2008 GMC Yukon, Reduced Power Mode, Code P0641 00 Stored

This 2008 GMC Yukon came in with the complaint of reduced power and the vehicle could not be driven over about 35 mph. There were a lot of codes stored but the only one of real relevance was Code P0641 Symptom 00, 5 volt reference circuit 1.  I looked at data under the TAC section of the PCM and found the circuit reading fluctuating between 0 and 5 volts. While the 5 volt reference circuit 2 was rock steady. This circuit provides 5 volt reference power to the MAP, FTP, ACP, APP2,  EOP and CMP sensors.

The wiring color codes are:

MAP, FTP, EOP  = Gray

ACRP = Gray/White

CMP = Dark Blue

APP2 = Tan

The 5 volt circuit is generated within the PCM and is bussed out through six different terminals.

PCM Connector X1 Terminal 33 = FTP

PCM Connector X1 Terminal 34 = ACRP

PCM Connector X1 Terminal 36 = APP2

PCM Connector X2 Terminal 41 = EOP

PCM Connector X2 Terminal 43 = MAP

PCM Connector X2 Terminal 44 = CMP

MAP (Manifold Absolute Pressure), FTP (Fuel Tank Pressure), EOP (Engine Oil Pressure), ACRP (Air Conditioning Refrigerant Pressure), CMP (Camshaft Position), APP2 (Accelerator Pedal Position 2)

This vehicle was a VIN “K”, indicating that it has four wheel drive. Since the FTP is at the rear of the vehicle and odd wiring issues commonly occur at the front drive shaft I decided to have a quick look. Pretty obvious that we a have a problem here.


Lifting the harness clearly shows that the harness had been rubbing on the front drive shaft enough that it had worn through the split loom covering.



I could see some signs of the internal wires being rubbed through the opening in the split loom.


Pulling the wires out and inspecting them revealed that the gray and the tan/black wires had been cut into by the front drive shaft.




I cut the wires right where they had been rubbed then installed heat shrink tubing and reconnected with a quality splice terminal.


Everything back together and secured. I positioned the harness in the clamp at the center of the next picture. I also wrapped that harness just below the clamp with a wire tie. This creates an obstruction on the outside of the harness that will prevent it from “walking up the clamp” from transmission movement.


Obviously I used a scan tool to monitor the voltage on the 5 volt reference 1 circuit and was able to locate this problem in a timely manner. One could also use a voltmeter and backprobe the respective wire at one of the easier to access sensors  or the PCM. Anything less than 5 volts would indicate a problem and a visual inspection of the harness would be needed. Particularly in the area noted above. If nothing was spotted the next step would be to disconnect all other sensors on this line until the voltage was restored. One at a time and rechecking voltage along the way. I have read reports of shorts with the CMP, EOP, FTP and ACRP sensors and their related wiring as also being causes of this code.

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