I've read a lot of posts about crank but won't start. I've replaced my distributor cap, rotor, and coil. They did not seem to require replacement but excessive corrosion on connection points in the distributor was cause for concern. I ohmed out the coil, it seemed ok but was out of range so I figured it was sending spark but not as hot of a spark as I would like so I bought a new one. Wires and plugs are all new within the past 15k miles so I left them intact. Fact is, the coil was tested and replaced, I put a test light inline with the coil wire and turned it over, nothing, not even a dim light.
BTW... I'm working with a 1991 Jeep Cherokee Sport with the 4.0L, a previous owner did something stupid and replaced the head with a '99 - '01 0331 (non-Tupy) head but reused all of the 1991 parts. He lost the temp sending unit hole in the back so he had to re-route that but everything else is stock with the exception of some lighting and aftermarket horn. I checked all of the ignition fuses, they are good. I swapped the relay for auto shutdown and auxiliary fan, they were the same exact relays that didn't make any difference. Several weeks ago the Jeep was starting up find, a bit of a rough idle but started right away every time once warm it idled fine, never cut out or stalled. Then we got some -10 weather a few nights and I went outside to start it and nothing. I kept a battery maintainer on the battery, new battery by the way, and it never had a problem with juice.

Since the CPS is not exactly easy to replace I would like to know what am I looking for on an ohm meter if I check the pins when unplugged. What am I looking for when it's plugged in, ignition on back testing the plug, if that's even possible with that round plug they use on the 1991 motor. Every other year seems to have a flat 3 pin connector. Can someone tell me how to diagnose this sensor while it's still attached so I'm not pulling off a good sensor? What resistance am I looking for, if any? Back testing and which wires I think there is a green one, black one, and another one I can't remember. Am I looking for 5VDC on one of those? Ground on another one? and is the other one a signal from the computer, a square wave 0V-5V signal that can be seen on a scope?

Any help to test my CPS using a ohm/volt meter would be greatly appreciated. I don't have a scope to see a square wave signal. So any tricks on using a meter to see if that signal is within spec is appreciated.

Thanks

 

I just did an engine swap on my jeep and have had some issues with a rough idle and have been chasing the issue and a partially failing crank sensor is certainly a suspect. I may have solved it with a new cam position sensor but unsure. Will find out in the drive to work tomorrow.

For the crank position sensor, it you unplug it you can confirm 5v and ground on the connector. I don’t remember which one was 5 volts but you can just test both outer ones as the middle one is the signal. So if one gives you 5v then the other one is ground. But that just tests your wires and a portion of you PCU. Even if you had a scope, unless there is another way, I don’t think you are gonna put your jeep on jackstands, turn it on, pop it into drive, and then start testing wires under it. I’m not sure about how to test the actual sensor though. Sorry.

Its really not that hard to replace though. Lots of extensions and a wobble socket make it doable in 15mins or so. But everything I looked into said that you need to get a mopar sensor. So before you buy an expensive sensor you can’t return, I would keep hunting around for other causes. Have you freshened up all the grounds?

 

I haven't "freshened up my grounds but I've tested my grounds all over, not just at the battery to body, block to body, and battery to block connections, I've tested my ground between the battery terminal and starter, coil, distributor, cam position sensor, coil mounting holes, and all test perfect. Yeah they are dirty on the outside but connection point inside are all good. My son in law removed the head and replaced the head gasket, had it prepped for reinstallation by a machine shop (not a good one) not knowing that one of the valve cover bolt holes had been drilled too deep by the previous owner and it leaks coolant and requires "artwork with RTV" to stop the flow until I can get around to what you are doing... engine rebuild and a new 1991 head casting and get rid of that 0331 POS. Moral of the story, the valve cover gasket leaks oil really bad because he not only reused the gasket I recently put on but he did not seal it properly.

I did notice that there is a small pool of oil inside the bottom of my distributor... would that foul the cam shaft sensor and prevent it from working 100% while the internals return good readings as if it were not malfunctioning. I believe it is a hall effect sensor so oil coating the outside would not technically prevent it from sensing a magnetic field passing by. These are extremely sensitive sensors, unless it's malfunctioning I don't know of anything that would cause it to not catch the element on the distributor rotor turning with the cam. I'm avoiding the removal of my distributor as I do not want to go through the trouble of re-installing it one tooth off and dealing with that, nor do I want to remove a spark plug to find top dead center...
I know, that's a long list of I don't want to's.
I already bought a Master Pro sensor from O'Rielly's Auto Parts for $77.00, I can't afford to wait weeks for the dealer to get one of these in stock, if they even can. Unfortunately I was in an accident in December with my daily driver and totaled it, It was an old 2001 Duramax so I did not have full coverage on it, probably should have now that I'm looking at trying to find a donor truck same year to drop my engine/tranny into, but they don't exist because the 2001-2004 are apparently very popular and expensive despite the milage. So, I'm driving my off roader / project vehicle that has no heat or AC (heater core is on the replace list), now as my daily driver. So, I'm stranded at my house pulling favors from friends to get a ride to the parts store for parts. I live out in the country, so it takes a week for an in-stock part to get shipped to me. I will check my PCU voltage on the plug today and make sure voltage and ground are OK. I did notice when I turned over the engine that the coil plug tested 12V on the voltage side and on the signal pin it only tested, I think it was 1.6V. I'm not sure if that's an average of various high and low voltages or if it should be a higher 5V signal voltage. I don't know much about the signal voltages of the ignition system. I do know you are supposed to back test the plug with it connected to the coil while turning over the vehicle, I did not do that because I'm not and expert at shoving things into the back of my connectors. ;-)

I think I'll stick with this easy to get Master Pro sensor, I'll order a Mopar one and when I start to have problems I will have it on hand to replace it, extensions, knuckle, and all. MAN!!! I have 10" of slushy snow all over the place, my Jeep has not been driven since it started snowing so it's clear underneath but I'm going to have to crawl through the mud to get underneath there. I am not looking forward to this. Time to put on the coveralls. I'm picking up the part tomorrow (Saturday, 1/27) I'll jump on it right away, weather permitting. I'll update the post with results.

I'd still like to test the sensor side, if anyone knows the ohm specs for the sensor that tell you if it's bad or good, let me know please. In my time working in manufacturing, we used hall effect sensors in some of our products and the ones we used were pretty indestructible aside from the plastic housing they were put inside of. Simple and sealed up, not much to go bad. Seams to me that even Mopar managed to mess this up if the CPS is a known failure point.
Thank you.

 

Working in snow is not fun! I’m up in Canada. I am fortunate to have a garage however it’s not well insulated and my heater is one of those diesel heaters so it’s not high on the BTUs. Dunno what the temp was for you last week but it was -35f here for days. Icy tools are awful, but I think working in snow is worse? Can you put a tarp down to keep you dry?

 

We've been hovering around 37 during the day which is far warmer than past winters, eastern Idaho is much colder, I'm tucked in a cold spot along a small mountain range that runs down the Washington / Oregon border (or Snake River). I have tarps but they are busy covering my firewood right now. even if I just borrowed one it's soaking wet. I do have some 6mil plastic I bought to fight bindweed, I could roll out a sheet of that on top of cardboard boxes. Lucky for me I have a 3.5" lift with 31" tires so no jacking is required. My lift is a long arm so that arm might be in my way, not sure.
I saw a video where someone tested the plug for the CPS while plugged in using a volt meter. For the signal wire he had someone just tap the key with short start bumps to slowly rotate the fly wheel, it was showing low voltage and then when the sensor element came around into position it showed 5 volts. As each magnet or hall element comes around it spikes to 5VDC for a very short time. I'm assuming this is a contact made in the sensor that returns the 5 volts to the PCU indicating speed, position, RPM, etc.

 

FWIW:

Hey, stuff goes bad, y'know?

IMO although CPS gets a lot of bad press, the failure rate isn't particularly high. The above test procedure isn't that indicative of a failed sensor and most people don't/can't test them the way they really need to.

 

Cause

 

Anyway. "IMO", your CPS is OK. The reluctor windows and blanks are going by too fast to register a peak signal unless you use a DVA so you end up with an average.

If you turn the engine over slowly by hand, ignition on, you should see some 5.0s.

Just for laughs do the same with the camshaft position sensor. Better chance of seeing 5.0s cause the signal dwells much longer.

 

BTW getting any engine codes?

 

Usual symptoms on the xj for a crank sensor are, no start or hard start. Recall trying to test mine, if I recall and I may not, very hard or impossible to test with an ohm meter. This is just one of those things you should replace "anyway". MOPAR is the only one I'd use. It's actually not so hard to change, you need a guy under the hood to guide the socket on, while you lay under with about 18-24 inches of 3/8" extensions and your ratchet.

 

What engine codes would you expect to see on a 1991 vehicle?
I do not see a check engine light or any other "dummy lights" lit up on the dash if that's what you are asking. No OBD in 1991, how would I check for codes?

 

I've followed that test procedure and have an open reading. The only other test would be the one I believe you are talking about that most people don't / can't do. Requires plugging the sensor in and back probing the plug to read pin A which (w/ ignition on) should provide a constant 5VDC, pin B should be a good ground, (which I've tested and my ground is good), and pin C should be the signal which using a scope should show a square wave ranging from 0V - 5V but using a volt meter if you use the key to turn the engine slowly you'll find the 5V spot if the sensor is good. if you don't find a 5V signal, sensor is bad. The 5V signal is so short compared to how long the 0V is that the volt meter average is down around 1.5V. If the sensor lines up with the tooth where it's supposed to send 5V and you only get a 4V reading, maybe the PCU will allow your engine to continue running, maybe it will not, I don't know what the computer's tolerance is for voltage range on those sensors.

I agree with you that the above test procedure is not 100%, it will tell you if your sensor is completely bad but will not tell you if it is still sensing but is just not sending 5V back to the computer, maybe something inside has worn but is still working but not properly, computer doesn't like what it sees so it shuts down everything.

I'm about done with this. If CPS doesn't get this thing running I'm going to use it for target practice!! GRRRRRR

 

 

Status update:
I replaced the CPS... NOTE to anyone reading this in the future... if you have a long arm lift kit, this gets a whole lot more difficult, plan on more extensions for your socket wrench and a helper to hold a flashlight from the top. Top and bottom stabalizer arms plus drive shaft are all in the same area right where your arm would go, and in your line of sight.
Anyway, beast will not start still. I realized that I tested the sensor pins to confirm no shorts but I failed to test the vehicle's plug for good ground and a solid 5V. AFTER new sensor installation and no start and confirm that I am still not getting spark I tested the CPS plug coming from the vehicle harness. Good ground. However, with ignition in the on position I am only getting 0.47V to the CPS. I decided to test my CAM Position Sensor as well. Same thing, 0.47V to that guy as well. When I tested the voltage on the coil plug, it's only two pins because it grounds itself to the engine mount. I read 12V on the power side and 1.6V on the signal side (I'm not sure what I'm supposed to see on that pin and if it pulses during turn over or not. I did not back test to see if it went high or not.

Am I looking at a bad relay to the PCU? Bad ground for the PCU. The CPS Ground is good but I measured 3.2ohms on the Cam Position Sensor ground may have a bad ground there. I think the plug side get's it's ground from the PCU. I don't have a drawing or schematic of the PCU for 1991. I've looked for the field servce manual and the best I could find was 1984-1993, I know there are differences between 1991, 1992, and 1993 and then there's the whole Renix phase... how can they put all of that in one book and call it universal. So I don't trust what I see in that book.

So... what's next? replacing all my relays and fuses? Replace the PCU (nice try there). I'm pretty sure the junkyards have been stripped of all PCUs.

I may have an idea what happened. when the Jeep decided it did not want to start anymore, I have had the hood ajar just a bit for a heavy duty extension cord to go in the front of the hood to power my battery maintainer. The snow made it's way in and then when I cleaned it off to open the hood and unplug, I got some inside by the firewall all over the main engine harness. I may have leaked some water the last time I drove it into the harness or PCU and caused a short? Doesn't make sense though, I've seen guys completely submerge these things and keep on Jeepin'.

I'm stumped. Automotive electrical is not my thing, if it isn't 120VAC, I'm lost.
Thanks for your help

 

So, was this running and then quit, or how did we get to this point? I’m pretty sure the cam and crank sensors, as well as the TPS and O2-sensors, all get 5vdc and a ground. If you don’t have 5v a frequent problem is the front O2 sensor wiring shorting out. Pretty sure they all get the 5v on the same ckt