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Intermittent "SERVICE FWD" on 2004 GMC Sierra K1500 Z21 Pickup Truck

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MikeMl

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I have been having an intermittent "SERVICE FWD" appear on the dashboard display of my 2004 GMC 2004 GMC Sierra K1500 four wheel drive Z21 Pickup (same as CHEV Silverado) for the past several months. Of course, it is out of warranty.

When this happens, the push-button 4WD will not shift into 4HI or 4LO. The Transfer Case Control Module (TCCM) seems to do a self-test during the engine start sequence, the self-test fails, and it turns off the LEDs, puts out the "SERVICE FWD" message, and stores a trouble code. It takes a special code reader to read the trouble codes which only GM Dealers have. They charge $95 to read out the codes, so that is not much help to me...

Searching the WEB, I found hundreds of hits where folks are having the same problem with various models of GM 4WD vehicles. On 2002 and prior models, the cause seemed to be poor solder joints in the dash-mounted Transfer Case Control Switch assembly. I took mine out, inspected it and checked for continuity through the switch contacts, and all was good. In 2003, GM began using a different supplier, and the Transfer Case Control Switches are good, but the trouble seems to have moved to the Transfer Case Shift/Servo Motor (TCSM, my shorthand).

Hundreds of posters on other web sites have had this same problem, a lot of which had the trouble while the truck was still under warranty. They took it to a GM dealer, which diagnosed the problem to the TCSM using their proprietary scanner, replaced it, only to have the problem reappear within 30 to 5000 miles. Several posters had the work done after the warranty expired, only to get presented with a bill of over a $1150, which breaks down to $850 for the new TCSM, and several hours of labor.

Hating dealerships, I am determined to fix it myself. Not having an electrical schematic/service manual for the truck, I poked around on the web and came up with **broken link removed** on how to remove the TCSM, replace an $80 sensor inside it, and reinstall. Some other folks also found problems in the wiring harnesses (water intrusion) or with the other sensors on the transfer case.

I jacked up the truck, removed the skid plate that protects the transfer case, and inspected the sensors that protrude from the transfer case. There are three magnetic reluctance speed pickups; two where the rear drive-line comes out of the transfer case, and one on the front drive-line. All three are identical. Each has only two terminals. If you connect them to a oscilloscope, and swing a ferrous metal tool past the pickup end, they put out a classical bipolar pulse of several Volts long as the metal object is moving past the end of the sensor. I'm guessing that one of the rear sensors is used as the main vehicle speed sensor, hence goes to the ECU, while the other two go to the TCCM, and are used only to detect if both drive lines are not rotating before the truck will shift into 4LO. Since the connectors were clean, and the sensors all seemed to work, I moved on to the TCSM.

The instructions posted above have you disconnect the front drive-line from the transfer-case; however, I left the drive-line alone and was able to get the TCSM off by carefully moving the front drive-line universal joint to clear the middle 15mm bolt. It helps to have a stubby 15mm box-end wrench. In my truck, my transfer-case was in 2HI when I took off the TCSM, which is significant, see below. As the TCSM comes off it exposes a short splined shaft with a key-way. Using a Sharpie, mark the position of the key-way on the transfer-case, so you can be sure it is in the same position when you reinstall.

I then disassembled the TCSM on the workbench as per the instructions. Without moving the output gear, I marked the position of the dimple on the output gear on the housing. Note that mine came off the truck in the 2HI position. My reasoning was that the position of the rotation sensor attached to that output gear is critical so I wanted to be able to reinstall the gear/sensor in the same position. Note that there is a | mark on the gear housing, but that did not align with the dimple in the 2HI position; I am guessing that it would align in the 4LO position, more on this later.

Next, I traced out the wiring diagram of the TCSM as shown in the attachment. I then used an Ohmmeter to measure the resistances as shown in the attachment. Note that there are three things inside the TCSM; A reversible DC motor, a DC powered clutch, and a resistive potentiometer rotation sensor. This is very useful information; I wish I had it before I started... I recorded the sensor resistances before disturbing the gear/sensor. Lo and Behold, as I started making the measurements, the path from the Brown-White wire (Pin 2, sensor wiper) to either the Green wire (Pin 1) or the Black-White wire (pin 3) was open. The resistance from the Green wire (1) to the Black-White (3) wire was 2280 Ohms.
I had found the root-cause of TCCM self-test failure...

When I removed the gear/sensor from the housing, and moved the sensor wiper by turning the gear, contact from the wiper to the resistance element was reestablished. Moving the gear back and forth must have "cleaned" the wiper contact, and it never opened again while I was testing it... I sprayed some electronic contact cleaner into the sensor while moving it back and forth. Note that the TCSM housing had some light oil inside it, which means the sensor is bathed in this same oil, so the wiper contact runs in oil. It would be interesting to know if this oil should be there???

I proceeded to record the resistance vs the gear rotation. However, I noticed something weird about the measurements. The sensor resistances are unlike any pot I had tested before, in that the sum of the resistance from the wiper to either end does not add up to the total end-to-end resistance. With the gear in the 2HI starting position, the resistance 1-2 was 2000 Ohms, 2-3 was 2453 Ohms, while 1-3 was 2280 Ohms.

I guessed that since this is used as a rotational position sensor, the TCCM puts a fixed voltage between 1 and 3, reading the voltage at pin 2. I applied a fixed voltage of 2.2V from 1 to 3, while measuring the voltage from 2 to 3. The second attachment is a plot of Vout vs gear position. I used the | mark on the housing aligned with the dot on the gear as the 0 degree reference, and turned the gear clock wise. There are eight holes in the gear, so it was easy to record the voltage out every 45 degrees. I converted the output as a percentage of the applied voltage. It is possible to turn the gear forever, so I went more than 360 degrees.

Note that at the 2HI position, the Vout = 59%. The 4HI position is approximately 22deg CW from the 2HI position, and 4LO is about 45deg from the 2HI position. In other words, the 2HI position is about 45 CCW from where the . aligns with |

I ran the motor, by connecting 12V between 4 and 5, and discovered it is necessary first power the brake (clutch?) 6 and 7, other wise the motor is stalled. The clutch/brake is likely used to suddenly stop the motor from coasting past the indexed position while shifting from 2HI to 4HI to 4LO. I haven't tested this, but I'm guessing that if either the motor or clutch windings were open, the TCCM wouldn't pass the wake up test. (Likely different trouble code, but same symptom as the open sensor wiper)

After closing up the TCSM, reinstalling wasn't difficult, provided that the gear was internally pre-positioned to 2HI, and the splined shaft was still at the original mark. If those hadn't been marked and positions noted, this would have been a *****.

Since the sensor began to work, I didn't replace it. I'm guessing that the means that the problem will eventually return. However, here is what I plan to do when it does: I'll remove the skid plate, and unplug the 8 pin connector between the truck harness and the TCSM at the top of the Transfer Case. I'll use an Ohmmeter to check the resistance from pin 2 to pin 3. If that is open, I'll take out the TCSM, and replace the $80 rotation sensor. If I have ~2450 Ohms, then I will check the motor (~1 Ohm) and clutch (~20 Ohm) resistances. If all of those are good, then I have another problem...

Total cost of this experiment: $9.95 for a set of stubby open-end/box mm wrenches and along the way, I bought a $20.00 Haynes Repair Manual #24066 expecting to find schematic diagrams. The book was sealed in shrink-wrap in the PEP Boys store, so I couldn't see inside. After buying it, and ripping off the shrink-wrap, there was no wiring information about the Transfer-case wiring, sensors, switches, or the TCSM, so thanks for nothing Haynes. What a rip-off!

Hope this information helps someone else with this problem... and btw- this is the last GM product I will ever buy. Those dips deserve to go out of business... With thousands of bad TCSMs out there, they are jacking people around to avoiding fixing them on their nickle.

ps: Title should say Z71, not Z21.
 

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Wow - that was a lot of effort, but I thank you for posting this! I own a GMC pickup, but its 2WD; however, knowing about searching the internet and elsewhere for a repair solution for an automobile, finding this kind of detailed information is invaluable. I'm sure someone out there having the same problem will find this page and be very, very thankful. You will have saved them a ton of cash! :)
 
we have a 2002 gmc sierra pickup, fortunately the 2-4wd hi-lo section is driven by an arm at flour of truck that goes directly in transfer case, no electronics at all = no problem!
 
Mike ML --- I just assisted a friend with a similar issue. Basically GM 4WD engagement is a 3-piece system consisting of the TCCM, the dash-mounted switch, and the small actuator motor that's mounted to the transfer case. Typically it's the actuator motor or the dash switch that have problems and the TCCM is pretty reliable... but there's always a departure from the norm for other parts to fail. In my friend's truck it was the motor that was at fault and $180 got him a rebuilt OEM motor. A rip-off IMHO, but his truck is back on the road. I myself would've disassembled the motor and tried to repair it if possible.

All of these vehicles have their quirky parts despite brand name. While my Dodge Ram has a reliable floor-mounted shift lever to engage 4WD, it's also got a dumb track bar up front that can cause all kinds of handling issues when they go bad. So it's a give and take regardless of truck brand. That track-bar aligns the front axle to the chassis making it a critically important part.
 
thanks mikeml for sharing so much info on this sensor, just as you did i marked my housing where the dot was on the output gear,according to your ohm readings mine read that it was in 2hi as i thought it was.My ohm reading between 1and3(your numbers) 2280 does not change no matter where i rotate the sensor. Is it supposed to? and do you know the ohm readings for 4lo and 4hi? thanks again,lonny.
 
thanks mikeml for sharing so much info on this sensor, just as you did i marked my housing where the dot was on the output gear,according to your ohm readings mine read that it was in 2hi as i thought it was.My ohm reading between 1and3(your numbers) 2280 does not change no matter where i rotate the sensor. Is it supposed to? and do you know the ohm readings for 4lo and 4hi? thanks again,lonny.

As long as terminal 2 is open circuit, then the resistance from 1 to 3 will not change as the gear is rotated. The readings from 1 to 2, or 2 to 3 will change as the gear moves.

I didn't happen to record the resistance in the other positions, since I knew my transfer case was in 2HI when I removed the servo. I was mainly focused on getting the gear back into the right position... I'm guessing that if you assemble the servo with the gear one tooth off, then the system will not work, but it may work if you have the dealer "reflash" the TCCM. That would reset the decision points...

The graph I posted shows the %voltage out as a function of gear rotation. Hook a 1.5V D-cell battery between 1 and 3, and measure 2 to 3, convert to a % of 1.5V, and you should be able to position the gear at either 4HI or 4L.
 
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Mike. another common issue with the 4wd is the engagement solenoid failing to fuly retract back into 2 wd. When this happens it generates an ratio error code and sets the light on the dash. Occasionally a good dissassembly and thourough cleaning aleviates the problem. Vehiclesdriven off road (mud) and snow bound areas seem to suffer more than the cowboy caddies that never get into 4wd or rarely. Did you record the codes? If so PM them to me and I may be able to point you in the right direction so you dont replace parts unecccesarily at high expense. I used to work in the GM and high line euro dealerships as well as the aftermarket before I became disabled and may be able to help you.
Bob
 
So far, so good. It has been three weeks and the 4WD works with no glitches. We have had a lot of winter weather, snow and slush on the roads.
 
So far, so good. It has been three weeks and the 4WD works with no glitches. We have had a lot of winter weather, snow and slush on the roads.

That'll be the one part that -never- fails again; the rest of the truck will fall apart, rust away, burn up, but that part will stay working - I can almost guarantee it! :)
 
Update as of 3-14-2011, still working without a glitch.
 
Mike. another common issue with the 4wd is the engagement solenoid failing to fuly retract back into 2 wd. When this happens it generates an ratio error code and sets the light on the dash. Occasionally a good dissassembly and thourough cleaning aleviates the problem. Vehiclesdriven off road (mud) and snow bound areas seem to suffer more than the cowboy caddies that never get into 4wd or rarely. Did you record the codes? If so PM them to me and I may be able to point you in the right direction so you dont replace parts unecccesarily at high expense. I used to work in the GM and high line euro dealerships as well as the aftermarket before I became disabled and may be able to help you.
Bob

Hi Bob, I have similiar issue with my 2002 GMC Sierrra 1500 ,, I changed out the dash switch ,, still nothing , It appears that the 4 wd did not disengage as you say .. no indicator lights on 4wd selector switch ..To be fair the "service 4wd " light was on for a while before it failed to shift in or out of 4wd and then all the lights came on.. I checked for codes but came up empty handed... I just wanted to make sure you were referring to the actuator on the transfer case, not the actuator on the front axle...any and all help appreciated...

thx Steve
 
Mike:

Is it possible that you just need a code scanner that's capable of reading the ABS trouble codes. Those scanners can be had.
 
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