How does the center diff lock work (1 Viewer)

[45Kevin]

First off, I have a 45 so no center diff lock for me.

But, I also have a chebby express 1500 that is AWD.
It has a BW4473 TC in it. It is a 1 speed chain drive.

It does not have a center diff lock and I am wondering why and if it is possible to add one like you can with the 80s.

School me on AWD transfer cases.

 

[I Lean]

Doesn't sound to me like it's an option. The Tcase has to be specifically designed to have a "selectable locker" in it, and this one's intended use doesn't indicate it has one, to me.

From a rebuilder site: "it's a single speed full time all wheel drive unit. This unit has a single operating mode 4 high full time, recommend fluid is Dexron VI."

 

[inkpot]

If you want a serious wheeler, you should look at what Chevy used in part time pickup trucks. It MAY be interchangeable. You need a 2 speed part time transfer case, and likely part time/ lockout hubs?

 

[Dirty Koala]

The HF2AV and HF2A transfer cases used in the 80 series are all gear driven, no chains. They have a center differential that allows the speed of the feont and rear drive shafts to spin at differing speeds when in qn open state. When thay center diff is locked both the feont and rear shafts turn at the same speed without and differentiation.

There is an electric motor in these cases that locks/unlocks the center diff. This motor is engaged automatically when the transfer case is put into Low range, unless the owner has done the pin 7 mod to allow low range unlocked driving. By adding a center diff lock button you are just adding a switch that allows you to manually engage the motor to lock the case in high range. Doing both the pin 7 mod and adding a center diff switch give the user the most flexibility/control over the case.

Unless the case you are referring to is designed the same way, and i doubt it us, adding a switch won't do anything.

 

[BILT4ME]

It essentially already has a "CDL" in that it is always connected from the front and rear axle all the time.
The following site has a little blurb about what to watch for and the first thing is mis-matched tires sizes and how they will kill this t-case.

The chain softens the "clunk" we experience with the gear-driven t-cases., but over time and miles, the chain stretches and can eventually cause gear failure, however, bearing failure appears to be the thing on this unit.

BW4473 Transfer Case Chevrolet Express and GMC Savana Vans AWD rebuilt replacements and Parts | Transfer Case BW4473

Switching it over to something else would be time-consuming, but if I were to do it, I would do it with an NP205 and a TH400 or TH700R, but you'd be doing a lot of other work as well.

And for God's sake, install some 35's.......

 

[45Kevin]

I have been driving this van for several years and the AWD has been great until the other day when I got bested on a scree pile.

I thought that maybe a CDL would have helped.

There are guys that have put the NP205 in the express AWDs. It's practically a bolt in mod with only a hole in the floor for the shifter and a shorter front driveshaft. I don't think I'll go to that extreme.

I have 32s on it and for what I do I think I'll keep it stock.

I will spend a little more time learning about the AWD TC though.

 

[SmokingRocks]

your chevy has a completely different system than the 80.

When you select 4WD in your van you effectively have the same thing as an 80 with the center diff locked. You'r chevy has a part time case, so when you put it in 4WD it "locks" the front drive shaft to the same output as the rear drive shaft. This is the same as when the 80 has it's center diff locked.

The Express's AWD is a RWD biased system that locks the front drive shaft to the rear when there is slippage. (there is a clutch that cushions the locking so it's not dangerous). Does your express not have a 4WD buton? just AWD and 2WD?


Auto 4WD = RWD until slips, then locks
2 HI = RWD
4 HI = CDL Locked 4WD
4 LO = CDL Locked 4WD Low transfer case gear

 

[SmokingRocks]

OK so I'm wrong, your's isn't like my old 1500's case. However based on the info here it should be able to split power 100% to each axle, so if it's not it may be an issue with gearing or the case could be broken. Here is the blurb on your case:


The Borg Warner (BW) model 4473 RPO NP3 transfer case is a 1-speed, full time AWD, transfer case. The BW 4473 provides 1 operating mode, 4HI full-time AWD. The transfer case has a planetary differential gear set, which provides a 35 percent/65 percent torque split, front/rear, full time. Both axles are constantly being driven for maximum traction, and the best balance of vehicle handling characteristics under all operating conditions is provided.

The BW 4473 case halves are high-pressure die-cast aluminium. The transfer case requires DEXRON®VI Automatic Transmission Fluid (ATF) GM P/N 88861003 (Canadian P/N 88861004), which is red in color.


Object Number: 851792 Size: LF
Click here for detailed picture of above image.

The power flows from the transmission to the transfer case input shaft (1). The input shaft (1) delivers the power to the planetary center differential (4), which splits the torque 35 percent through the differential pinion gears (Cool to the front sun gear (7), through the sprocket drive (2), via the chain (10) to the front output shaft (9) and to the front propeller shaft. 65 percent of the torque is delivered to the rear output, which goes through the differential ring gear (6), which is connected to the rear output shaft (5), and to the rear propeller shaft.

The viscous coupling (3), torque biasing device, has a series of opposing discs which are attached to the front and rear output shafts. 1 set turns with the front axle and the 1 set turns with the rear axle. The viscous coupling has a sealed housing, which is filled with high viscosity silicone fluid. When there is a speed difference between front and rear axles, the inner and outer plates of the coupling turn relative to each other. The silicone fluid provides resistance to the speed difference, and torque is transferred to the axles to equalize the speed difference. The tuning of the coupling is such that binding is not felt in tight turns, on dry surfaces, while being high enough to bias torque quickly to the wheels that have traction in a vehicle slip event. If the speed difference between the axles is high, the viscous coupling can go into hump mode. This occurs when the heat generated by the high speed difference expands the viscous fluid in the coupling, which results in pressure between the plates, forcing them in to contact each other, similar to a clutch pack. In the hump mode, the coupling can bias torque 100 percent to 1 axle. To prevent damage to the viscous coupling, the following should be avoided in the vehicle:

• Do not tow with only 2 wheels on the ground.
• Do not drive with 1 propeller shaft removed.
• Do not drive with a mini spare tire for any extended period of time.
• Do not operate the vehicle on a hoist to diagnose the driveline components.

Customers may have concern that the transfer case is not operating properly, because 1 set of tires spun for a brief period. It is normal for 1 set of tires to spin until the viscous coupling engages. Turning off the traction control switch, if equipped, enhances the function of the viscous coupling. The viscous coupling, as described above, engages by heat. Allowing 1 set of tires to spin or slip for a very brief period, generates heat in the viscous coupling. The engine speed should be kept at a constant speed during the brief spin of the tires. Feathering the engine speed, or hard acceleration, will not allow the viscous coupling to operate properly.
Power Flow - No Wheel Slip


Object Number: 851796 Size: LF
Click here for detailed picture of above image.

During normal operation, 100 percent engine torque is delivered to the input shaft (1) from the transmission. The torque is split 65 percent to the rear output shaft (5), and 35 percent to the front output shaft (9), by the planetary differential (4). Because there is not a loss in traction, or slip in the front or rear wheels, the viscous coupling (3) is locked in place and there is no shear mode or hump mode involved.
Power Flow - Front Wheel Slip


Object Number: 851797 Size: LF
Click here for detailed picture of above image.

When traction is lost at the front wheels, the viscous coupling (3) works in conjunction with the planetary differential (4) to bias the torque more to the rear wheels. The rear torque goes higher than 65 percent, up to 100 percent torque to the rear output shaft (5). The torque at the front output shaft (9) is lowered from 35 percent, to as low as 0 percent torque.
Power Flow - Rear Wheel Slip


Object Number: 851798 Size: LF
Click here for detailed picture of above image.

When traction is lost at the rear wheels, the torque is biased to the front wheels. The torque to the front output shaft (9) goes higher than 35 percent, up to 100 percent. The torque at the rear output shaft (5) is lowered from 65 percent, to as low as 0 percent torque.