4WD (four wheel drive) button not illuminating dash light

I have a 92 Hiace Super custom 3L with 4WD locking hubs. It had been a while since I used Four Wheel Drive. When I went to use it, I locked the hubs, then pressed the 4WD dash button. The indicator light on the dash did not come on. Any ideas what this means? I didn't really test to see if 4WD Hi range was working despite the indicator light not going on, but I'd like to figure this out. I did shift it into low and it moved over smoothly, but am unsure if it was operating in 4WD, or just still in 2WD (rear). Luckily, I was just putting the van into 4WD as precaution because I was in a snowy area, but didn't really need it. A question is, does 4 wheel drive work if the hubs are locked regardless of the light coming on? Or is there something going on that won't allow 4WD to work?

I read somewhere else that the vacuum tube to the front differential may be have been disconnected or bad. I'm don't really know how to identify vacuum tubes, but I took a peak underneath and saw a wire disconnected that goes into the bottom driver (right) side of the motor, it plugs in close to where the oil filter is, but into the side of the motor. Anyway, I plugged that back in, but still didn't have any luck with the 4WD light going on.

I'm not super mechanically inclined, so try to keep responses in simple terms please.

It sounds like the connector you plugged back in was to the oil level sensor. Ideally we should be engaging 4wd on our vans (and, on a side note the AC compressor) once a month to keep the transfer case and diff gears/seals coated in oil. Hopefully someone can chime in on the issue you're having!

Thanks for the reply, I'll definitely keep that in mind. That makes sense about the oil level sensor. I've been having the oil indicator light come on for a second when I start up the van, maybe because that was unplugged.

You could have an issue with the vacuum line that runs to the Transfer Vacuum Actuator that is attached to the transfer case. Check and make sure the line is connected, you may also want to buy a vacuum tool to see if you have a leak in that line. Not sure if there are any electrical connectors at the t-case or at an upstream vacuum actuator (electronically-controlled valve that allows vacuum to be applied/blocked to a downstream part) that needs to be checked. There's probably some general info on IH8MUD that could give you ideas on how to troubleshoot.

My assumption is you hit the 4wd button, a vacuum actuator opens its valve, which sends vacuum down the line to the vacuum actuator at the transfer case, which engages the gearing that drives the front prop shaft.

Transfer Vacuum Actuator diagram:
JP-CarParts.com

Transfer Vacuum Piping diagram:
JP-CarParts.com

So my 4WD was actually working fine. The dash light for whatever reason just won't come on. I also noticed that the 4WD button itself does not illuminate when the headlights are turned on; the other buttons in that area, such as rear heat, and rear defrost do illuminate. While the 4WD light does not illuminate at all (remains dark). So I don't know if that is related to all of this. However, my 4WD is working. Anyone know why the 4WD button as well as in dash 4wd indicator light both remain dark?

A test to see if actuator is working is while someone is pressing the 4WD button (with van on and hubs locked), another person is under the van at the transfer case, listening for the actuator to make a noise, which is what engages the 4WD. If you don't hear a sound, try banging a small hammer on the round diaphragm looking thing (the vacuum actuator) bolted to the transfer case. Sometimes this get stuck, and some banging helps free it up. If that doesn't work, a mechanic told me you might have to take a part the 2 small black vacuum actuators, clean and lube them.

Great news that your 4WD is working! As far as the dash lights go, you may have to check for a loose connection or burned out bulb. I believe there are guides on the forum to removing the instrument cluster. For the 4WD icon on the cluster, there may be a sensor at the Tcase that detects when the 4WD is mechanically disengaged, as that light doesn't always turn off the instant you press the 4WD button, so it could be an issue with either the light bulb, it's electrical connection, or the sensor itself. Thanks for the troubleshooting tips on the actuators!

Just a side note, because I've had this happen. In my case, the light did not come on (or 4WD activate) because I had a bad solenoid under the van where the actuator is. I can't find the solenoid part anywhere so I have to swap the vacuum tubes from where they go from the two solenoids to the two ports for the actuator everytime I want to go into 4wd . Works in a pinch I guess!