AWD Engage/Defeat switch?

[MountainManJoe]

If you're driving on dry pavement then the AWD will not engage. The 2 mpg would be a coincidence.

[Cobra]

that's not necessarily true i know for a fact my AWD kicks in at least twice a day on my commute to/from work and it's been dry as can be lately. Proving an increase in mileage would however be difficult but i believe it's possible

[mdmead]

Everyone talks about gaining mpg from doing this... and I just don't see it. All those front end parts have to spin whether you are sending power to them or not and that is going to hurt mileage. If the t-case doesn't send power to the front, (or does so very rarely,) in normal dry condition driving, how can pulling the fuse or using a switch make a difference?

[MountainManJoe]

If the t-case engages, then I imagine there would be a little bit more drag on the engine caused by friction in the front end components. But that second or two will affect your fuel economy about as much as an extra red light.

Does your daily commute involve gravel roads? If not and your AWD is kicking in, then I would say something is wrong because it should only do so if your rear wheels slip.

[Cobra]

how do auto makers get their mpg numbers?

I can be a rather rough driver sometime my tires chirp every morning just trying to get out of my driveway. I'm in a major city no gravel that i've seen also my transfer case is the viscous one so engagement might be slightly different. The only thing i could think of that could cause any change in mileage would be the addition drive train losses but like timeless said i'm sure it would be minuscule. Losing any power though in my mind is a bad thing

[MountainManJoe]

If yours tcase is viscous coupled, then pulling the fuse will do absolutely nothing because it is engaged all the time. I thought we were talking about the electronic AWDs here.

[Cobra]

we are talking about the electric
the only reason i mentioned it was because i can only really speak of my experience

[lockdoc]

we are talking about the electric
the only reason i mentioned it was because i can only really speak of my experience

Where is your experience coming from Cobra? You don't have an electric tcase. You have a 98. Unless you are talking about another van you have.

[Cobra]

i believe it's possible

I never said i've done it i only stated my belief we do have a 2001 but never tested it

[lockdoc]

i believe it's possible

I never said I've done it i only stated my belief we do have a 2001 but never tested it

You were talking about how yours engaged all the time. I figured you thought you had the electric one because I can't see the connection you were trying to make. My bad.

I would ASSUME that the electric ones engage more often than people think on dry roads during turns. I base this solely on the fact you have to use that special GM fluid in the case to prevent the dragging/chattering that happens occasionally.

[MountainManJoe]

The electronic transfer case tries to engage during low speed turns, because of the big difference in front and rear shaft speeds, and that is what causes the binding with the wrong kind of fluid. Under normal conditions, NV136's should only engage when there's rear wheel slippage.

[MountainManJoe]

Taken from: http://transmissiondigest.com/tech/TD20 ... 20HTML.htm

With active or automatic transfer cases, the driver electronically shifts through the modes and ranges by means of a motor/encoder that shifts the transfer case and applies an internal clutch pack to split the power between the axles. These units have an “automatic mode” for 4WD in which the transfer-case control module (TCCM) automatically sends power to the axle that needs it without any input from the driver. The computer reads prop-shaft and vehicle road speeds and, when they differ, will actuate the clutch pack until the shaft speeds are equalized. In the automatic mode this usually means that 95% of the torque is going to the rear axle until a slip is detected, and then the computer will send a pulse-width-modulated (PWM) signal to the encoder to apply as much torque as needed to the front axle. If the driver selected the 4W High range, the computer would lock the clutch pack to apply a 50/50 torque split to the axles with no modulation of the duty cycle.

Before examining the motor/encoder technology, we need to correct some common misunderstandings and myths prevalent with your customers. The following will save you a lot of time and money:

1. In any of the classes of 4WD transfer cases, it is NEVER all right to drive the vehicle in 4W High or 4W Low range on dry pavement. This will result in crow hop, high steering effort, wheel shake, internal transfer-case noise and popping, and – ultimately – internal damage. In snow, sand, mud, gravel, loose dirt or grass, there will always be some tire slip in the 4W High and 4W Low ranges. Dry-pavement operation causes internal components to bind because of spline lock, since there is no wheel slip. If your customer is driving on dry pavement in the 4W high or low range, fix the customer before he destroys the transfer case.
2. All transfer cases are sensitive to variations in tire size and pressures. Begin every diagnostic routine by measuring the tire circumference with a stagger gauge or a tape measure. All tires must be within 1/4 inch in circumference. All tire pressures must be equal and correct for the rating of the vehicle. This simple beginning will eliminate at least 50% of your diagnostic concerns.
3. All transfer cases are sensitive to fluid level and quality. All units that are run low on oil or use an incorrect oil will suffer damage quickly. Units that are run low on oil will overheat the drive chain immediately. The chain will stretch and damage the cases or the drive and driven sprockets. Any time you take a transfer case apart and find the chain with a light-brown baked-on color, the unit has been run low on oil. There is no accurate way to measure chain stretch, even on good units. During an overhaul, selling the customer a new chain is cheap insurance.

These are complex multifunction parts and are expensive. The motor itself is connected to multiple sets of internal planetary gears that can provide sufficient torque to make the shifts and apply the clutch pack. The encoder part of the motor is an internal position sensor that varies voltage to the computer to inform the computer of the transfer case’s gear position.

The encoder section of the motor on the active transfer cases is more complex, because it has to apply the clutch pack at the correct duty cycle and shift and lock the motor to the correct gear. Figure 3, showing motor/encoder operation for an NV 136 transfer case, also applies to the NV246, except that the 246 has two relays instead of one. The motor side of the encoder consists of the same four-channel, 5-volt system as the NV233, but we will now review the clutch-apply side.
To apply the clutch pack at the desired duty cycle, the TCCM will energize a relay on its circuit board, sending 12 volts to the motor. To regulate clutch apply the TCCM will pulse-width-modulate the ground side of the motor, controlling the amount of current flow the motor will see and controlling the amount of clamp load the shift fork applies to the clutch pack. This in turn regulates the amount of power and torque split to the prop shafts. The duty-cycle amperage will vary from 0 amps=0 clutch apply to 20 amps=100% clutch apply.

The TCCM determines slip by reading prop-shaft speeds through a set of vehicle-speed sensors mounted on the transfer case. It makes continuous comparisons between the prop-shaft speeds and the vehicle road speed and decides by using a set of ratio comparisons how much duty cycle to apply to the clutch pack. At 0-10 mph a slip must be above 25 rpm before the TCCM takes action. The greater the road speed, the larger the amount of slip must be for the clutch pack to be applied. These units are capable of applying 100% duty cycle within 72 milliseconds (0.072 second) and can revert to zero duty cycle within 48 milliseconds.

This is why tire size and pressures are so critical in your diagnosis. If tire circumferences vary by more than 1/4 inch, the computer will see that as a slip and set a code or activate the system incorrectly. All the tires’ having the same side-wall labels doesn’t mean a thing. Only an accurate measurement will keep you from wasting valuable time and labor resources.

[doyoulikeithere]

this is probably a stupid question, but could you add an s-10 front diff with a locker with that switch?

Not a stupid question at all.
One member in this thread mentioned saving 2mpg by switching the AWD off.
Now I have NO experience with any astro newer than 95, but what I know about mine is that Astro Front Axles are always on. While S10 Axles have a mechanical on/off which can be actuated many ways. electric, vacuum, or cable to disengage the axle, just like unlocking the hubs on an old school 4x4.

If the axle is disengaged, no power will get to wheels if the t case is in 4wd, and no wheel power can travel up to the transfer case in 2wd. The front drvshaft stays still when not used.

I think the van i'm building will get excellent mileage compared to any AWD van even with the AWD disabled.
Its a 95 engine & AWD Frame running TBI in an 87 van with a clutch and a 5spd, np231 , with an S-10 axle 3.73, cable posi-lok actuator.

[MountainManJoe]

tons more juicy technical info here:

http://www.rsgear.com/products/transfer.asp

[Meterpig]

Timeless-excellent tech resource you provided.

The way I read that..is....if I want to have a "full time awd override" then I must put 20 amps to the encoder motor. Is that right?