So the connection tested fine - 12V at one lead, 5V at the other while in 2Hi; 5V lead switched to ground when 4Hi was pressed. I guess this means it's the vacuum solenoid, but I'm afraid to replace it [again] since only a month ago I replaced it with a new OEM unit to no avail. I might just go through the winter with the solenoid bypassed.

Any ideas would be appreciated - I tried tapping on the solenoid and it didn't help. I have it in the house currently - thinking perhaps it's frozen and seeing if warming it up a bit will help... but I doubt it.

 

You tested the connection at the solenoid valve? If so, then it would appear that the valve is the culprit. You can bench test the valve with a 12V power source by applying the appropriate power and ground...

 

We didn't have a 12V DC power inverter, but we had a 12V AC one... hooked that up and the thing vibrated (as it should, being alternating current), so the vacuum solenoid appears to be functional. Put the solenoid back in, pressed 4Hi with the engine off and the solenoid clicked, so we started it up. The "Service 4WD" light didn't come on immediately, but the 4Hi button blinked as it always does (never went solid). After a spin around the neighborhood, 4WD didn't feel like it was engaged and the Service 4WD light came on.

If bypassing the switch gives me 4WD and the switch is, in fact, working, I have no clue what the problem could be.

 

not that it matters...are the vacuum hoses hooked up correctly? I can't remember off hand but I did mention in a previous post-just above. If correctly in place, I agree with Kyle-gotta be something wrong with that solenoid. Make absolute sure hoses hooked up correctly-on wrong and that unit vents thru the top and may not allow enough vacuum to pull the servo/cable/front differential in. JP

 

We've tried it both ways. If we let the constant vacuum go to the top connector, you can hear it drawing in air from the relief valve. Therefore, we concluded that the vacuum has to go to the bottom connector (I believe I made this same determination earlier in this thread).

 

I would exchange that solenoid...BTW did the new/replacement wheel bearing resolve your issue? Had an 01 in Friday with a missing 4mph signal from the left front sensor(had all other mph's from 3 to whatever except 4mph on that side only). New Timken 513124 bearing and whola....no more activation of the ABS at low speeds. JP

 

Yep. I replaced the wheel bearing this morning and the ABS problem was resolved (I had no doubts that would resolve my problem).

I'll speak with the stealership and see if they'll exchange the vacuum solenoid. Worst case, I go through winter with the jerry-rigged 4WD system. At least I can force it to work if I need it.