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Crank..PCM..the BCM, Air Bag, and the trans

Customer Question
Hi I have a 2003...
Hi I have a 2003 JGC Laredo and the Key Security light comes on and the vehicle will not start. It will crank but no start. When I place my Snap ON Verus computer on this it will not communicate with the PCM but will communicate with the BCM, Air Bag, and the trans. I am able it get it to clear the security key sometimes and do not know what I do but then it will crank and run. The PCM will them communicate and does not show any codes. I have gone through the On Demand trouble shooter and have tested all 5V sensors and all check OK. The PCM B+ fuse in the Junction block under hood 10amp will also blow and I can replace it sometimes it will blow and other times it will be ok. I have removed the PCM and did not see any burn spots on the inside board. DO you think this is a bad PCM or a security Key problem?
Submitted: 8 years ago.Category: Jeep
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Answered in 1 hour by:
6/9/2010
Jeep Mechanic: jim, ASE Certified Technician replied 8 years ago
jim
jim, ASE Certified Technician
Category: Jeep
Satisfied Customers: 1,524
Experience: ASE A6,A8,L1 and California Licensed Smog Technician
Verified

Make the following checks.

 

Test powers and grounds at the Powertrain Control Module (PCM) on the C1 connector. Pins 2 and 22 are power, then check pins 31 and 32 for ground.

Next, test the 5 volt power supplies at pin 17 of the C1 connector and pin 31 of the C2 or middle connector for shorts to ground. Sensors on the circuit are the Camshaft Angle (CMP) sensor, Crankshaft Angle (CKP) sensor, Throttle Position Sensor (TPS), Manifold Absolute Pressure (MAP) sensor and the Oil Pressure Sensor

Unplug the sensors on the 5 volt power supply, one at a time, while monitoring the 5 volt supply.

When the shorted sensor is unplugged, the PCM will wake up and scan tool communication will return.

 

The wiring harness could also be shorted to ground on one of the 5 volt power supply wires. Cut the wires at the PCM on pins C1-17 and C2-31, then check for the 5 volts on the PCM of the connector or checking for any continuity to ground with the sensors unplugged at the PCM connector on C1-17 and C2-31 to check the wiring harness.

 

The fuse powers up the fuel injectors and the Powertrain Control Module (PCM). Check around the fuel injector wiring harness for damage.

This fuse also feeds the alternator field wire. This is done through the Powertrain Control Module (PCM) so it is not represented in wiring diagrams. Be sure to disconnect the alternator and check alternator field wiring for a short to ground.

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Customer reply replied 8 years ago

 

i have answered your questions below and did not find anything

 

Test powers and grounds at the Powertrain Control Module (PCM) on the C1 connector. Pins 2 and 22 are power, then check pins 31 and 32 for ground.

 

Pin # XXXXX

Pin# XXXXX

Pin 31 Ground

Pin 32 Ground

Next, test the 5 volt power supplies at pin 17 of the C1 connector and pin 31 of the C2 or middle connector for shorts to ground. Sensors on the circuit are the Camshaft Angle (CMP) sensor, Crankshaft Angle (CKP) sensor, Throttle Position Sensor (TPS), Manifold Absolute Pressure (MAP) sensor and the Oil Pressure Sensor

Pin#17 7V

Pin #31 7V

I also unpluged all sensors but nothing changed


Unplug the sensors on the 5 volt power supply, one at a time, while monitoring the 5 volt supply.

When the shorted sensor is unplugged, the PCM will wake up and scan tool communication will return.

 

The wiring harness could also be shorted to ground on one of the 5 volt power supply wires. Cut the wires at the PCM on pins C1-17 and C2-31, then check for the 5 volts on the PCM of the connector or checking for any continuity to ground with the sensors unplugged at the PCM connector on C1-17 and C2-31 to check the wiring harness.

 

Checked the harnes and did not find anything

also C1-17 and C2-31 is NO

 

The fuse powers up the fuel injectors and the Powertrain Control Module (PCM). Check around the fuel injector wiring harness for damage.

This fuse also feeds the alternator field wire. This is done through the Powertrain Control Module (PCM) so it is not represented in wiring diagrams. Be sure to disconnect the alternator and check alternator field wiring for a short to ground.

 

Ok on this i used a test light with it connected to power and tested each wire and got a very faint light. I checked it with test light attached to ground and nothing.

when i test it with a DVOM it reads 11.94

Jeep Mechanic: jim, ASE Certified Technician replied 8 years ago
Next, test the 5 volt power supplies at pin 17 of the C1 connector and pin 31 of the C2 or middle connector for shorts to ground. Sensors on the circuit are the Camshaft Angle (CMP) sensor, Crankshaft Angle (CKP) sensor, Throttle Position Sensor (TPS), Manifold Absolute Pressure (MAP) sensor and the Oil Pressure Sensor

Pin#17 7V

Pin #31 7V

I also unpluged all sensors but nothing changed

 

You should not have 7 volts. Check the power and grounds going to the computer. If they are good, you more than likely have a problem with the computer. The computer sends out the 5 volt referance signal.

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Customer reply replied 8 years ago
I had the connectors unpluged from the computer when i got the 7v would this make a diffrence? I also pulled the Alt. and had it tested after you told me about the field wire. The Alt. tested ok but they were not required to put a load switch on to see if it would take a load. they said something was shorted inside it. I replaced it and a new battery and now the Battery Gauge reads 14V my Verus reads 13V and by handheld tester reads 12V?
Jeep Mechanic: jim, ASE Certified Technician replied 8 years ago
The computer supplies a 5 volt signal to the sensor, if you unplug the computer connector you should have 0 volts.
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Customer reply replied 8 years ago

 

Does the harness on the far left supply the power to the computer. And if the other harness should not supply power when all sensors are unplugged what could supply power to this? I have not found any breaks or shorts in the wires.

Customer reply replied 8 years ago

 

Jim, I went and retested everything today and here is what I came up with again.

1. C1-02 (ING SW out) is 12.97 volts with key switch on only

2. C1-22 (Fused B+) is 12.97

3. C1-31(ground) is ground

C1-32 (Ground) is ground

I then cut the wires C1-17 and C2-31 as you said and here is what I got:

1. C1-17 (Fused +B) when cut gets 11.82 on the PCM side and nothing on the continuity to ground on the sensor side also all sensors were unplugged

2. C2-31 (+5V) when cut gets .030 volts on the PCM side and nothing on the continuity to ground on the sensor side

I checked the fuel injection harness and found nothing to show any damage

I rechecked the alternator and did not find a short to ground on the C2-11 (Gen field DRV) wire.

My question now is do you suspect that I may have a bad computer and also why does the security key light stay on when there is no communication with the PCM.

Customer reply replied 8 years ago
Do you have any other help that might assist me??
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