Four wheel drive not engaging

Can you tell us which transfer case you have? At this point we need to get into the electrical testing but let's get the detail on your vehicle and let me review the operation detail before we go down that road. Thanks

I’m pretty sure it’s the mp1625 transfer case. My truck is listed above Duramax 6.6 liter. I’m thinking electrical also but wasn’t sure. I even thought about changing the front actuator wiring harness.

Okay. The next step is we need to check for codes. Refer to the third attachment for the detail. It says, if the service 4WD indicator comes on, check for codes. If no codes then we have a possible fuse or circuit issue.

Let's see if there are codes and we can go to the next step. Thanks

Sorry it took so long to get back to you. I just tried it and no codes but now when I turn it to 4wd it flashes for about a minute and the light stays on 2wd even with the selector on 4wd.

Okay. This normally means it is attempting to make the shift and it is not able to so it times out and remains in 2WD.

This means we need to clarify what you replaced so far. I understand the knob is the selector in the cab of the truck and the actuator on the front axle is the FAD. But the actuator on the transfer case, I assume you are talking about the encoder motor. We also have a control module that causes the light to flash when it receives the request from the shift knob. If the encoder is not completing the needed circuit to show that the 4WD is engage then it just flashes until it times out. Since it appears you have replaced all the components involved, I suspect you have a control module issue.

Yes sorry, I meant the encoder motor on the transfer case. I just had it to the garage. It’s popping up service drive and everything electronically is working properly but 4wd won’t engage. The mechanic talked about a short shaft in the front differential that engages the 4wd. Could this be the issue?

It is possible if the front axle is not actually engaging. Can you tell if the front drive shaft is spinning. If it is then your issue is in the FAD. If this is the case then I suspect he is correct.

Is the actuator on the front drive shaft the FAD? Only reason I ask is because I already changed that twice. The mechanic talked about a splined shaft in the differential? I’m really outside of my simple mechanical knowledge, lol.

Yes. The FAD is the Front Axle Disconnect actuator. The splined shaft inside the differential is most likely the pinion. I am not sure what other splined shaft he would be talking about. If we can ask him for more detail I will be able to better answer.

Okay, I’ll ask him. I greatly appreciate all of your help. So if the pinion was worn down that would make it so the 4wd wouldn’t actually engage? When the mechanic and I were talking the other day he said he had to change the pinion in his Dodge pick up but it sounds like a big job. Is that correct?

Yes. It is a big job. The actual installation is not that big of a deal but it has to be measured and shimmed properly or it will make noise.

It depends on how it failed. I can't say it is a common failure so I suspect he was talking about something else but as I said, I am not sure what he may have been talking about.