My 98 Blazer will not go into 4wd. 3 button on the dash switch. Truck is on 4 jack stands.

1. Vacuum actuator and cable are working fine. Tried pulling cable all the way out and locking with vice grips and still no 4wd.
2. TC Switch replaced yesterday
3. Pulled and inspected TCCM for corrosion. Looked clean. Both fuses, 10A in dash and 20A under hood are fine
4. Dash switch lights indicate everything is working fine.
5. Front drive shaft spins when switched to 4wd, does not spin in 2wd.
6. Battery disconnected and reconnected several times to clear codes, including overnight.
7. Audible clicks when shifting into 4 hi and typical clunk going into 4 lo, just that front wheel/s do not start turning.
8. There was small amount of trans fluid residue in the vac lines. Blew them out when I replaced the TC switch. Did not pull the vac canister out of the fender well to see if that was full, but front dif did not have excess fluid when fill plug pulled.

I think it may be something mechanical in the front differential (collar not locking), but maybe it is a sensor or something?

Can anybody point me towards the next step? Appreciate any help.

Thanks,

 

Sounds like a problem in the diff. If the drive shaft is spinning and you pulled on the cable with vice grips, there's a translation issue in the diff.

 

Guess I will start tearing into the differential then. I found a decent explanation with pictures of how the shift fork moves the collar to engage the gears. Maybe I will be able to see something obvious once I get a look inside.

Thanks

 

this video shows some of the internals to check