I got a cheap 98 Cherokee Sport 4.0 of some guy who said he couldn't get it running. When I first got it the thing had switches installed and everything just to get it to turn over. I rewired everything back to factory, uninstalled the security system, replaced the ignition switch along with the ASD and Fuel Pump Relays just cause I had a parts Jeep with them in it anyways. So now the thing at least cranks like a normal jeep.

My Issue is that after everything I've done I still get no spark. I checked the Camshaft Position Sensor and that is functioning fine (The previous owner replaced the distributor). I re-did the timing since he had it all out of wack but still nothing. I tested my NSS and that is also functioning properly.

My ultimate question is about the CrankShaft Position Sensor seeing as it is one of the last things I have to test. When I test the resistance from B to C I get around 22kOhms, not sure if that's really that bad? Also, I tested the feed voltage and it was a steady 5v even during crank. The thing I thought was weird is how the signal wire was also reading ~5v all the time. I even disconnected the sensor and that was still the case. Is the Crank Sensor supposed to pull down the 5v on the signal line? I thought it was supposed to supply a signal voltage back. Also, the ground tested good on the sensor as well.

 

Here is a video Eric O (South Main Auto) dealing with a CPS on a 4.0 I6 (on a grand cherokee). He does describe it as a pull down sensor. I think it would show 5v on signal until being pulled down.

He gets into scanning the sensor at about the 5:30 mark.

 

Alright cool, so it does pull the signal low. I was a little confused. His pulls to ground right when it's plugged in, mine never pulls to ground. Guess that means I'll be swapping out the Crank sensor tomorrow and see how it goes. I appreciate the video btw, I was looking for a decent one.

 

Glad to help. Let us know how it turns out.

I've been meaning to scope my own CPS while it is happy, so that I have a known good image of it. That way if I ever suspect it, I can just compare it to the good one. Some of the tests for CPS are good if it has completely failed, or has a general connector/voltage problem. But the one that is much harder to troubleshoot is where it tests fine for resistance and voltage, but it is send bad signals to the computer due the magnet getting weak, or the flywheel being damaged, or just quick transient disruptions in the signal (I think signal problems have to persist a certain amount of time in seconds before the computer throws a code related to the sensor).

 

Yeah, I'll definitely give an update tomorrow. I'm an Engineer so I only have a desktop Oscilloscope for when I'm doing circuitry work. I might take it out to the jeep and grab a signal off of it though.

 

So changed out the Crankshaft Position Sensor and spark was back. Turns out the fuel pump was also on it's way out so fuel was weak. Swapped that out and now we are up and running. I appreciate the help!

 

Most excellent.

 

Hope it was a Bosch fuel pump. Airtex are junk. Delphi are junk. Carter are now made by Airtex.