Back wheels are spinning but front wheels aren't

[niterider40]

I just got a great astro van but it doesn't get any power to the front end. I flushed the transfer case and changed the differential fluid and to be sure I am not getting power I jacked up the rear and the wheels just spin in gear without any sign of power to the front. What should I do?

[Herbie]

You say 'jacked up the rear' - are the fronts off the ground too?

What year van? (i.e. vicsous TC or ECU controlled?)

[niterider40]

Its a year 2000 and the front wheels are on the ground and the back wheels are off the ground just to see if it will engage the front wheels when the back wheels spin in gear

[Mmusicman]

Its a year 2000 and the front wheels are on the ground and the back wheels are off the ground just to see if it will engage the front wheels when the back wheels spin in gear

I've tested my 2000 this way and it's a little jerky, but works.

To the best of my knowledge, here are the basics: (99-05 models)
1) There are speed sensors on both driveshafts (at transfer case) that tell the controller that the prop-shafts are turning at different speeds.
2) The controller sees this variation, and sends PWM signal to "encoder-motor"
3) The "encoder motor" is told to activate.. and forces pressure on forward clutch packs.
4) Clutch packs are engaged, and forward drive-train is activated.

If there is a breakdown at ANY of these 4 points, you obviously won't have AWD.
This is assuming the fuse is good, as well as all connections.

In other words, if the speed sensors are bad, your controller may not know to send signal.
If the controller itself is bad, no signal will be sent.
If a signal is sent, but the encoder-motor is bad, then the clutch packs won't be engaged.
If everything is working correctly and the clutches are bad, then no AWD either.

The most likely and most common failure, is the encoder motor.
It's a mechanical device that is well known to fail.

Start by checking your AWD fuse (under hood) ... and go from there.

There is much more info available in service manuals, and on the web.

Good luck.

[doyoulikeithere]

Man, I sure love my old school system.
Pull the cable to physically lock the gear so the front axle is engaged.
Shift the transfer case into 4wd Hi, Lo or 2wd with 1 Lever.
No relays, actuators, encoders, de-encoders, modulators, ecu's thingamabobs or whatchamacallits.
lol.

[niterider40]

thanks alot for the information. I think I will replace the encoder motor and see if that fixes it.