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I can't really find any real information about what the PTU does (at a mechanical level) or how it's controlled. Can anyone enlighten me?

I know that it's responsible for connecting and disconnecting the rear wheels to the drivetrain, but that's it.

My guess is that it's just an electronically controlled clutch. But I've also read that it diverts variable amounts of power to the rear wheels, and the only way I can see it accomplishing that is by letting the clutch slip. I'd think that would burn up the clutch pretty quick, but maybe I'm wrong on that. Anyone know the inner workings of the PTU?

Any idea how much torque it can handle?

And what controls the PTU and tells it to engage or disengage? The ECM? TCM? Something else? I know that pulling a fuse can disable the unit entirely.
 

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Think of the PTU on a simple level as the transfer case that would be used in a part time 4x4 system, but it's mounted to the transmission of a FWD vehicle to route a driveshaft to the rear of the vehicle.

At the rear The (RDM) has clutches that control the power going to the wheels.

The PTU can stop and start the driveshaft. It can spin the driveshaft and not send power to the rear wheels, the actual power to the rear wheels is handled by the clutches in the RDM.

The Drive Train Control Module (DTCM) controls the PTU and RDM action. You can in the PCM tell the PTU to not be active as well.
 

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Makes perfect sense, thanks. The previous explanation I'd read made no mention of the RDM and implied everything was handled by the PTU.

So if I have this correct now, the PTU just handles hi/lo switching and disconnects the driveshaft (to reduce parasitic losses when in FWD). There's also a front PTU which handles hi/low as well. The RDM has a clutch attached to the driveshaft which it can engage to varying amounts to divert up to 50% of engine power (at full engagement) to rear. Left/right is brake controlled or locked by RDM. Front/rear (while engaged) is also brake controlled. Do I have this right now or am I still missing something?
 

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So if I have this correct now, the PTU just handles hi/lo switching and disconnects the driveshaft (to reduce parasitic losses when in FWD)
That is correct
There's also a front PTU which handles hi/low as well. The RDM has a clutch attached to the driveshaft which it can engage to varying amounts to divert up to 50% of engine power (at full engagement) to rear
There is only 1 PTU in the Cherokee. The clutch isn't directly connected to the driveshaft, its located inside the RDM module. Fun fact the Jeep MK platform (Patriot and Compass) have the ECC clutch mounted on the RDM and the driveshaft connected directly to it but the Cherokee does not. You are correct only up to 50% torque can be sent to the rear wheels

Left/right is brake controlled or locked by RDM. Front/rear (while engaged) is also brake controlled. Do I have this right now or am I still missing something?
Torque on each side of the axle is (on the rear axle) controlled by the electronic locking feature. If it is not locked(or not equipped with a locker) then the ABS system controls individual wheel speed.

On the front axle then the ABS system controls individual wheel speed.

Power transfer front to rear is not handled by brakes or the ABS system.
 
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