does, and is there a test to know if it works. It is an open circuit between the mounting tab and the single pin. Maybe it is supposed to be that way. I just put in a new long block and am now troubleshooting some electrical issues. The other place that it might mount is on a valve cover stud near the back of the valve cover on the dipstick side of the engine.

 

Welcome to CF!

I can't tell you for sure because my 2000 doesn't have one that I can find, but it might be a knock sensor. If the fuel charge detonates instead of burning smoothly, the cylinder walls vibrate like a bell from the pressure wave. A knock sensor tells the PCM to retard the spark advance to eliminate it. From the wear pattern on the mount, it looks like it goes under a head bolt or ground connection.

 

The hole in the mounting tab is for a 1/4" or 6mm bolt. Much too small for a head bolt. I thought it might be either a knock sensor or a thermal limit sensor. but neither shows on any diagram that I have.

 

Okay, I found it on mine. It mounts on a stud screwed into the side of the head just above the gasket surface under the coil rail. It's a capacitor that protects the ignition system from voltage spikes from the ignition coils as the fields collapse causing the spark. IDK if it's large enough capacitance to react to a charge/discharge test. I know large caps can be charged, then shorted and cause a spark or flash a test light. I'm just assuming that's what it is because the 4.7 has 2 of them and that's what they are for. One on each head. The 4.0 has 3 coils that fire all 6 plugs in pairs, one on compression and one on exhaust.

I wondered about the possible coil backlash because of the way the spark is created. The coils are energized constantly, grounded thru digital drivers in the PCM and ungrounded in sequence to fire the plugs. There used to be about a 40 volt flashback in the primary of the old can coils when the magnetic field collapsed and I wondered how the drivers could handle that. Now I know! You learn something every day!

Now we BOTH learned something! Without that cap in the circuit, you could conceivably fry the drivers.

 

Good research, thank you. With the part number you provided I found it on Mopar Overstock Online (plus other sites). You are absolutely correct (the parts guy at the local Jeep dealer said he had no clue and had never seen one before). For the price, I will probably just get a new one to be safe. There is not much point in testing a capacitor if you don't have all the specifications for it.

 

Here is a picture of location of 'ignition capacitor'. 2000 WJ 4.0

 

Just for your own information, back in high school (1963), my friends and I were trying to build a transistor ignition system using the large power transistors that had just come out. We kept toasting them until we learned about the backflash. A diode and capacitor took care of that. We still used the points, but they only carried the trigger load of the transistor. A teacher was going to help us apply for a patent, but someone beat us to it!

 

"It is an open circuit between the mounting tab and the single pin. Maybe it is supposed to be that way."

Yes, true, a capacitor will look like an open circuit to a continuity tester.

I've got to ask...Are/were you able to hear your ignition through the AM radio?