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Robert R
Robert R, ASE Certified Master
Category: Dodge
Satisfied Customers: 3930
Experience:  Dodge Master Certified, Dodge Sprinter Certified
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How do i test the signal voltages from the PCM to my

Customer Question

How do i test the signal voltages from the PCM to my sensors? I have crank but no start. 1999 dodge avenger 2.5. Checked the 5 wire connector to the distributer, 12v supply is only there when i jumper the contacts for the ASD relay. I have no spark, no power to the fuel pump and no signal voltage to the cam sensor.
Submitted: 1 year ago.
Category: Dodge
Customer: replied 1 year ago.
More specifically, I am uncertain if the PCM is good or not...
Expert:  Robert R replied 1 year ago.

Hello, welcome and thanks for asking your question. My name is*****'m a Certified Auto Technician with 28+ years experience. I will do my best to assist you. please keep in mind that I may need to ask questions of you before I can answer. When you reply please feel free to add as much detailed information as possible pertaining to your concern.

Do you have access to a code reader or scanner? From what you are describing, you may have one of a few possibilities causing this concern.

If you have access to a scanner, can you post any codes that were retrieved here? If it hasn't been scanned, that would be my first recommendation. Any codes will DEFINITELY steer us in the right direction.

From what you are describing so far, it sounds like you may have either a failed PCM that is not responding, or you may have a short in the 5 volt supply circuit causing no response from the PCM.

The clue is that you are not getting voltage at the cam sensor. The cam and crank sensor and other sensors should all receive a 5 volt reference voltage from the PCM. If this 5 volts is shorted out somewhere it will cause the symptoms you describe.

To find out if a sensor is internally shorted out, disconnect the Throttle Position sensor and check the 5 volt reference signal wire for voltage. If no volts present, start disconnecting each of the other sensors one at a time and check for the presence of 5 volts at the TPS. if after disconnecting one of the sensors, you recieve 5 volts, you have found the shorted sensor. Replace that sensor and the vehicle should start. If none of the sensors result in the return of 5 volts, you will need to continue by checking the PCM power and ground circuits... If power and grounds are good, then the PCM has likely failed...

It is normal for the 12 volt supply to only be present to the distributor when the ASD relay is engaged, you can check for this engagement on the first initial 2 to 3 seconds while first turning on the key. It should also have battery voltage present there while the engine is cranking. If voltage present while cranking, asd is ok and PCM is likely not dead....

Let me know if you have additional questions or need addditional diagnostic info, I should be able to help you get this figured out if you have access to a scanner and a multimeter and test light...

Keep me posted...

Thank you,

Robert

Customer: replied 1 year ago.
I have been using a digital multi-meter to test voltages. I have connected this to a scanner. I have not returned any codes. Also, all 5 of the wires in the 5 pin plug are grounded... I have continuity between all 5 and ground at battery. I also get 12v when connected between any of the 5 wires and battery positive.
Customer: replied 1 year ago.
Where can i find the schematic for this 5v supply? Is this provided through a voltage regulator or from the pcm?
Customer: replied 1 year ago.
I have a pinout of the PCM connectors... Should i check all the grounds and B+ supply to? Throttle position sensor, crank sensor, cam sensor, what other sensors should i be looking at? Thanks! I have already replaced the crank sensor.
Customer: replied 1 year ago.
Please see attached pinout
Expert:  Robert R replied 1 year ago.

Sounds like you may have a short to ground in the harness that goes to the Distributor. Have you tried to trace the harness to it's source and see if it is rubbed through anywhere? Look carefully at the engine harness for issues and post back any results...

As for the 5 volt supply circuit, this is provided by an internal voltage regulator IN the PCM. Let me see if I can find some info on diagnosing the 5 volt supply circuit. I'll need some time to gather the necessary info for you.

Post back the results of the wiring harness inspection and look carefully for rub through and also for areas that the wiring can be melted together such as areas where wiring runs near the exhaust manifolds or crossover pipe.

Yes, PCM pinouts will help you check the power and grounds. It will also help with the 5 volt supply. To test the 5 volt supply, the PCM connectors must be connected, you can back probe the PCM connector for 5 volts and also check the 5 volt supplies at the sensors themselves. These are spliced together at some point so if you receive 5 volts at one sensor, they should all be receiving this same 5 volts unless a wire is open somewhere to one of the sensors...

Keep me posted...

Thank you,

Robert

Expert:  Robert R replied 1 year ago.

5 volt sensors include, cam sensor (in Distributor), crank sensor, TPS, MAP/MAF sensor, A/C Pressure Sensor, Coolant sensor and Knock sensor are all the ones that come to mind.. there may be others...

Expert:  Robert R replied 1 year ago.

CHeck all pins that supply B+, all pins that supply Ground, and any that supply ignition run and /or start. The PCM can be disconnected to check these wires.

also check all 5 volt supply pins, these must be tested with pcm connected and backprobed.

If no 5 volts present at one of the 5 volt supplies, you can try to open the circuit by cutting the wire a few inches behind the connector so that it can be resoldered together. IF you receive 5 volts at the wire cut coming from the PCM, then you definitely have a short somewhere in the 5 volt circuit that will need to be traced and repaired...

Customer: replied 1 year ago.
Asd relay is functioning properly... I have a .3v signal on all my sensor wires... Including cam sensor... When I plug the same connector in then probe from the back... The .3v dissapears
Customer: replied 1 year ago.
Currently going to check the 5v signal straight from the pcm... Back of connector
Customer: replied 1 year ago.
Positive supply and grounds are good to the computer... 5v signal wire clipped from pcm connector yields .3v to ground and 13 between it and batt pos
Expert:  Robert R replied 1 year ago.

Hello, sorry for the delayed response. Since you get .3v to ground at the PCM with the wire clipped it means that the PCM has failed internally and will need to be replaced....

Keep me posted and let me know if you have any additional questions...

Thank you,

Robert

Customer: replied 1 year ago.
Is there any other definitive check to tell if the pcm is bad?
Expert:  Robert R replied 1 year ago.

That would be the definitive test. If the PCM is only outputting .3 volts on the 5 volt supply circuit with the wire clipped eliminating a short in the wiring downstream, it will DEFINITELY condemn the PCM...

Of course, the PCM power and grounds will have to have already been tested to be sure the PCM is receiving battery voltage AND a good ground path..

I have seen this in the past and replacing the PCM fixed it every time...

Keep me posted and let me know if you have any additional questions...

Thank you,

Robert

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