A strange thing going on here...
Some time ago I have realized that my vacuum system does not have an electrovalve in between vacuum source and the actuator. When I got the car, there was an electric plug is dangling down the wire harness, later on I've figured out it's for the valve. I have one solid piece vacuum hose from the T-connector at the brake booster till the actuator, no intermediate joints or anything like that. The system is 4 push button type and operates as expected. Why? Could someone enlighten me here, please? Is this electrovalve necessary?
Brgds, Mike

 

Might have the previous owner deleted it either because they could not get a replacement or because they wanted the front axle engaged at all times?

 

That was my first thought. But:

• there is a position sensor that reads the 2WD/4WD and
• I can switch to any selected position and it is confirmed by a steady light on the selector panel. I guess steady light comes on when a position sensor is in accordance to the pushbutton request.

That hardly makes any sense to me...

 

Mike,

I would think it is possible for that indicator light to go on irrespective of whether or not the solenoid valve is connected to the electrical circuit. The indicator lights might only be there to inform the drive what "should" be going on, not what is actually going on.

BTW, we are talking about the solenoid valve that hangs above the middle of the firewall in the engine compartment, right?

 

Exactly that one.
As for the lights, I assume they have a "blink" state on pending request and "lit" once the request is executed.
I remember I had a vacuum leak once, and was unable to switch 2WD->4WD (or opposite, don't actually remember) and had them just blinking. And I think front differential was equipped with a position sensing switch.
Hm... I guess I'd have to be more specific here. Tomorrow I'll try to make some observations and write back. Still it does not make any sense to permanently supply the vacuum to the actuator...

 

Ah yes, see, you are assuming that whoever did this did it for a good reason or knew what they were doing!

More seriously, it might have been a "stop-gap" solution that was never later addressed properly, perhaps for a lack of a replacement part. To me, having vacuum applied to the axle actuator would be the equivalent to keeping the manual hubs locked on a front axle. Having them locked with the transfer case disengaged doesn't really harm anything but it can add a bit of wear to the system over time.

 

I don't think it was a "stop-gap". I've been replacing the vacuum hoses, and the old one was truly an old one, solid piece, with no shunt in the middle, connecting the two pieces at the electrovalve ends.
From what I have seen today:
- the drive was on 2WD
- i switched to 4WD Hi and I've heard one "clang"
- then i followed to 4WD Lo and there was another "clang"

Do I understand correctly, that:
1. While 2WD there is a fixed shaft from a transfer case to the rear differential and the rear hubs.
2. For 4WD You have to engage two things:
- a front shaft linking front differential and the transfer case
- a front differential to the hubs by that actuators

and in my case the drive shaft was disengaged, while the hubs remain engaged cause of the missing electrovalve...?

Luckily I've already ordered that valve, preowned for $20 :-)
Thanks Christine for Your help.

 

Crazy. Well, when you get the new valve installed and do a check, I'm sure this will all make sense. For one thing, we'll learn if the push-button switch "knows" if the solenoid valve is connected or not. It is probably not worth the effort but I'd be curious to check if the hubs were engaged when the engine was running and supplying vacuum to the front axle actuator even when 2WD was selected.

This would require lifting the front tires off the ground with the engine running to see if turning one would turn the other the opposite direction. I am one to normally avoid running the engine when the truck is lifted. I just hate the idea of it accidentally getting into gear while I'm under it! Ack! But if you have a hand vacuum pump, I guess this test could be done with the pump supplying the vacuum instead of the engine. But again, probably not worth the effort unless you are a nerd like me and like doing experiments. LOL

 

Since I was a kid I've been disassembling toys and things to find out how it's done and I can't grow out of it. I will gladly do the test tomorrow - it should bring me some more understanding. I'll jack up & support the front, then I'll start the engine. No need to get under. I will just be aside trying to turn the wheel. I've been running the engine on a lifted car even today. I had to turn wheels side to side and I don't like the idea of doing it without a steering pump off. The bloody grease fittings on the inner tie rods are very difficult to clamp. Probably as difficult as getting 1/4 inch angled fittings in a metric country

 

A new experience gained.
I've jacked up front.
1. Did not start the engine, grab the wheel and turning one was making the opposite one turn into an opposite direction -> front differential locked. The shaft from transfer case remains steady.
2. Started the engine. Went throughly from 2Hi to 4 Hi and 4 Lo. In all the positions the behavior was as in pt. no 1.
3. To disengage the front differential I've stopped the engine[1]. and pulled the vacuum hose off the T-connector. The front differential got disengaged and wheels could turn freely.

The conclusion here is that the indicators on the transfer panel not going to indicate whether the front differential is in right or wrong position.

[1] It is not easy to pull the vacuum hose off the connector while vacuum is on. These plastic T-connectors are prone to breaking, I've already had that pleasure of making one of them. I made it from brass.

P.S. I left the car jacked, as I am expecting the electrovalve arrival today. Gonna put it on line and test if it works properly. That makes a tempting possibility to check the SRS front sensors...
EDIT:
I took my chance.
Passenger's side SRS sensor ok, around 1k. Same resistance can be read on the driver's side plug (A+C). Driver's side SRS sensor - A+B zero resistance, which is good. C+D fluctuates around 4-10k, should be around zero = busted. As You said, not easy to find these...