93 Cherokee Crank Position Sensor test

[seanzy13]

I am working on my neighbors 93 that just suddenly quit running much like my own 88 with a bad cps several months ago. The problems sounded the same so I checked it out.

According to the manual I'm supposed to get infinite resistant between the B and C terminals. I tested this as per a diagram I found online and I appear to be getting ~33 million ohms between B and C. Is this likely a bad CPS? I don't really want to chance it because its a $65 part and its not my jeep, so I'd like to be sure.

Other background information is that its not sparking, but it is getting power to the coil. So it narrowed down to a bad coil, distributor problems, or a bad CPS I believe. Anything else in there that could be causing it? Plug wires appear to be in good condition.

 

[MudderChuck]

Not real familiar with the 93 I have a 96, many of the tests are the same, just a slightly different layout.
Says in my troubleshooting list that the resistance should be infinite using the 1K-10K scale, this can be important. The Higher the ohm scale used, in my experience, the more error I'm likely to have.
Sounds simple, but keep your fingers off the metal part of the probe (you may be measuring the continuity through your body). Make sure your connector (plug) is clean, I use a quality electronic contact spray.
In the 93 you should be able to use the key method and retrieve the MIL codes, counting the number of times the CEL blinks. If you get a "no crank reference signal detected" code, it is likely a faulty sensor, dirty connector or the bad continuity in the wiring. Codes can fool you though, the 5 volt reference signal is shared by other sensors and can give a false reading on one sensor or another.
When my CPS went bad, the fuel relay would close for a few seconds to prime the fuel rail, as soon as the key was turned to run on the way to crank. Then wouldn't close again as the engine was cranking over like it should. During cranking I'd have very little, to no fuel pressure, along with no spark during cranking (starting).

 

[seanzy13]

The meter I'm using is digital and has no range selection, it does that automatically. Thats why I was curious if this meant that over 10k ohms is good, even though it its not exactly infinite.

How do you retrieve the codes using the "key method"? I'm not at all familiar with this model of jeep.

 

[MudderChuck]

The meter I'm using is digital and has no range selection, it does that automatically. Thats why I was curious if this meant that over 10k ohms is good, even though it its not exactly infinite.

How do you retrieve the codes using the "key method"? I'm not at all familiar with this model of jeep.

When the computer fails to receive a (pulse) signal from the CPS during starting, it will stop spark and open the fuel pump relay. The fuel pump relay will close for a few seconds normally when the key is turned to run, it seems to be on a timer. If your not getting spark and no fuel (significant pressure and fuel column at the test valve on the fuel rail) while you are cranking (starting) it is a sign the CPS is bad. I've also heard a faulty sync sensor can cause spark grief, never had that problem myself, I'm not exactly sure of the symptoms. The sync sensor and the CPS share a 5 volt supply circuit, so if one fails (shorts), it can affect the other.
I've had faulty CPS, that tested fine, open between the "B" and "C" pins, but still turned out to be bad. The ohm test doesn't seem to be definitive, but any sort of ohm reading between "B" and "C" has got to be suspicious. Hopefullly not a meter gremlin. I occasionally get some questionable ohm readings when testing any sort of coil, is there a coil in the CPS?

Turn the key to run, then to off, then to run, then off, then to run. Count the blinks at the CEL (Check engine light), code 12 and 55 are normal codes, anything else may be problem. They say the codes repeat, mine don't. A typical code will be five blinks, then a momentary pause then five blinks, code 55 (end of test). I'm pretty sure a faulty CPS is code 11. The onboard diagnostics are often not diffinitve, but give you a starting point.

It may take a couple of tries to get the on board diagnostic to work for you. I found I have to use a rhythm, too slow it deosn't work, too fast it doesn't work. May just be a quirk of my system, like my computer not repeating the codes.

 

[seanzy13]

Well I've got some more time to work on this jeep again so I'v got it in the garage ready to start replacing. I went ahead and checked computer codes, and all I get is a code 55 and it doesn't repeat for me. I still have a feeling its the CPS, so were going to replace that first.

 

[seanzy13]

Well I changed the CPS sensor, but it did not restore spark. I removed the coil and tested it per the instructions I found here:
How to Test the Ignition Coil on a 1993 Jeep Cherokee | eHow.com

and if this is correct, the coil appears to be good.

What else could be causing no spark? I checked the cap and rotor and both appear to be in fairly good shape actually, I dont see anything unusual under there.

 

[MudderChuck]

The dark green and orange wire going to the coil should be hot with the ignition in the run position.
The black/gray wire is the trigger wire and is fired/triggered (grounded) at the PCM.

You can hold the high voltage wire near a ground, about a quarter of an inch is usually good and ground the black gray wire momentarily. All you need to do is brush it on a ground, this collapses the field and genrates the high voltage spark, don't hold it on a ground. Possible the coil has an internal short, that only shows up when it has high voltage and won't show up on a simple ohm test. Or take the coil down to Auto Zone and have it tested on the machine. The coils mounted down on the block are known for overheating and failing.

If the ASD relay is acting up you won't get fuel or spark. You may get a three second prime when you first turn the key to run, the fuel pump relay will close momentarily, but not enough fuel for a start.

Does the vehicle have a remote unlock? Some remote unlocks are hooked up to the ASD circuit. I'm kind of reaching here, just some off the wall possibilities.
If the PCM doesn't get the right signals while you are cranking, it won't generate spark.

There are some off the wall things I've hard about but never seen myself, like a shorted or way out of value engine temperature sensor or a faulty sync sensor, but these should set a code.

I'd ohm test the cap to coil wire, I've had these mess up before.

I'd also see if I had good fuel pressure while it was cranking, If you get no fuel and no spark while it is cranking, chances are it is a sensor or wiring problem. Don't mistake the little squirt of fuel from the fuel rail from the prime as the fuel pump running like it should during cranking and really pressuring up the fuel rail.

 

[seanzy13]

Will check out all of these things today. I tested the coil to cap wire and it comes out arond 6k ohms I believe it was. Would have to check again to be sure, but it seemed to be within limits. I will test the coil manually today and see what happens. For this test do I need the coil mounted to the block for proper grounding?

If this test proves sucessful, and the coil works, How can I test that the coil is getting the signal to fire? Will my ohm meter connected between a ground and the negative coil wire work? I'm guessing I should get high resistance while the car is off but i should see a pulse while cranking?

 

[seanzy13]

Well good news, I tested the coil manually and couldn't make it spark, yet it passed the ohms test. Went ahead and replaced it and it fired right up. Thanks for all the help in putting another jeep back on the road!