scope103

On my '94 (3VZE, 4wd) I have a Black/Orange check valve in the line from the 3-port "filter" on the plenum to the furthest-aft VSV for the PAIR (Black towards the plenum). I have a Blue/White check valve in the line from the "filter" to the pair of VSVs that drive the ADD (Blue towards plenum).

So far as I can tell, they are just check valves, not vacuum delay lines or such. This makes sense; the ADD line connects to the vacuum reservoir (found in the right wheel well), and you wouldn't want that reservoir back-feeding into the plenum. I'm not as sure why it is in the PAIR line, but it could be as simple as the need to keep the PAIR valve open even if vacuum is fluctuating.

I don't know why the two check valves are colored differently, but there is probably a reason.

70elcoss

dont want to hijack the thread but im having a similar issue maybe you guys can help me out with.... when i shift into 4wd (5speed 1990) the light on the dash doesnt turn on but the car kinda shakes and wanders hard to the drivers side. anyone have any idea what this might be?

RJR

Tell us a little more about your vehicle. Does it have ADD (automatic differential disconnect), or does it have automatic hubs out on the wheels? How long have you had it? Did this behaviour just start recently, or has it been that way ever since you owned it?

You can get that behaviour if there is a lunch-box locker in the front and the ADD is not engaging, but that's kind of a wild guess at this point.

Best thing to do with any 4wd problem is to jack up both front wheels, chock the rears, set the ebrake, put the vehicle in neutral, and start it up. Cycle it into and out of 4wd and see what the front wheels do when you try to turn them. That will tell you more about what's connected (or not) then any of us can from behind our computer screens. In 4wd, if you have open diffs, when you turn the left wheel forward the right wheel should turn backwards, and the front driveshaft should not turn. If you have any different experience than that, something is wrong.

70elcoss

its 100% stock... open diff with ADD. I've had it for about 7 years and 4wd used to work but this was the first time in about 4 years I've turned it on. I won't have a chance to work on it until Sunday but I'll jack it up and see what I find then