C0327 & Service 4WD Msg

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04 Z71:


Well,  Here you Go.....  I have 04 Silverado Crew cab Z71....  Lately I will get in and the truck will already be in 4 low or sometimes I will get in and it will show that the indicator light will say nuteral....  Though it still runs like it is in 4 high.. 

Then My "" Service four wheel drive"" pops up in my information center.   When Service 4 wheel drive comes on I have no controle of my buttons to switch to 4 high or 4 low and all  that.  But If I turn my ignition off and turn it back on the thing works for a little while till the ""service 4 wheel drive"" comes back on...

Here is what I know.  I have the same problem as you but I know something you do not know. 

There is a sensor inside the encoder motor.  I purchased the sensor today at a chevy dealership.  I cost 100 dollars.  to put it in myself. 

I suggest you all go out and buy the encodor motor sensor ... don't be fooled into buing the encoder motor itself... 500 dollars... The guy at the parts counter almost sold it to me by mistake.   But I know better..

Hope the best for you all... Have a great New Year... I only came on here today to tell you all this.. I am going to remove myself... now  Bye Bye...

sperry:
Try this: LINK

Although the position of this on your Silverado frame may be different in some ways, this may be where your transfer case gets it's ground.
It didn't clear my intermittent ABS light but I hadn't had an intermittent Service 4WD since I cleaned the frame contact area and the contact parts. Also, both of my fasteners here were loose.

BTW, it's easy access, a 13 mm wrench (if I remember :2: ) and some steel wool to remove the frame paint under the lugs and washers.

sperry:
BTW, that frame area shown in the link is under the driver floorboard/firewall.

CoveredAV:
Preamble

I don't post that often, but the subject of this thread, and the two other threads like it (that sperry linked above), was one of my main reasons for joining CACFC... I haven't found any other GM truck website as informative on this and other technical topics (like Lux Nav, etc).


Mucho Props

Much credit is due to the moderators (like sperry) for their attention to detail and organization of the information here, as well as to the many members for adding their experiences.  As a member, it is time that I give back and add to the experience pool:


On Topic

I had an intermittant Service 4WD message, throwing code C0327, as early as only 10,000 miles in my 2005 3/4 ton that I bought brand new.  The drivetrain is a 6.0/4L80E/4x4/4.10 gear with the NP8-NVG246 transfer case.

To fix it, what the dealer replaced on my vehicle was the encoder motor SENSOR,  not the entire encoder motor.  The encoder sensor is a very small donut shaped disc (about 2" diameter) that has an encapsulated circular printed circuit going around the circumference, that tails out into a connector.   

I know what the part looks like because I went to the parts department before the back counter released the new part to the technician, and asked to look at it.  They handed it to me, I looked it over, I handed it back, they handed it to the tech.

The GM part number to the Encoder Sensor is 88962315.

The GM Technical Document that outlines the procedure the dealership used to diagnose and repair faults from code C0327 is GM Document # 1720739.

The parts department already had the sensor in stock 88962315...  I was told that it is very common for these sensors to have to be replaced.

While my repair was covered entirely under the original factory warranty, I believe that the Encoder Sensor is considerably less expensive than the entire Encoder Motor, so if you are not under warranty, and a check of all connections still did not resolve the problem, you might consider replacing only the encoder sensor before buying a whole encoder motor assembly.


Ancillary Information

The scan tool that is free to borrow or rentable at AutoZone will NOT reveal this chassis code (C0327) stored in history.  Many Body (B0xxx) and Chassis (C0xxx) codes are not part of the federally mandated OBDII diagnostics system, as the diagnostic fault codes do not materially effect emissions.

Furthermore, many software scan tools (like Auto Enginuity, Auto Tap, etc) are incapable of retrieving certain Body and Chassis codes for vehicle specific ancillary systems.  I am very impressed that the Hypertech programmer was able to pull up the code.  I tried several code readers (including the best ones at AutoZone) which did not reveal the code.

However, if you subscribe to ONSTAR, believe it or not, they can pull the code, and give you the code number, if you ask.   It helps if you already know what the possibilities are, as the Onstar operator may not know what to tell you.  You can say, "do you a number like 'C0327'?"  This gives them guidance as to what to look for on whatever screen they are looking at.

To help you help them, or help yourself if you have access to a REAL scan tool (like a Tech II or a professional scan tool), here is a list of Diagnostic Trouble Codes (DTCs) that would cause a Service 4WD Message to light for the NP8 - NVG246:

B0790
B2725
C0300
C0305
C0306
C0321
C0327
C0359
C0374
C0379
C0550

Note that these are all Body and Chassis codes, which require a more sophisticated code reader endowed with the software to pull them up.  One more type code related to the transfer case shift module is a Communication fault code:

U1026


On Intermittancy

It is unwise to assume that just because the Service4WD message light goes out on it's own, that the problem fixed itself or went away.  It is equally unwise to assume that because the message appears, then disappears, then reappears days later, that the problem is intermittant.  That could be a trap of wishful thinking that might end up costing a lot more than a sensor down the road.

The intermittancy could be because the conditions for the computer to rediscover and report the fault and trigger the message light are intermittant, while the fault remains.  When the start cycle or drive cycle encounters a particular condition, and the data returned doesn't add up, then bing, the light comes on.  After that drive cycle is complete, and the next drive cycle is begun, it may well be that the computer hasn't encoutered the condition during the next drive cycle that would cause it to report the fault.  But the underlying problem could still be there.

The safest course of action is to have the Av scanned as soon as possible after the first Service 4WD message light appears, even if the light disappears after the next ignition cycle.  It is best to retrieve whatever codes were set and stored in history, so that you don't continue to drive in such away that a sensor part grows into a hard part breakdown, because the sensor was giving the wrong data to the motors that move the hard parts.


A Possible Root Cause

I've given a lot of thought to this Sevice 4WD / Encoder Motor / Encoder Sensor issue, and have tried to figure if there is anything I might have done... any type of driving behavior or otherwise, that could have caused the fault.

Surely, after the GMT800 chassis was already 6 years in production, GM would have noticed the warranty claims and would have curtailed this liability by adjusting the part to make it more robust.  I'm thinking they did that, but there is something that we driver's might do that GM engineers have not anticipated.

Here's what I DIDN'T do:  No Offroading, No Mudbogging, No brake stands, No drag racing, No chips, no programmers, no exhaust, no intake, no nothing.

My vehicle was bone stock, exactly as it was when it left the factory, and is driven conservatively by someone with graying hair, who excerised the 4WD system to keep it maintained and lubricated by operating it on a level, loose gravel parking lot once in a while.

But here is what I DID do, that MIGHT have caused the error: I changed the oil.

Don't laugh.  I'm serious.  And while I was changing the oil, I rotated the front driveshaft with my hand.   Why did I do this?  And why might it matter?

First, why it might matter... (but I do not know that it did... below is just an untested, unproven hypothesis of mine, revealed here for your challenge or consideration):

The circuit description for the transfer case encoder states that the encoder is an analog sensor that returns a signal that represents the physical position of the transfer case sector shaft.  I rotated my front drive shaft by hand, and I do not know what that other components inside the transfercase also rotated and changed position from where they were before the front of the vehicle was lifted in the air for me to change the oil.

The front driveshaft would otherwise have no other way to change it's rotational position unless commanded to engage by the transfer case control module.  If this theory holds any water, and there was a connection between the front driveshaft and the encoder position sensor, and the last known position state was altered unexpectedly, causing a data fault that could not be resolved, then maybe I should't be playing around with my shaft.

So, why did I rotate the front driveshaft while changing the oil?

Simple.  I wanted to see if the front drive shaft in my "Texas" assembled truck was made in China, like the rear drive shaft is (Pudong Province, to be specific).  I was rotating the front shaft to find a label of manufacturing origin.

After that oil change, my Service 4WD light appeared for the first time.

Now, think about when you change the oil, or when you bring it to the local Oil Changer's place for them to change it.  Do you use the front drive shaft to "pull" yourself under the truck as you roll along in a creeper? 

I used to do that, because the driveshaft is the cleanest thing to grab, as the frame has all that black sticky goo anticorrosion wax all over it.  I sure don't anymore.  Whether my hypothesis is right or wrong, I don't touch that driveshaft now.  Do you know what the oil changer guy is grabbing a hold of when he is under there?


Conclusion

Hope some of the earlier information helps.  It took a long time to gather it all, well over two hours to type it all, and I know this is an unusually long post, but this site was very helpful to me when I had this particular problem, and maybe this info can be added to the other info to make a one stop shop for future AV and GM truck owners with this common problem.

CoveredAV


sainiak:
Thanks to everyone ( especially sperry and CoveredAV) for the detailed information.

I have a 2003 Avalanche that has been getting the SERVICE 4WD message "randomly" for the last month.  When I took it into the dealer, there were no active messages.  History showed C0306, C0327, C0379.  They cleaned all connections and found no issues.  But the error reoccurred a few days later.

CoveredAV - Assuming your hypothesis is correct (rotating the front shaft caused the SERVICE 4WD message to appear, would the replacement of the encoder motor sensor fix the possible misalignment?  We usually drive in Automatic 4WD but with the bad weather recently, we have been using the 4WD High setting.

The only reason I ask is I had an oil change/4X4 service a few months back, and I am wondering if we did not expose this problem until we started to use the 4WD system.

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