In this test step, you're gonna' verify that the crank sensor is getting power.
The crank sensor on your Jeep SUV needs an external power source to start creating its magic (the crank signal). Without power... well you know what will happen.
IMPORTANT: The PCM is the one that supplies the crank sensor with juice (5 Volts), so take care no to short this wire to power (12 volts) or you could fry the PCM
You'll be testing for a DC voltage supply, so keep your multimeter still in Volts DC mode.
Now, probe the wire identified with the number 3 in the image viewer, with red multimeter lead. Do not probe the front of the crank sensor connector to verify this voltage. Pierce the wire with an appropriate tool.
Ground the Black lead of the multimeter to a good ground point on the engine.
When everything is set up, turn the key to the on position and have your helper turn the key to the On position... no need to crank the engine with the starter motor.
If all is good with this circuit, your multimeter should register 5.
OK, let's interpret the multimeter test results you just obtained. Choose from the CASES below that best match your specific results:
CASE 1: The multimeter registered 5 Volts DC when the key was turned On. This let's you know that the CKP sensor is being fed with power from the PCM. There is still one more test to do, before we can condemn the crank sensor as BAD, and that is to test the ground circuit. Go to CKP TEST 3
CASE 2: The multimeter DID NOT register 5 Volts DC when the key was turned On, recheck your connections and try the test again...
If the multimeter still does not register 5 Volts, then you now have confirmed that the lack of power is the reason why the crank sensor is not working (and so replacing it will not help solve the problem).
Although it's beyond the scope of this article to find the cause of this lack of power (to the crank sensor)... solving the issue that is causing these missing 5 Volts will solve the No Start Condition of your 4.0L Jeep equipped SUV.
In this test step, you're going to verify that the crankshaft position sensor is getting ground. As mentioned earlier, the ground path is completed inside the PCM.
IMPORTANT: You got to be very careful not to short this wire, since you run the risk of damaging the PCM. Also, do not use a test light to verify this voltage... use a multimeter.
Alright, this is what you'll need to do:
With your multimeter still in Volts DC mode, probe the wire identified with the number 2 in the image viewer, with black multimeter lead. Do not probe the front of the crank sensor connector to check this ground. Pierce the wire with an appropriate tool.
Connect the Red lead of your multimeter to the battery positive terminal.
When everything is set up, have your helper turn the key to the On position.
If all is good with this circuit, your multimeter should register battery voltage (12+ Volts).
Choose from the CASES below that best match your specific results:
CASE 1: The multimeter registered 12 Volts DC when the key was turned On. Then the CKP sensor is getting a good ground. This results confirms that the crankshaft position (CKP) sensor is BAD. Replace the CKP sensor.
Here's why: A good working CKP sensor, when it receives power and ground and the engine is cranking... will generate an On/Off 5 Volt signal. Now, in your particular case:
CASE 2: The multimeter DID NOT register 12 Volts DC when the key was turned On, recheck your connections and try the test again...
If the multimeter still does not register 12 Volts, then this result let's you know that the CKP does not have a good sensor ground. You have now eliminated the crank sensor as the cause of the No Start Condition, since without this ground, the CKP sensor will not work.
“A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.”