Slight "clunk" On Acceleration

[GMCanuck]

This is my first 4wd truck, so I am not sure what is "normal" for sounds and function.

 

Quite often when accelerating from a stop (usually fairly slow - I don't pound it), there seems to be a very slight delay and a bit of a "clunk" from the drive line. It's quite soft, and hard to tell if it's coming from the rear diff, or further forward.

 

I usually drive the truck in "Auto 4WD", so I'm not sure what it does in 2wd.

 

can anyone relate to this experience? Is this something normal?

[oubobcat]

The clunk is on every 99-07 truck. Normal sound.

[oubobcat]

also, it will make the noise in 2wd to.

[stormrider]

The clunk is on every 99-07 truck. Normal sound.

 

 

I would add 08's as well

[grayghost7]

Yep....somewhat alarming the first time or two, but my mechanic buddy told me it was normal.

[700 sportsman]

That would most likely be the slip- yoke, do a search, and your will see, that many have had problems with it, none causing any real harm, just an annoyance, also it can be greased and that will quiet it up in most cases

[Zembonez]

It sounds like the slip yoke clunk. It's very common. See the link in my siggy for the how to on fixing it.

[fm2176]

That same sound bugged the hell out of me until I found the cause and solution here. Twenty minute fix (including cleaning off the rust and old grease) and no noise now for a few weeks.

[4x4txed]

so, we can drive around with "auto 4wd" ? do you all recommend that? or would driving around in 2wd be ok? Do any of you put it in 4wd every now and then for maintenance?

[GMCanuck]

also, it will make the noise in 2wd to.

 

Thanks for putting me more at ease with all the input - I appreciate it and will definitely be looking into the slip yoke issue.

 

And yes, it is doing it in 2wd as well (just spent some time running around and checked this).

[MS3DALE]

so, we can drive around with "auto 4wd" ? do you all recommend that? or would driving around in 2wd be ok? Do any of you put it in 4wd every now and then for maintenance?

Yes,No,Yes,Yes.

[GMCanuck]

so, we can drive around with "auto 4wd" ? do you all recommend that? or would driving around in 2wd be ok? ...

 

Well, I see your from the great (and snow-less) state of Texas. I assume there is no need for 4wd in on-road dry conditions there. Here in Canada (and no it's not cold here all the time ) we have snow and ice on the roads diuring the winter months. There's no telling when the 4wd will be needed, so I've hear that most guys up here are in Auto 4wd. It works very well.

 

I expect that come summer time warm (occassionally as hot as 100 F ) conditions, I'll be in 2wd most of the time, since the 4wd would only be needed off-road.

 

...Do any of you put it in 4wd every now and then for maintenance?

Well I used the full time 4wd just the other day, but that was just when I was axle deep in a snow-filled ditch, being pulled out by a friendly fuel delivery truck... . But yes, I suspect I will be doing the occational 4wd (H and L) just to keep her excercized and the parts conditioned. But that's just me (backyard Mechanic). I don't know what the technical recommendation would be...

[MS3DALE]

This should answer any questions on the 4WD system...

 

Transfer Case Description and Operation

General Operation

The New Venture Gear model NVG 246 RPO NP8 transfer case is a 2-speed automatic, active, transfer case. The NVG 246 EAU provides 5 modes, Auto 4WD, 4HI, 4LO, 2HI and NEUTRAL. The Auto 4WD position allows the capability of an active transfer case, which provides the benefits of on-demand torque biasing wet clutch and easy vehicle tuning through software calibrations. The software calibrations allow more features such as flexible adapt ready position and clutch preload torque levels. The technology allows for vehicle speed dependent clutch torque levels to enhance the performance of the system. For example, the system is calibrated to provide 0-6.78 N·m (0-5 lb ft) of clutch torque during low speed, low engine torque operation, and predetermined higher torque for 40 km/h (25 mph) and greater. This prevents crow-hop and binding at low speeds and provides higher torque biases at higher vehicle speeds, in order to enhance stability.

 

Transfer Case Shift Control Switch

The NVG 246 EAU transfer case features a 4 button shift control switch located on the instrument panel. When the ignition key is in the RUN position, the transfer case shift control module monitors the transfer case shift control switch to determine if the driver desires a new mode/range position. At a single press of the transfer case shift control switch, the lamp of the new desired position will begin flashing to inform the driver that the transfer case shift control module has received the request for a new mode/range position. The lamp will continue to flash until all shifting criteria has been met and the new mode/range position has been reached, or has been engaged. Once the new mode/range position is fully active, the switch indicator lamp for the new position will remain ON constantly.

 

During normal driving situations, the transfer case can operate in the Auto 4WD mode. In the Auto 4WD mode, the transfer case shift control module monitors rear wheel slip speed, based on the inputs from both the front and rear propshaft speed sensors. When the vehicle experiences a rear wheel slip condition, the transfer case shift control module sends a pulse width modulated (PWM) signal to an electronic motor, which is the transfer case encoder motor. This motor rotates the transfer case control actuator lever shaft, applying a clutch pack. This clutch pack is designed to deliver a variable amount of torque, normally delivered to the rear wheels, and transfers it to the front wheels. Torque is ramped up to the front wheels until the front propshaft speed sensor matches that of the rear propshaft speed sensor. Torque is ramped down to the front wheels. The process would repeat if rear wheel slip is detected again.

 

The NVG 246 EAU transfer case has the added feature of also providing the driver with 3 manual mode/range positions:

 

• 4HI - 4 Wheel Drive high range

 

• 2HI - 2 Wheel Drive high range

 

• 4LO - 4 Wheel Drive low range

 

The driver may choose to select any of these mode/range positions while driving the vehicle. However, the transfer case will not allow a shift into or out of 4LO unless the following criteria has been met:

 

• The engine is running.

 

• The automatic transmission is in NEUTRAL.

 

• The vehicle speed is below 5 km/h (3 mph).

 

This transfer case also has a NEUTRAL position. A shift to the NEUTRAL position allows the vehicle to be towed without rotating the transmission output shaft. NEUTRAL position may be obtained only if the following criteria has been met:

 

• The engine is running.

 

• The automatic transmission is in NEUTRAL.

 

• The vehicle speed is below 5 km/h (3 mph).

 

• The transfer case is in 2HI mode.

 

Once these conditions have been met, press and hold both the 2HI and 4LO buttons for 10 seconds. When the system completes the shift to NEUTRAL, the red Neutral lamp will illuminate.

 

The NVG 246 EAU case halves are high-pressure die-cast magnesium. Ball bearings support the input shaft, the front output shaft, and the rear output shaft. A thrust bearing is located inside of the input shaft gear to support the front of the rear output shaft. The transfer case requires Auto Trac® II Fluid GM P/N 12378508 (Canadian P/N 10953626) which is blue in color. The fluid is designed for smooth clutch application. An oil pump, driven by the rear output shaft, pumps the fluid through the rear output shaft oil gallery to the clutch and bearings.

 

There are two versions of the NVG 246 EAU, which depend on the transmission applications and vehicle applications. If the vehicle is equipped with a transmission RPO M30, the transmission splines in the input gear will have 27 teeth. With this application the planetary carrier assembly will have 3 pinion gears. If the vehicle is equipped with transmission RPO MT1 or MN8, the transmission splines in the input gear will have 32 teeth. The planetary carrier assembly on this application will have 6 pinion gears.

 

 

Some of the changes to the transfer case, that will take place during this model year, include the following components to the high/low range shifting mechanism:

 

• The range shift fork pads change from a Peak® 3-piece design to a Vespel® 2-piece design.

 

• The 3 notches in the high/low range shift fork are deeper for the Vespel® pads.

 

• The high/low range sleeve collar is wider for the Vespel® pads.

 

• The high/low range sleeves can be identified by the grooves in the collar.

 

- The 1st design high/low range sleeve that uses the Peak® 3-piece pads has 2 narrow grooves in the collar.

 

- The 2nd design high/low range sleeve that uses the Vespel® 2-piece pads has 1 wide groove in the collar.

 

2WD Power Flow

When the NVG 246 EAU is in the 2HI mode, the power flows from the transmission to the input shaft gear. The input shaft gear is connected to the rear output shaft by the high/low range collar. The range collar outer teeth are engaged with the input shaft gear inner, high-speed position teeth. At the same time the range collar is slip splined to the rear output shaft. The rear output shaft delivers the power flow to the rear propshaft. The position of the control actuator lever shaft allows no clutch engagement. The high/low range shift fork, is in the high-speed position on the control actuator lever shaft.

 

4HI and AUTO 4WD Power Flow

In the 4HI mode, the power flow to the rear propshaft is the same as it is in the 2HI mode. To deliver power flow to the front propshaft during the 4HI position, the transfer control module commands the encoder motor to apply the clutch to a calibrated torque. The encoder motor turns the control actuator lever shaft. A brake in the encoder motor holds the control actuator lever shaft in the full clutch position. The control actuator lever shaft is cam designed and the cam action moves the clutch lever. The clutch lever pivots on the clutch lever pivot studs and moves toward the clutch apply plate, to engage the clutch. As more pressure is applied to the clutch apply plate, the clutch discs are compressed. Using inner clutch discs, which are engaged with the clutch hub, and the outer clutch discs, which are engaged with the clutch housing, the power flow is delivered to the clutch housing. The clutch hub is splined to the rear output shaft, and the clutch housing rotates on a needle bearing on the rear output shaft. The chain drive sprocket is splined to the clutch housing. The power flows from the drive sprocket, through the chain, to the chain driven sprocket. The driven sprocket is splined to the front output shaft. The power flow is delivered to the front propshaft through the front output shaft.

 

During the Auto 4WD mode, the power flow is the same as it is in the 4HI mode. Except, during the Auto 4WD mode, the encoder motor rotates the control actuator shaft lever to the correct torque level positions. Rotating the control actuator to the various positions changes the clutch torque level. When a difference of front propshaft to rear propshaft speed is recognized, the transfer case control modules command for more, or less clutch torque.

 

4LO Power Flow

When shifting the transfer case to the 4LO mode, it commands the encoder motor to turn the control actuator lever shaft, to move the high/low range shift fork. The shift fork moves the high/low range collar, which is slip splined on the rear output shaft toward the rear of the transfer case. The range collar outer teeth disengage from the input shaft gear inner teeth. The range collar outer teeth then engage in the planetary carrier teeth. The power flow is now from the input shaft gear planetary teeth to the planetary gears in the carrier. Rotating the planetary gears , which are engaged in the annulus gear, the planetary carrier rotates. The planetary carrier engaged to the range collar, drives the rear output shaft, providing a 2.72:1 reduction to the speed of the rear output shaft. The power flow to the front propshaft is the same as it is in the 4HI.

 

A NEUTRAL position is obtained when the range collar is not engaged to the input shaft gear or the planetary carrier. NEUTRAL position is used for towing the vehicle.

 

Service 4WD Indicator

The Service 4WD indicator is an integral part of the cluster and cannot be serviced separately. This lamp is used to inform the driver of the vehicle that a transfer case system malfunctioned. The Service 4WD indicator is controlled by the transfer case shift control module via Class 2.

 

Transfer Case Encoder

The encoder is mounted to the transfer case motor/encoder assembly and is replaced as an assembly. The encoder converts the shift detent lever shaft position, representing a mode or range, into an electrical signal input to the transfer case shift control module. The module detects what position the transfer case is in by monitoring the voltage returned on the encoder signal circuit. This voltage translates into AUTO 4WD, 2HI, 4HI, NEUTRAL, and 4LO or in transition between gears.

 

Transfer Case Motor/Encoder

The transfer case motor/encoder consists of a permanent magnet DC motor and gear reduction assembly. It is located on the left hand side, driver's side, of the transfer case. When activated, it turns the shift detent lever shaft of the transfer case, clockwise or counterclockwise, to shift the transfer case. The motor/encoder is controlled with a PWM signal by the transfer case shift control module. This circuit consists of a driver on both the Motor A and Motor B circuits. The encoder motor is bi-directional in order to allow the motor to shift the transfer case from 2HI or 4HI to NEUTRAL and 4LO positions.

 

Transfer Case Motor Lock

The transfer case motor lock is used to provide a 2HI, 4HI, and 4LO lock-up feature. When the lock circuit is energized, the transfer case encoder motor is allowed to turn. When the transfer case is placed 2HI, 4HI, or 4LO the motor lock circuit is de-energized and the lock is applied. This assures that the transfer case remains in the current gear position until a new gear position is requested. When AUTO 4WD is selected, the motor lock remains applied until an adaptive mode, torque being applied to the front propshaft, is required. During an adaptive mode the motor lock circuit is energized and the motor lock is released, enabling the encoder motor to turn and apply or release torque at the front propshaft.

 

Transfer Case Shift Control Module

The transfer case shift control module uses the VIN information for calculations that are required for the different calibrations used based on axle ratio, transmission, tire size, and engine. The system does not know which calibration to use without this information. When the vehicle is in the AUTO 4WD mode, the transfer case shift control module monitors the speed of the front and rear propshaft, in order to detect wheel slippage. When wheel slippage is detected, the module applies a clutch pack contained inside the transfer case. This clutch pack is used to lock-in and apply the front propshaft, transferring torque to the front wheels. The clutch pack is applied by a motor/encoder assembly. When slip is no longer detected by the transfer case shift control module, the clutch is no longer applied.

 

Transfer Case Speed Sensors

There are three speed sensors on the automatic transfer case (ATC), two on the rear output shaft and one on the front output shaft. Each speed sensor is a permanent magnet generator. The permanent magnet generator produces a pulsing AC voltage. The AC voltage level and number of pulses increases as speed increases.

 

Vehicle Speed Sensor

One of the two speed sensors on the rear output shaft is the vehicle speed sensor (VSS) input to the powertrain control module (PCM). The PCM sends this information to the transfer case shift control module via the Class 2 serial data bus.

 

Rear Propshaft Speed Sensor

The transfer case shift control module converts the pulsating AC voltage from the rear transfer case speed sensor to a rear propshaft speed in RPM to be used for calculations. The rear propshaft speed can be displayed with a scan tool.

 

Front Propshaft Speed Sensor

The transfer case shift control module converts the pulsating AC voltage from the front transfer case speed sensor to front propshaft speed in RPM to be used for calculations, and to monitor the difference between the front and rear sensor speed. It is also used in the AUTO 4WD mode to determine the amount of slip and the percent of torque to apply to the front axle. The front propshaft speed can be displayed with a scan tool.

[grayghost7]

This should answer any questions on the 4WD system...

 

Excellent info....thanks. I didn't know you could get the transfer case into neutral with a pushbutton....good to know!

[GMCanuck]

It sounds like the slip yoke clunk. It's very common. See the link in my siggy for the how to on fixing it.

 

Just read through your "fix" thread - great info, but I have one question...

 

You do not mention to mark the slip yoke-to-tranny relationship (as you did the diff). Is this not necessary to avoid some trial and error when reinstalling the yoke? I realize that as long as the rear does not move, everything should end us as previously alligned, but would marking the front as well not make it just a bit easier? Or is the yoke splined in such a way as to only allow it to go back together in one position?

 

Just curious - thanks again.