jeep cherokee 4.0 petrol 1996. Non Start

[avc]

This would not start one morning was fine when parked up, I have tried to communitcate with engine ecu and had no comms with three different tools,   so i have done some manual checks. First thing i noticed that when the ignition is switch on i only get the parking bake light and abs (abs goes off after 5 sec as normal)  but i don't have the "Engine Check Light"  or battery/ign light. After doing a few check i had no spark and no fuel,  i then check Crank sensor (three wire) i should have a 5v feed from the ecu pin (A4) with ignition on I have 0.3v with ignition on,   also the engine relay and fuel pump relay do not recieve the earth switch from the ecu pins c19 and c3. i have also checked earths to the ECU and have bat and ignition feed

Would i be right in thinking that the ECU could be at fault here?  As i can not see what would supply the 5v to crank sensor apart from the ecu it self.  I also thought that the ecu would switch the relays once the 5v is supplied.   Am i missing something here?  appreciate any thoughts


Regards Darren

[avc]

Update on this one, the 5v from the ecu to crank also supplies the TPS, map and some other components, now if i disconnect the crank sensor my 5v from the ecu is there and also at TPS, map and so on. I don't have any info on testing the crank sensor any one help me out here??

Regards Darren

[Shuggy]

had one a while ago immobilise problem ? does this one crank ?

[KevMT]

Darre,

Are all the guages on the dash dead?

Try this ,remove the ECU cover and then try starting it!

[avc]

Shuggy it does crank fine,

KevMT,  the fuel gauge is the only one that works, i can't test the others so i presume they are OK.

Further to my investigation, if i disconnect the crank sensor my 5v feed is there, also at TPS, Map and so on, and from the info that i have found terminal A & B on the crank sensor should be 175 - 225 ohms  i have .3 ohms, so is it possible that the crank sensor is short to earth and the ecu drops the the voltage to stop it from being damaged??   I have ordered i new sensor.  Just trying to understand why i had .3v with it connected. 

Regards Darren

[KevMT]

Darren

If you have 5v at the connector disconected and 0.3v at the pin with the connector in place the sensor is shorting to earth.  It becomes a series circuit and the sensor when connected is acting like load in the circuit, what you are seeg is the last of the volt drop to earth.

If a new sensor dose not cure it, try removing the ECU cover (exposing the PCB) and then try starting it.

[dodgemnz]

What tools have you tried to scan this with? Ive used my launch on them ok.

[avc]

Fitted crank sensor, now i have my 5v feed with sensor connected, but still no go, I have tried to open the ECU and expose the board but its not easy to separate and i don't want to damage it.  Basically the fuel pump relay is not pulling in nor the engine relay, but if i introduce earth to the earth of the relays they will pull in and i get fuel pump running and power on the injectors, the earth  to the relay comes from the ECU i think, any suggestions 

Regards Darren

[KevMT]

Darren,

Yes ECU will provide the earth for pump relay.  During cranking are the injectors pulsing?  Would reccomend a final volt drop test on all supplys and earths.  If still no go then the ECU cover removal test may prove usefull.  The cause of this no start can be atrebuted to one of the ECU cover screws shorting the PCB.  The test is to remove the cover and start the car without the cover, if the car starts and runs you then with the engine running replace the ECU cover and slowly close each cover screw.  If this is the fault one particular screw will short on its way in and thats the screw to replace with one of the same pitch but half the original length.

[avc]

Thanks KevMT,   Do you think that the CPS could of caused problems with the ECU,  from the customers point of view it looks like i have fitted a CPS and it still does not work! but the crank sensor was definitely faulty. I even connected the old sensor back in to double check my diagnosis.

I will volt drop an all earths and supplies, if they are ok and try again with the ECU cover removal.

Regards Darren