The ol' "check engine" light

[PrestonP]

It is bright and colorful, and kinda draws you in - but I need help with my check engine light.
My 94 Jeep Grand Cherokee will not start if the check engine light does NOT light up when I turn the the key to the "on" position. However, if the key is turned to the "on" position and the check engine light DOES light up, then the Jeep starts without a fuss.

check engine light = deep Jeep purrrr
no light = turn over but no start

any advice?
thanks - This forum has been a life saver! Thanks
Preston

[dksob81]

bad CPS.

it could the PCM as well but the CPS is the known culprit.

email me I have a test you can do (do this test when the CHECK ENGINE light does not come on - in morning when first starting)

Joe
[email protected]

[PrestonP]

Joe, Thanks for the help. Sorry, but what is a PCM and CPS and where are they located? I live in BC, Canada where it rains all winter long. I have the feeling that the cold/rain somehow affects my problem - will this test rule out moisture as a factor? Also, my alarm has been giving me troubles, could that also be related? Thanks for your help - this is a great forum.
Preston

[dksob81]

PCM - PowerTrain Control Module (aka: ECU, ECM) - Computer

CPS - CrankShaft Position Sensor.

No, this test will not rule out Moisture as being the problem. No I don't think the alarm is the problem.

- one way to determine a bad CPS, is when the CHECK ENGINE light DOES NOT come on, pullone of the wires off of a plug and hold it close to something metal (bolt) and have someone crank the engine while you look for spark - NO SPARK, Also check to see if the fuel Pump is kicking on, take gas cap off, Place you ear next to the filler neck, have someone turn the key to the ON position and listen closely for the fuel pump to kick on (whirring sound).

NO FUEL PUMP and NO SPARK = Bad CPS, it could also be the PCM but like I mentioned about most likely the CPS they are rpone to go bad causing these and numerous other problems. If you would like the test for the CPS email me.

The PCM is located in the Engine Compartment on the Passenger Side firewall (behind the Coolant reserve bottle - on 5.2L/5.9L engines, not sure about 4.0L).

the CPS - on 4.0L is located on the driverside of the Transmission Bellhousing. On 5.2L/5.9L it is located on the PassengerSide Engine block right behind the right Exhaust manifold.

[1BADGTO]

Yep, PCM is also in the same spot for the 4.0L. I agree with the CPS diag. I think what happened to mine (4.0) was where the CPS comes out of the Driver side bellhousing was close to the body and fatigued the wire. My wire was in bad shape. I think due to heat and vibration. Mine would just quit while going down the road.

I bought a new CPS and wrapped the wire in black tape to stiffen it. Been fine since. Preston, you might check the wire and connector for damage where moisture could screw things up. Good luck!