Confirming Emergency 4WD is Actually Engaged

[Alleycat]

I bought a really well preserved 76' J20 equipped with "Emergency 4WD" which I realize is basically a center diff locker. This truck is almost 50 years old and I'm wondering if there is a way to determine if the locker is actually engaging. I'd like to find out now rather than getting stuck and finding out the hard way that the system is toast. After I replaced and correctly routed the vacuum lines, the Emergency 4wd light - lights up at the dash. But does that actually mean that the locker is engaged and functioning? I'm just considering the possibilty that over the course of the truck's 48 year life that some knucklehead could have engaged the system and driven for a while on hard pavement and trashed it. When I actuate the switch I hear a "swoosh" sound for just a second when the light comes on at the dash which I guess is a good sign.

[Srdayflyer]

the vacuum actuator is at the transfer case and i believe if i remember you should see the engagement shaft move so you should see if engagement occurs when selected, once engaged it stays in position (Engaged) until deselected loss of vacuum has no effect on disengagement till selected

[Alleycat]

Very helpful - thanks!

[Stuka]

Another quick test is to make a sharp turn while in e-drive on gravel. If its in 4wd (e-drive) you will hear it wanting to spin the inside rear tire. It will do that because the rear tires travel a shorter distance, and with no slip between them, it has to make up the difference.

In regular AWD mode, the slip will typically not allow this, unless your clutch cones are maybe brand new and really tight.

[tgreese]

There is a procedure in the TSM for testing the center differential, the "torque bias test." I presume you understand that the unlocked differential is a sort-of limited slip differential (LSD) that lets the front and rear driveshafts slip wrt each other when driving on the street. You should also be able to compare the slipping mode with the locked mode using this same setup.

Also, the switch for the e-drive light is activated by a groove on the shifter shaft that moves the fork on the lockup hub. It would require an internal failure of the transfer case for the shifter shaft to move, light come on, and the hub not lock.

[Alleycat]

Thanks for additional great info

[OldFarmTruck22]

You CANNOT see the vacuum pod shaft move at the transfer case on a 1339. The shaft is completely inside the T-case.
Get on some grass field or dirt road. Ride for a bit in the current position, notice how it feels when you make turns. Then put the transmission in neutral just to make it easy on the T-case, Turn the E-Drive dial all the way to the other direction and wait a couple seconds. Shift into drive, drive on the same loose surface and notice if you feel a difference. If it's working properly I promise you, you will feel a difference.