Hodadical

So I've got a 2009 Suburban with the 4WD dial (not push button) on the dash and an MP3023 transfer case (2HI-auto-4HI-4LO-N) that is driving me up the wall on diagnosing 4wd problems. Its currently throwing a C0387-59 Unable to Complete a Shift and that is the only code. The dial position indicates 4HI, but from jacking the car off the ground and turning components, I can tell you the front output of the tcase is not engaged (but front axle actuator is, so I'm 2HI with the hubs locked). Now like everyone, I assumed the shift motor was my problem even though this tcase with the shift motor on back and separate sensor, not the one i read about the most that cradles under the front driveshaft. Anyway, I got a new one from Autozone, but its made no difference. I've already replaced the dash button as well as the separate rotational sensor on the tcase, again no difference. I've bench rule the front actuator and it works.

I've got access to GM Tech2 and been able to make these observations:
- when button is in 4HI, button and tcase position reads 4HI, but if i move the dial, tcase position will read "invalid". When the dial is on a position other than 4HI, eventually the lit up dot will disappear and not be lit in any position. If dial stays on 4HI, dot rill remain lit and it "thinks" its in 4HI, though mechanically its for the front hubs locked but hasn't engaged the front driveshaft.
- when I try to "learn" the tcase positon, either by command with Tech2 or the manual process, the learn "fails" and nothing changes.
- the rotational sensor and IPS both read in align with each other, about 36* which tells me the ATC motor and case seems to be on the same wavelength
- If I try to command the ATC motor forward or reverse with Tech2, I hear a click, but the motor does not turn.
- If I remove the motor for the tcase but leave it plugged up, and again command it forward or reverse, the gear does not turn.
- If I remove the rotational sensor in the tcase, stick a flat blade screw drive in the slot, I can turn counter-clockwise somewhat, but it springs back. I can't turn clockwise at all.

If it wasn't for already having tried a brand new shift motor that exhibits the same, I'd say the shift motor is at fault. I get good telemetry from the ATC using Tech2 and it seems to pass its calibration, but no idea why I can't instruct it to move or why the learn procedure fails. I'd love to see the shift motor turn, but its a pulse width modulated and I have no idea how to bench test that.

Any suggestions? i'm at whit's end with this 4wd and long for the days of a good ole twin stick coming out the floorboard!

Thanks
-Rob

repairman54

Spring a couple bucks for a alldatadiy.com manual for yours. Money well spent. They list component check out for motor and it's control module.
It will not copy and paste for me and it's a pretty long page.

Lambert2828

mines doing the same thing, everything you have mentioned. I even took the motor off and turned the little geer it springs back.. I'd say there's a geer in there could manually be pushed to the right position id say its stuck in neutral or something... try this.. push your emergency break down then put foot on break put vehicles shifter in neutral then put your 4 wheel drive shifter turn it to the neutral position hold over on it for 30 seconds. It'll put the t case in neutral position. Then let off to 4 low. Then turn key off. Put in park. Start it put the shifter in neutral again. Then push emergency break down then foot on break then turn from 4 low to 2 hi.. see if that helps take it out of neutral mode.. it didn't work for me.. then again I bought a t case to big for my vehicle it was for a 6 speed not 4 speed.. so that might be your problem to if you bought another t case and its not working for you? My truck is 4 speed and I had no idea it was... because when you put the t case on the transmission the shaft went up in the t case but it wasn't even touching the t case for it to move. I think that's your problem.. look up your vin if you bought a t case. There's vin decoders online that will tell you if it's 4 speed or 6 speed etc.. you might have to go to several different links on there but one will tell you whats in it.. again please let me know if this information helped you!?