05 Trailblazer BAD! PCM question?

[MrGiggles]

What are the signs of a PCM going or gone bad?

Getting random start vehicle issues were I get P0651 and P0107 codes. The map sensor is brand new also installed new Throttle body, new gas pedal with the sensor, New fan clutch and water pump. Yesterday I drove the Trailblazer and stopped at the store for 5 minutes returned and the Trailblazer would not start got the P0651 again. To get it started I had to unplug the fan clutch plug and clear the codes then it started and plugged in the fan clutch again no codes since or issues since. This has happened two times before the wiring to the fan clutch seems on and wires are not crushed.

Could this be a bad PCM or Wiring harness maybe?

How or what wires do I check on the PCM to make sure the PCM is putting out the correct 5 volts for reference?


Don't want to buy a PCM if I can avoid it and understand you have to have a relearn for the cam shaft positioning sensor and let the vehicle set with the key on for the security system to sync up with a new PCM for 30 minutes if I was told correctly.

Any diagrams on wiring or advice is appreciated.

 

[mrrsm]

Good Morning...

Start your Diagnostics by pulling the Three Vari-Colored PCM harness Connectors... then Inspect the Green "Gooey "O"Rings for any signs of rubber gasket folding or misalignment... Get your hands on a can of CRC Lectro-Motive or similar Electrical Circuit Cleaning Solvent...and spray out the 'innards' of the myriad contacts and connectors of the PCM AND inside of each Connector...and after letting them thoroughly Dry Out...carefully thread them back nice and snug. You will probably require an Odd-Ball 6.5mm or 7mm Slender Deep Socket to access those unique fasteners... so rifle your Tool Box in 1/4" Socket Drawer ahead of time and you won't be disappointed.

https://www.napaonline.com/en/p/CRC091313

If it becomes necessary to Back Probe any of the PCM Harness Connectors... these PCM Pin-Out Images will prove Helpful:



THIS is a Likely Problem Area of the Fan Clutch Wiring that CAN Short the 5 Volt Circuit to Ground:



This excerpt is just an EXAMPLE from this Link that should shine some light into the Dark Corners of the PCM’s “5 Volt Reference Signal” Ruler-Like Architecture and point the finger at the likely cause(s) of what you have mentioned involving the Fan-Clutch Wiring Harness, etc:

http://www.fixya.com/cars/t8944798-truck_dtc_p0651

Hi and welcome to FixYa, I am Kelly.

Here is the Description of the Chevrolet P0651 code: (Reference 2 - circuit out of tolerance)
P0651 CHEVROLET Description Circuit Description
The Powertrain Control Module (PCM) provides 5 volts to the following sensors:
The air conditioning (A/C) pressure sensor
The fuel tank pressure (FTP) sensor, if equipped
These 5-volt reference circuits are independent of each other outside the PCM, but are bussed together inside the PCM. Therefore a circuit condition on one sensor 5-volt reference circuit may affect the other sensor 5-volt reference circuits. The PCM monitors the voltage on the 5-volt reference circuit. If the PCM detects that the voltage is out of tolerance, DTC P0651 sets. Possible causes - Shorted sensor AC pressure Sensor or Fuel Tank Pressure Sensor
- Shorted sensor circuit (wiring to AC pressure Sensor or Fuel tank Pressure Sensor)
- Failed ECM (PCM) Most likely there is a short somewhere... that is ... Grounding out the 5V reference Voltage. This will take some serious work with a meter to determine where the ground has manifested itself. I would start with the AC sensor and then check the fuel pressure sensor & wiring. This code will reset itself after 3 successful key strokes and there is no longer a short / loss of 5 V reference 2 voltage.
Thanks for choosing FixYa,
Kelly

Here is an EXAMPLE of the Diagnostic Flow Chart being followed on a similar PXXX Code:

HOW TO... with DTC P0654 fault code: 2008-12-18_205440_p0654

As for the New MAP Sensor... This Video shows the means for testing it AND TESTING THE 5 VOLT REFERENCE SIGNAL ... regardless of whether or not it is an OEM ACDelco flavor as called for by GM. LOOK FOR SHORTED OR DAMAGED HARNESS WIRES AS THIS MIGHT BE CAUSING THE P0651 CODE:

 

[Mooseman]

For the schematics, you can get them from the link to the manuals in my signature.

What brand fan clutch did you get? The store/unknown brands as well as Dorman are junk. Hayden or Behr/Hella/ACDelco are recommended. The fact that you unplug it and it works is pointing you in that direction. You can drive with the fan unplugged if it's not too hot out and see if this theory holds (others have driven with non-functional clutches without overheating) I'd check for good consistent continuity between the fan connector and the PCM. Should also check the others on that same 5v reference circuit.

 

[TJBaker57]

This document lists many of the 4.2 codes and the conditions which cause them to set the particular DTC.

Your issue clearly seems to be a short to ground of some component of the 5 volt reference #2. Could be bad wiring or a failed component. As is almost always the case, the code DOES NOT point the finger at any specific part. Only the entire circuit is identified.

https://gsi.ext.gm.com/gmspo/mode6/pdf/2004/2004file2_LL8.pdf

 

[TJBaker57]

The Torque app with a cheap bluetooth elm327 clone can display the values of the two 5 volt reference signals.

Lacking that one could backprobe a connecter to display a 5 volt reference then go wiggling and jiggling wiring at the various connections and such watching for meter fluctuations.

 

[MrGiggles]

All the pdfs posted are helpful thank you also answer the question about the fan clutch it's ac delco also ordered that electrical clean going to try all suggestions does not hurt to do them all. I found the the Bluetooth scan you suggested on over stock on sale for $8 so ordered one.

Thank you every one I will post how it turns out.

 

[MrGiggles]

Tried cleaning all connectors and wiggling them while idling and watching 5 volt ref with my scan tool no changes. Leaning to bad PCM unless anyone has any ideas. Wonder if the previous owner may have tried to reprogram the PCM and screwed it up because I have to click the unlock button on the remote then turn the key to start the trailblazer. If you don't push the remote unlock button the dash lights and accessories all go out when you try to start the truck and stay out until you release the key. I installed a new ignition switch so that's not it.

 

[TJBaker57]

Tried cleaning all connectors and wiggling them while idling and watching 5 volt ref with my scan tool

Does your scan tool show two different 5 volt reference signals? If so did you try looking at both? In the past I have read about more than one 5 volt reference signals from the PCM and the thought that maybe they are tied together inside the PCM. I have no idea if they are tied together or not except that the Tech 2 reports 2 different PIDs for 5 volt references so maybe they are not tied together.

 

[mrrsm]

A Few Reasons NOT to Condemn the PCM:

(1) The Vehicle-Engine Starts and Runs... Nominally.

(2) The PCM is Properly Reporting Fault Codes (P0651 and P0107).

(3) A Visit to THIS link will describe a problem similar to yours as a P0641 ... BROKEN REFERENCE VOLTAGE ON CIRCUIT A

P0641 Sensor Reference Voltage A Circuit Open

While the P0651 and P0107 codes You are experiencing are likewise occurring EXTERNAL to the PCM as a BROKEN REFERENCE VOLTAGE ON CIRCUIT B

P0651 Sensor Reference Voltage B Circuit Open

(4) Perusing the Digital GM OEM Shop Manual suggested by @Mooseman will provide you with PAGES of PXXXX Fault Codes... and their Diagnostic Flow Charts Specific to Your Year-Make-Model Vehicle.

(5) Widen your Investigative Gaze... and employ a DMM (Digital Multi-Meter) as well as observing the On Screen information in common between OTHER GM Vehicles when Using OBD2 Scanning Software (Torque APP) during the attached Video.

(6) Most critically, Observe The Diagnostic Principles repeatedly mentioned by @TJBaker57... IN ACTION. These ideas cross GM Platforms and Engines ...and yet such Videos STILL Have Instructional Value. Here is a PERFECT Example demonstrating this approach specific to the Problem of Diagnosing a P0641 Error Code... But these principles also apply for the P0651 Code:

The Simplest Explanation of ANY Problem ...is ALMOST ALWAYS ... 100% Correct..." Occam's Razor

 

[budwich]

Your problem suggests more of a problem external to the PCM since unplugging a wire allows you to start the vehicle. Further, the problem suggests a "weakness" such that it might occur as a result of the engine warming as opposed to a "cold start". It would be wise to carry some form of meter such that If the problem occurs, you can do a resistance measurement of the "unplugged" clutch fan to ensure that it is not seeing a very low / shorted resistance. Of course, checking the same resistance at others times would provide a "baseline" to compare against.

 

[TJBaker57]

(5) Widen your Investigative Gaze... and employ a DMM (Digital Multi-Meter) as well as observing the On Screen information in common between OTHER GM Vehicles when Using OBD2 Scanning Software (Torque APP) during the attached Video.

(6) Most critically, Observe The Diagnostic Principles repeatedly mentioned by @TJBaker57... IN ACTION. These ideas cross GM Platforms and Engines ...and yet such Videos STILL Have Instructional Value. Here is a PERFECT Example demonstrating this approach specific to the Problem of Diagnosing a P0641 Error Code... But these principles also apply for the P0651 Code:

[/B][/FONT][/SIZE][/COLOR]

In the video the author incorrectly describes an open circuit. An open circuit and short circuit are not the same thing and effectively troubleshooting the two will be different.

 

[Mooseman]

Download the manuals and check the schematics for the 5V reference. Could be a short or break somewhere that affecting it.

And unplugging a sensor might not help if it's something like the pedal sensor or the throttle body, both of which are on the same 5V reference.