4WD engages by itself. I need help to diagnose.

Hello all,

Im new to the forum and need to see if anyone can help me with diagnosing the "auto" portion of my 4WD.

I have a 1999 Merc Mountaineer and when in the "Auto" mode it feels like the car is stuck in 4WD.

I have placed it in "Neutral", swithched the unit to "4WD", listened to it engage, and drove forward. Then placed it back in "Neutral", switched it back to "Auto", placed it in "Reverse" for about "10 feet" to ensure a unlock but it feels like im still locked in.

Sometimes it frees itself then other times the car seems to lock into 4WD by itself.

Any help would be apprecieated.

Mark

welcome mark

Thanks!

Welcome to this forum! I've moved your thread into the transmissions & transfer cases section.

Check your sensors, is your ABS light on?

The way "auto" works is it detects if your rear wheels are spinning while your fronts are not turning, if your rears are spinning and your fronts not then the transfercase is engaged and sends power to the front.

The easiest way to fix what you're describing (and probably cheapest) would be to Brown Wire Mod it. But if you don't want to do that, I would check sensors, then transfercase, then TCC, and blah.

Lets us know what you find out. And Welcome btw!

Brad,

Yes, my ABS light is on and I will check out the sensor tomorrow. You mentioned, "to Brown wire it", do you mean just to jumper the sensor out?

Let me know.

Mark

Markart12 said:

Hello all,

Im new to the forum and need to see if anyone can help me with diagnosing the "auto" portion of my 4WD.

I have a 1999 Merc Mountaineer and when in the "Auto" mode it feels like the car is stuck in 4WD.

I have placed it in "Neutral", swithched the unit to "4WD", listened to it engage, and drove forward. Then placed it back in "Neutral", switched it back to "Auto", placed it in "Reverse" for about "10 feet" to ensure a unlock but it feels like im still locked in.

Sometimes it frees itself then other times the car seems to lock into 4WD by itself.

Any help would be apprecieated.

Mark

Click to expand...

Mark,

I'm guessing you've owned an older Explorer or been given some advice from someone who did. The Control-Trac system on your V6 Mountaineer is a little different than the 4x4 of the 1st generation Explorers.

The "10ft" procedure on the early rigs was done to unlock the automatic hubs. The Control-Trac system on your '99 does not have hubs that need to lock or unlock - the axle shafts always turn with the wheels.

There are two elements to your Mountaineer's 4WD system. There is a magnetic clutch that couples the front driveshaft in the transfercase, and there is a reduction gear to provide low-range. There is an electric motor that shifts the reduction gear.

When the dash selector switch is in "auto" the magnetic clutch is not energized unless the brain detects wheel slippage. It's detection is done by comparing the speed of each wheel. When it sees that one or more wheels is turning faster than the others, it momentarily energizes the clutch to transmit power to the front axle. It then releases the magnetic clutch and checks to see if the wheel speeds match. If they don't match, it energizes the clutch again.

When you move the selector switch to "4x4 High", the magnetic clutch is no longer under the automatic control of the brain. In "4x4 High" the clutch is engaged all the time. You can switch between "Auto" and "High" while driving because nothing changes mechanically, other than the magnetic clutch changing from always on (High) to sometimes on (Auto).

Selecting "4x4 Low" requires that the vehicle be stopped and the transmission placed in N. This allows the shift motor to engage the reduction gear. In Low, the magnetic clutch is on all the time.

The "brown wire" of the brown wire mod is the wire that sends power to the magnetic clutch. Some people want to be able to run 2WD and disable the "Auto" mode. '95-'97 models don't have "Auto" as a choice. Their Control-Trac selector is 2-High, 4-High, or 4-Low. If you interrupt the brown wire with a toggle switch, it prevents the magnetic clutch from engaging.

BradR asked if your ABS light was on because a bad ABS wheel speed sensor will cause the Control-Trac system to believe that one wheel is not turning the same speed as the others and will keep engaging the magnetic clutch.

So in summary, you are probably correct that the vehicle feels like it is stuck in 4WD. In all likelihood there is nothing wrong with the 4WD system, it is just reacting to a failed ABS sensor.

The 'brown wire mod' won't fix any underlying electrical gremlin that could be causing the problem, and definitely won't fix a mechanical issue; and that's the first thing we need to find out.

Lay underneath it, disconnect the 16-pin connector near the transfer case, and see if the problem persists. If the problem goes away, then we know it's an electrical issue causing the problem. If the problem persists, we know it's a mechanical issue with the case.

But, as mentioned, if the ABS light is on, I would start there. A common cause of problems is the rear axle speed sensor, as the sensor and its wiring the most vulnerable of the three.