4wd help

myHD truck · August 18, 2019

2003 duramax - front tires not spinning in 4hi or 4lo - I can hear shifting from dash button, i can see cable shifting but wheels will not turn - I did get all 4 tires off ground to see if wheels spin - (caveat never done this before - but nothing turned when engage in Drive - 2wd or 4 wd - maybe something to do with resistance not being there?)

but testing 4wd hi lo nothing happens -

please help me further diagnose - perhaps cable is snapped and not actually engaging? in transfer case...? how would i test this? and should tires spin if on jack stands or lifted and all 4 tires free to rotate?

thank you in advance

davester · August 19, 2019

Does the light go solid when you select 4wd, after flashing for a bit?

When you shift into 4wd, reach under the truck and try to spin the front driveshaft with your hand.

If it rotates freely, then the transfer case didn't shift into 4wd for whatever reason (wiring problem, encoder problem, something broken inside the TC)

If it doesn't rotate, then the TC, then jack up one of the front wheels (leaving the truck in 4wd) and try to rotate the wheel.

If the wheel doesn't rotate, then you are in 4wd.

If the wheel rotates then look at the front driveshaft while you are rotating the wheel

If the front driveshaft is rotating, then the front axle is properly locked, but the TC has failed (most likely the chain is broken).

If the front driveshaft doesn't rotate, then somethings wrong with the front diff (wiring problem, actuator has failed, broken locking gear or axle, or 4wd sensor failed in TC)

myHD truck · August 19, 2019

thank you for the steps - I will check it out this week - or weekend - time permitting

carkhz316 · August 19, 2019

Seconded with davester to see what the indicator lights on the switch are doing. If they flash a few times then stay lit on the appropriate button, then the system is working electrically (and you have a mechanical problem). If the light continues to flash, then there is a fault. You can listen for two distinct sounds when 4WD is commanded. The first will be a quick whirrrring click of the transfer case encoder motor emanating from underneath the truck. The second will be a high pitched whirring drill like sound that lasts a bit longer than the TC. That is the front axle actuator. You can to a quick test of this but unscrewing it from the front differential and plugging it back in to see if the plunger extends and retracts when 4WD is commanded.

myHD truck · August 20, 2019

ok thanks - I have not checked lights on switch panel - thank you

myHD truck · August 20, 2019

thanks all for the assistance - placed truck in 4wd hi and lo and drift shaft engaged - in 2wd the driveshaft spins freely -

replaced actuator but did not take the step to confirm 12V at plug - - but tried to climb a rock - still not spinning -

so actuator not engaging ?

next step..is check 12v at actuator and then - what?

thanks in advance

davester · August 20, 2019

unscrew the actuator from the front diff, plug it in, then shift between 2wd and 4wd, to verify that the actuator extends/retracts.

If it does, then something is busted in the front diff itself.

If it doesn't (and at this point, I would plug in the old actuator and see if that works), then the actuator is defective, there's a electrical problem (fuse/short/break), or the 4wd sensor in the transfer case is defective).