> Main > Tech > Maintenance > 4X4 Engagement
94+ Ram 4 Wheel Drive Operation
(& Diagnosing 4WD Engagement Problems)
Visit Geno's Garage
for Truck accessories.
Geno's Garage Truck Accessories
CAD operation CAD alternatives Hard Low range shift 4x4 Engagement Problems Limited Slip Test
From the 1998 Owners Manual:   Transfer Case Shifting Procedure: 

2H to 4H or 4H to 2H 
Shifts between 2H and 4H can be made while the vehicle is moving or stopped. If the vehicle is moving, shifts can be made at any speed up to 55 mph. This shift will be easier if you momentarily release the accelerator pedal. Apply a constant force while shifting the transfer case lever. NEVER drive in 4H on dry, paved roads. 

NOTE:  During cold weather (below 30 F) it may be more difficult to shift the transfer case while the vehicle is moving. If so, stop the vehicle and shift. Move the vehicle slightly forward or backward to complete positive engagement of the gears. 

4H to 4L or 4L to 4H 

Slow the vehicle to under 3 mph (5 kmh), shift an automatic transmission to N (Neutral) or depress the clutch on a manual transmission. Shift the transfer case lever to the right and pull firmly down into 4L. Do not pause in transfer case N (Neutral). 

NOTE:  Pausing in transfer case N (neutral) in vehicles equipped with an automatic transmission may require shutting the engine off to avoid gear clash while completing the shift. If difficulty occurs, shift transmission to Neutral (N), hold foot on brake, turn engine off, and THEN make the shift to desired mode. 

Do not attempt to shift to or from 4L while transmission is in gear or clutch is engaged.

Front Axle CAD Operation
> Is the front diff always engaged and 4WD controlled only by the
> transfer case? If not, then how is the front diff engaged?

  As you may have noticed, there are no hubs (manual or automatic) on the 4x4 Ram.

  The drivers side front wheel is permanently connected to an axle shaft which runs to the differential.

The passengers side has a two piece axle, one half permanently connected to the wheel, one half (intermediate axle shaft) connected to the differential. The two halves meet in the middle of the passenger side axle tube (that bulge on the axle tube is where the axle disconnect lives).  The two halves are connected together for 4X4 operation by the Center Axle Disconnect mechanism shown at the right. CAD mechanism, click for larger image
The inner axle shaft has a nose, which fits into a socket in the end of the outer axle shaft. The photo at the right shows the two ends which meet at the shift collar. A needle bearing in the outer axle shaft socket allows the inner and outer axle shafts to rotate independently. Axle shaft ends

 In 2WD, the axle disconnect keeps the two passenger side axle halves independent. The driver side axle happily spins the side gear in the differential, but because the differential is open and there is no solid connection on the passenger side, the side gear simply spins the diff's spider gears, which then spin the passenger side inner axle shaft backwards. Neither the differential's ring gear nor the front driveshaft will turn while in 2WD.

With me so far? OK, now when you shift into 4WD it gets a little more complicated...

  1. A synchronizer in the transfer case spins the front driveshaft up to speed
  2. The 4x4 shift lever locks into place and engages a vacuum switch
  3. Vacuum lines swap the High/Low vacuum signals to the vacuum solenoid on the front axle
  4. The vacuum solenoid (shift motor) slides the splined shift collar
  5. When the splines on the two axle stubs align, the collar locks into place
  6. Now that the axle ends are locked together by the shift collar and you have 4WD 
CAD vacuum system diagram  
  Left:  CAD Vacuum System 
  Below: CAD shift motor components
CAD components


Front Axle CAD Alternatives

So, you don't like the vacuum CAD system because it doesn't allow 2WD-Low, or doesn't engage fast enough, or you really want manual hubs. What can you do?

More 4X4 operation notes from the mail lists:

Subject:     Re: 4WD Question
Date:         Wed, 28 Jan 1998 09:52:55 -0800
To:            cummins

> When I am operating my Suburban on the street with 4WD disengaged, my
> front axles do not turn. However, I notice that on the Dodge trucks, on the
> street, the axles are turning. So what is happening here?

In 94 and newer Dodge trucks, the front axles always turn. There are no hubs at the front wheels to engage/disengage. Inside the front axle housing, on the passenger side, the front axle is split. There is a spline coupling that slides back and forth to connect/disconnect the front axle. This spline is controlled through the transfer case shift lever.


Subject:    [RAM] Axle Disconnect
Date:        Wed, 04 Jun 1997 11:31:59 +0000
From:       Chris Siano <>
To:           ramtruck

You are correct that the axle disconnect does wear more than the old hubs. Basically, you are spinning everything but the diff and the front driveshaft. The outer axles spin with the wheels.  The side gear on the front diff is turned by the axle on the driver's side.  This side gear spins the spiders and the opposite side gear.  The central axle shaft is then spun opposite to the wheels by the diff.  (same action as rotating a tire on a lift and watching the other turn backwards.)

When you shift, a mechanical interlock activates the front driveshaft.  This in turn slows the central axle and then rotates it in the same direction as the wheels.  At this time, a vacuum line is switched that places pressure on a diaphragm behind the front axle on the passenger side (looks like a little power brake diaphragm).  As the central axle reaches the same speed as the outer axle on the passenger side, the pressure on the diaphragm slides a connecting collar over the splines of the two axles and links them.  Similar to the way a hub moves the splined collar over the end of the axle.

When you disengage, it all works in reverse.  You don't have to stop, back-up, or anything else.

Sometimes, the 4WD light stays on, or fails to come on.  This is a great indicator.  What it means is that there is a difference in speed between the two axle ends, and the vacuum can't move the collar.  Burping the throttle, twisting the wheel side to side, or shifting the 4WD again generally releases the pressure.

2 things.  1) It is easier to shift while moving.  Even into 4LO, a slight roll makes the shift easier.  If you are stuck, and it won't shift, turn the steering wheel side to side as you burb the throttle to get the axles to line up.  When at a stop, it is very tough to line up non-moving splines.

Pros and Cons

Fairly strong.  The size of the axle shaft at the disconnect is significantly stronger than a hub end could ever be.  The outer axle splines can have a much tighter tolerance and thus distributes the load better.  The weak point is the bearing carrier design instead of using discrete inner and outer bearings.  In the early Jeeps, there was close to 4" between the inner and outer bearings to support side loads.  In the Ram, there is only 1/8" between them.  This makes rim backspace selection critical, and requires much higher tolerances in the bearings themselves.  Also, with no way to service the bearings without a 20 ton press, you have no way to prevent failure without spending $300 for a new carrier.

Since the use of 4WD requires vacuum, a leak, or destruction of the lines can render the 4WD inoperative.  However, the lines are fairly well protected, and failures are rare.  The vacuum switch is a known weak point.  The switch is a little tower on the transfer case with four lines coming from it.  The switch is activated by the transfer case when a lever inside the case presses a plunger at the base of the switch.  Then, a spring in the switch extends the plunger when the lever is retracted.  The problem is that the plunger is rounded to ease the lever's action.  This rounded part can be easily jammed inside the switch by the standard gunk in the transfer case.  When this happens, the interlock and the 4WD light will not disengage.  It is an easy fix by just un screwing the switch and cleaning it, but when it happens, you can feel the extra drag of the front driveshaft and associated parts.

Maintenance is very easy.  A fluid change every so often is about all it requires.  No spindle bearings to wear out, no dial to regrease, and no seals to leak.  (actually the axle seals can leak, but it isn't too common.  The part that sucks is that the passenger side seal is a bugger to replace as the seal must be pressed from inside the disconnect housing.)

This is my third vehicle with the disconnect system and I've never really had any major problems with it.  Sure, I've had seals leak, and the switch jam, and other annoying things, but when I need 4WD, it has always been there.

I've also had vehicles with manual and auto hubs, and this is still an easier and more reliable system.  It is only when you HAVE to get the system engaged while stopped that it can be difficult.  For me, I just engage the 4WD earlier.  Besides, it goes with the tread Lightly ideals.

Just another message from the RamHack

4WD Hard Low range shift
Symptom:  Difficult shifting from 4HI to 4LOW or from 4LOW to 4HI  - usually affects Rams with the automatic transmission.

This is a common problem with the automatic transmission. Unlike a manual with the clutch depressed, the auto is trying to spin the output shaft unless it is in park (many transmissions still "windmill" the gears even in neutral).

When the vehicle is stopped to shift into (or out of) lo range, the transfer case goes through the neutral position while the automatic transmission tries to spin the gear cluster. This causes the grinding and difficult shifting. Some people try (with mixed results) to shift while rolling at very low speed so that everything is turning. It helps to perform the shift as quickly as possible so the gears don't have time to start moving independently (I didn't say it was easy!).
From Mike Di Giovanni <>

What I have found that works pretty good is, put it in neutral and "WAIT 20-30 SECONDS" for the tranny and transfer case internals to come to the same speed, then pull or push the shifter lever aggressively. (depending if you want to apply 4lo or disengage 4lo.)
Mike Di Giovanni    Port Coquitlam, B.C.   Canada

Diagnosing 4X4 Engage Problems

If you like to four-wheel, brush on the trails can rip the vacuum lines from the axle CAD. Carefully check the lines between the frame and the CAD diaphragm, for damage or oil contamination. Numerous people have ripped the lines loose and one person has found that transmission fluid dripping onto the vacuum lines caused them to swell and leak. If there is no obvious damage, proceed with the diagnosis below:

> I have a 97 RAM 1500 Club Cab Sport 4 X4 and I am having
> a problem with the 4 x 4 High and Low engage light.. it just
> will not come on when I put the truck into 4 wheel drive......

If this truck was mine I would:

  1. Shift into 4WD and move truck a couple of feet
  2. If light is still off, park truck, block the wheels, set the park  brake, jack right front wheel, and try to hand spin the wheel
  3. Attempt to hand spin the front driveshaft If the driveshaft WILL spin, the transfer case is not engaged Check the transfer case shift linkage, problem is in linkage (or in the tran$fer ca$e internal$ <-- yes this means $$,  ouch!)
  4. USE CAUTION!      With transmission in neutral or park, start engine, measure vacuum on the two vac lines at front axle shift motor. One line should have approx 20" vacuum, other line should have none.
  5. Remove vacuum shift motor from the axle, attach vac lines to shift motor, engage/disengage 4WD. Does shift motor plunger move at least 1/2"?

Good luck.   Dave

Checking For Limited Differential Operation

> How can I find out whether my RAM has a limited-slip differential or  not?

The only sure way to tell if a LS differential is installed AND working is to:

- block the wheels
- put the transmission in neutral
- release the parking brake
- jack one rear wheel until the tire clears the ground
- try to turn the tire by hand.

* If it turns freely, the differential is open  (build codes, stickers, axle tags, etc can be wrong) or the LS clutches are worn out.

*  If it requires at least 50 ft-lb to turn the tire, there is a traction aiding diff of some kind installed (or a bad bearing somewhere).


 Last Update: November 1, 2000