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2012 Service High Voltage Charging System with WOT's Sensor Defeat Plug in place ????

29837 Views 55 Replies 23 Participants Last post by  Rollmann
Kudos to WOT for developing his battery coolant level reservoir sensor replacement/defeat plug. This device essentially mimics the electrical characteristics of a properly functioning OEM coolant reservoir sensor. The stock/OEM sensor is a cheap and flakydevice that can and does fail, causing the setting of fault codes that require dealer level tools to clear the codes.

My early 2012 developed the Service High Voltage Charging System error message months after having installed the WOT sensor replacement. This occurred after a successful recharge while still connected. I scanned for the DTC fault codes with a tablet running the Torque app. Initially the codes were B2AAA, P0AA6, P003E, P1FFF, P1FFF. I tried to clear the codes with the Torque app but as WOT has previously posted only dealer level tools can clear all these codes. The clearing process is in fact a reflashing of the software for the HPCM2 module. After my attempt at code clearing another scan reported P0AA6, P1E00, P1FFF.

I could not charge the battery with either my L2 charger or the stock L1. I uninstalled the WOT defeat plug and reinstalled the original stock sensor prior to bringing the Volt to my original new selling dealer service department. All recalls and servicing have been performed by this dealership. The dealer's diagnosis/repair efforts stopped at square one when they discovered that their Fluke 1587 multimeter was missing and was a required tool in the GM diagnostic procedure. Surprising to me was that the next closest Chevrolet Volt dealer also did not have the required instrument. I guess this is a reflection of how few Volts they service. I had to travel to a dealer a hour away who had the required tool.

I was thinking that there must be some real failure, separate from the reservoir coolant sensor issue, that was casing the faults. Possibly a harness issue as a result of poor workmanship when the battery compartment structural enhancement recall was done a couple of years ago. I had asked the dealer service writer to have the tech perform a reflash of the HPCM2 and Battery Energy Control Module to clear the latching error codes as a first step to see if the faults would then reappear. They refused saying that they needed to follow the documented GM diagnostic protocol steps.

They called a day later to report the Volt was now repaired with nothing other than module reprogramming. The notes on the invoice are:
"Scanned for P0AA6 and P1FFF. Isolation test resistance 250K. Bulletin PIC59206 requires to check coolant level, check moisture in battery as well inspect plugs. No issues found. 2880268 reprogram HPCM2 and BECM"
"The HPCM2 is up to date but not BECM. Programmed BECM and tested resistance. Now at correct specs 3000K. All OK at this time. Programmed CD0D3."

I am glad the Service High Voltage Charging System error message is now gone and I will be able to recharge the battery. But, all of the above is a bit baffling to me. With the WOT coolant sensor defeat plug in place for months without issue and the coolant level at the revised proper level, what triggered the error codes? How could reflashing/reprograming the HPCM2 and BECM cause the isolation resistance measured with the Fluke 1587 multimeter to change form an apparently to low 250K to a proper 3000K? This sound illogical to me. And what is the CD0D3 that was reprogrammed, and what does it have to do with the fault? I cannot find anything about the CD0D3.

Should I know reinstall the WOT sensor coolant defeat plug or leave the apparently correctly functioning coolant reservoir sensor in place?

WOT, if you see this post I would appreciate your take on this and guidance.
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I'll believe mpmoore1979's fix works (kudos!). A question remains though is first, why it's necessary, and second, why it works.

My hypothesis: grounding and ground loops.

For about 100 years so far, manufacturers of cars (and airplanes) have cheaped-out by using the vehicle frame to cut the expense, weight, and complication of wiring electrical systems by nearly half. Assuming a negative ground system, a single positive wire supplies a current path from the battery to the device and the path back to the negative battery post is mostly through the body/frame. In ancient times BE (before electronics), this worked fine. In more recent times BD (before digital), less so. Today, AD (after digital), it's a mess.

Why? My old BD airplane is a good example. The ground/frame was a birdcage of welded steel tubing. Sitting on the instrument panel was a good ol magnetic compass. When you turned on the master switch, the compass would swing. Why? Current flowing through the frame would create a small magnetic field around each tube which affected the compass. Okay, that can be adjusted out, right? Sortof. But with each additional load, such as a radio, switched on, the current flow changed which in turn changed the compass reading. The point? With a negative ground system, one has a return path to the battery, but has no clue in how it accomplishes that task, other than it will, like a lightning bolt, take the path of least resistance including paths that may go through other-than-intended devices.

Experiment: With the car running, use a DVM to measure the voltage directly across the battery terminals, say 14.2V. Then start measuring across red & black wires and red-to-ground at other locations. Don't be surprised to get readings other than 14.2V. In the majority of cases, it's because the load in that circuit is pulling the voltage down. Now get long extensions for your DVM leads. Connect one to the (-) battery terminal. Put the other at locations around the car where there are grounding points to the frame. Good chance you will find some showing a voltage reading between that point and the battery terminal when they should be the same. The term for this is ground loop.

The unit Volts is always a reference between two points. Volts (the car) have batteries in the back and most of the electronics in the front. Since there are not individual ground wires for each device, and many of the devices are "grounded" at different places, the situation is ripe for ground loops, with currents flowing between places that should have been at the same voltage potential (namely 0).

BE, a volt and half an amp is unlikely to matter. BD, more so. AD and millivolts and milliamps going where they shouldn't can take the car down.

Compounding issues is that GM and others seem to have forgotten how to do basic electricity. The Volt's ground connections are crappy. The body is thoroughly painted before ground connections are made and they depend mostly on little star washers to cut through the paint to metal. My car had a host of increasingly weird problems until I pulled apart every ground connection (disconnect the battery ground while doing this) I could find, sanded off some paint and reconnected with a little dab of dielectric grease to protect against moisture and rust. Ever since = perfect.
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