Service 4WD - 2020 RAM 3500, - Stuck In 4HI?

[rhs98jeep]

The truck is stuck in 4hi.

I need insight, please.

I put the truck into 4hi, and after about 30 seconds to a minute of driving, the serv 4wd light came on.

It was then locked 4hi and would not shift out of it, no matter what button I tried.

I disconnected the batteries and reset everything, then was able to switch to 2WD.

I got home, cleaned the actuator plug, and added dielectric grease.

It switched into and out of 4WD with no problems then (I didn't drive with it, just hit the buttons sitting still).

I then later needed 4WD again.

I switched into 4wd hi, no issues, then about a minute up the path, I got the damn serv 4wd light again.

I tried all the same tricks but couldn't get it back to 2WD or anything else. I'm stuck in 4hi, 8 hours from home.

So I pulled off the actuator, split it open, manually wheeled the fork to 2wd, and slid the axle collar over to 2wd.

I tried plugging it back in (before I installed it), and it immediately slid back to the 4WD position. Once I turned on the ignition, it would not let me slide back to 2WD via the button.

I opened it up and manually cranked the fork over to the 2wd position, reinstalled it on the axle, and then did NOT plug it in.

That enabled me to get home; the truck still shows in 4hi on the dash.

I looked for codes via the tuner, and none show; I will try to scan it tomorrow with a friend's scanner and see if it sees anything.

I hope it is just the actuator on the axle or the transfer case electric motor, but I hate to assume and throw parts at it.

All the wiring looks good; I checked the plugs, etc.

Does anyone have any insight on this?

TIA

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[AK Bob]

Hopefully, a full system scan will reveal a trouble code; the tuner only scans the PCM.

Please keep us posted; I am glad to help with the trouble code information you need.

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[rhs98jeep]

Hopefully, a full system scan will reveal a trouble code; the tuner only scans the PCM.

Please keep us posted; I am glad to help with the trouble code information you need.

I appreciate that, Bob!

I did buy a BlueDriver pro scan tool, which came today.

Here is a list of the codes.

C0031 left Front wheel speed sensor failure

C0034 Right front Wheel speed sensor failure

C0020 ABS pump motor control circuit voltage out of range.

There are a few other odd things I do not think are related—mostly low voltage alerts.

I did replace the batteries a few months ago as they were dying; I assume some of this is left over from that.

So I did read the speed sensors could cause this Serv 4wd light.

I am debating plugging the actuator back in and seeing if clearing the codes allows me to swap back to 2WD.

Or if the speed sensor codes return, replace those?

It makes sense, as it didn't ping the light till after I was driving in 4WD for a bit both times.

Does this sound like the right track?

TIA

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[AK Bob]

I appreciate that, Bob!

I did buy a BlueDriver pro scan tool, which came today.

Here is a list of the codes.

C0031 left Front wheel speed sensor failure

C0034 Right front Wheel speed sensor failure

C0020 ABS pump motor control circuit voltage out of range.

There are a few other odd things I do not think are related—mostly low voltage alerts.

I did replace the batteries a few months ago as they were dying; I assume some of this is left over from that.

So I did read the speed sensors could cause this Serv 4wd light.

I am debating plugging the actuator back in and seeing if clearing the codes allows me to swap back to 2WD.

Or if the speed sensor codes return, replace those?

It makes sense, as it didn't ping the light till after I was driving in 4WD for a bit both times.

Does this sound like the right track?

TIA

Yes, I would go ahead and clear the codes to see if it allows you to swap back into 2WD.

On the wheel speed sensor trouble codes, I am showing C0031-1D, 2F, and 62, the same for the C0034.

If you have the whole code, I will send the specific ones or all six if you wish.

On the C0020, I have the C0020-01, 16, and 1C.

So let me know on this one or if you want all three.

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[rhs98jeep]

Yes, I would go ahead and clear the codes to see if it allows you to swap back into 2WD.

On the wheel speed sensor trouble codes, I am showing C0031-1D, 2F, and 62, the same for the C0034.

If you have the whole code, I will send the specific ones or all six if you wish.

On the C0020, I have the C0020-01, 16, and 1C.

So let me know on this one or if you want all three.

Thanks, Bob; it does not give me the whole code, just the C0031 and so forth.

I will hook up to my buddy's scanner tomorrow and see if I get any more in-depth information.

I will then, once home, plug in the actuator and clear the codes to see what happens.

If it allows me to go back to 2wd, I will select 4hi again and drive around the farm a bit, see if it triggers the Serv light again, locking me into 4hi.

Do you think it is a speed sensor on the wheel issue?

I am having difficulty finding the front wheel sensors; I am just seeing the rear.

I will keep you posted; thanks!

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[AK Bob]

Thanks, Bob; it does not give me the whole code, just the C0031 and so forth.

I will hook up to my buddy's scanner tomorrow and see if I get any more in-depth information.

I will then, once home, plug in the actuator and clear the codes to see what happens.

If it allows me to go back to 2wd, I will select 4hi again and drive around the farm a bit, see if it triggers the Serv light again, locking me into 4hi.

Do you think it is a speed sensor on the wheel issue?

I am having difficulty finding the front wheel sensors; I am just seeing the rear.

I will keep you posted; thanks!

When we get the specific detailed trouble codes, we will know more.

You must replace the hub and bearing assemblies for the front speed sensors.

Correction, they call them Anti-Lock Brakes Sensors and are available for the front wheels.

The part in the link below can be fitted to either the left or right wheel.

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[Corkey]

This is excellent information to be knowledgeable about.

I haven't had an issue with mine yet, but gathering information like this thread allows me to be ahead of the problem when it occurs.

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[AK Bob]

This is excellent information to be knowledgeable about.

I haven't had an issue with mine yet, but gathering information like this thread allows me to be ahead of the problem when it occurs.

I agree; we never stop learning as we work through members' questions and concerns.

A big thank you to the OP, who gave a great detailed description of the issue and his logical troubleshooting procedure.

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[rhs98jeep]

Ok, so few things I learned.

The front wheel sensor is in the Hub, like the Hub, and the sensor is one unit and a pricey one.

That is how the new 3500s are set up.

I tried my buddy's scanner; it was no additional help past what mine said.

So what I did: I plugged the actuator back in, cleared all codes, and it immediately went back into 2WD, and the Serv 4WD light went away.

I put into 4WD and drove up and down our gravel driveway, trying to simulate the distance I drove up our cabin road before the SERV light-triggered.

It didn't ever come back on. However, I'm not sure it was an incredible simulation.

I did try switching it back and forth from 4hi, 4lo, and 2wd; all worked well.

I visually scanned all the wiring to the front wheels to see if I could spot any damage or irregularities; everything looked perfect.

The truck only has 22k miles, so everything is clean underneath and has easy-to-follow wires.

Visual checking is not as good as a multi-meter, but I did not get into that.

I plan to see if this reappears or gets put to rest. I will make sure I have the scanner with me at all times so I can reset things if it does lock me out again with the Serv light.

At that point, I may consider changing the hubs/sensor units in the front at $300 bucks a pop.

Hopefully, it will stay off and work right; I will keep you all posted; not going down to the cabin till mid-April, though.

I wanted to post an update so if anyone else had this problem, it may help them out.

Thanks, Bob; I appreciate the help!

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[AK Bob]

Ok, so few things I learned. The front wheel sensor is in the Hub, like the Hub and sensor are one unit, and a pricey one at that. Apparently that is how the new 3500s are setup.

I tried my buddies scanner, it was no additional help past what mine said.

So what I did: I plugged the actuator back in, cleared all codes and it immediately went back into 2wd and the Serv 4wd light went away. I put into 4wd and drove up and down our gravel driveway trying to simulate the distance I drove up our cabin road before the Serv light triggered. It didnt ever come back on, however, not sure it was a great simulation. I did try switching it back and forth from 4hi, 4lo, and 2wd, all worked well. I visually scanned all the wiring to the front wheels to see if I could spot any damage or irregularities, it all looked perfect. The truck only has 22k miles on it, so everything is clean underneath and easy to follow wires. I realize visual checking is not as good as a multi-meter, but I did not get into that.

I plan to just see if this reappears or gets put to rest. I will make sure I have the scanner with me at all times so I can reset things if it does lock me out again with the Serv light. At that point, I may consider changing the hubs/sensor units in the front at 300 bucks a pop.

Hopefully it will stay off and work right, will keep up all posted, not going down to the cabin till mid april though.

I wanted to post an update so if anyone else had this problem it may help them out.
Thanks Bob, appreciate the help!!!

Thanks for the update, pleased to hear that simply clearing the codes has resolved the issue for now.

I am still confused on the wheel speed sensor availability for the front wheels.

I was also thinking the hub assembly needed to be replaced, but then I found the ABS sensors available for a 2020, 3500 (see post #6).

If anyone has additional information on this, it would be appreciated.

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