View Full Version : Eagle's Select Drive System

Big Bad AMX
10-13-2004, 01:32 PM
Inside Eagle's New Select Drive System

Does this new system from AMC make a good thing even better?


It may seem that AMC is doing a turnaround about why people should buy a four-wheel-drive Eagle. When the Eagle was introduced as an '80 model, the pitch was that the vehicle offered full-time four-wheel drive and thus did not require any shifting or unshifting to get from point A to point B, no matter the terrain.

So what does AMC do for mid-year '81? It introduces a system called Select Drive which lets you shift into two-wheel drive in the interest of economy when 4wd isn't needed. A little like talking out of both sides of the corporate mouth? Well, if you didn't look into it any further, that would be a logical asumption.

But what AMC actually has done is make a good system better without giving up anything. Not a bad tradeoff, no matter how you look at it. Let's take a close look at just what Select Drive is and what it does.

First off, it should be mentioned that the AMC Eagle is still full-time 4wd when in 4wd. The aftermarket kits that convert full-time 4wd Chevys, Dodges, and Fords with the New Process 203 transfer case to part-time 4wd do just that. The system is either in 2wd or 4wd but not full-time 4wd. In other words, the vehicle cannot be driven on hard surfaces in 4wd anymore. The Eagle can.

The heart of Select Drive is a vacuum-actuated spline clutch installed in the right-front axle shaft (p***enger side). The clutch is engaged during the 4wd mode but releases torque to the right axle shaft during the 2wd mode. Releasing torque to the right axle, allowing the right wheel to turn freely, also relieves torque to the left axle due to the differential action of the hypoid gear set just as if both axle shafts had been disconnected.

To complete the Select Drive system there's a vacuum-operated mechanical shifter located within the transfer case which operates another spline clutch. As the shifter slides the spline clutch along the transfer case's main shaft it will either engage power to the front drive chain sprocket when moved rearward or disconnect the chain sprocket when slid forward. When the shifter is moved forward (to 2wd mode), the mechanism also locks the transfer case differential so that all power is delivered to the rear axle. In 2wd, the main shaft and transfer case differential ***embly turn as a unit while the front output shaft, chain, and sprockets remain stationary.

It should be noted that the rightfront axle disconnect does not release all front drivetrain components in the 2wd mode, such as locking hubs do in a conventional part-time 4wd system. The half shafts and differential side gear continue turning with the left front wheel but because they're turning freely under no load, the energy loss is negligible.

All this mechanical action is controlled by a dash-mounted two-position vacuum switch. Because the switch from 4wd to 2wd can be made only when the vehicle is stopped, the dash switch requires two separate movements to activate to prevent accidental engagement when the vehicle is moving. A release pin must be pulled downward before the switch lever can be moved laterally to either the 2wd or 4wd position.

The Eagle can be operated for any length of time in either mode or switched as often as needed and can be driven on any surface in either mode. It's strictly up to the driver. Seems to be the best of both worlds - convenience and economy, and fulltime 4wd capability.

By the time you read this, dealers will have Select Drive Eagles on their showroom floors. The system is available on all '81 Eagles with all engine/ drivetrain combinations. PV4 will be testing this new system and making comparative fuel economy tests between 2wd and 4wd just as soon as an Eagle can be delivered to us.

The heart of the Select Drive system Is Jeep's center disconnect front axle, built around a spline clutch on the right-front axle shaft With it disconnected, there is no load on the front axle. Another spline clutch in the transfer case engages and disengages the front drive chain as needed, all at the flip of a switch. The vehicle must be stopped when the switch is thrown.