Dan Rudd

My 4runner is a 98 Limited, 6cyl auto. Last year here in Michigan I used the 4WD a lot and it always worked like a charm. This spring while pulling a boat out of the boat launch my son called to say the 4runner wouldn't go into 4WD. He said he tried all he could think of but it wouldn't work. He said no light on dash and no "click" when he pushed the 4WD button.
When I got home from work I tried it and it went right in. Light came on and clicked into 4WD.
Well, a week or so ago I went to put it in 4WD and nothing, no light or no click and no 4WD. I can put the transfer case on L and it does go but I get nothing at the front diff. I'm thinking the click I'm used to hearing was the relay giving the 4WD it's power?

I've searched here on Yoda Tech and other places and read quite a bit about the "ADD" (what's ADD stand for?) relay and how it could be the issue. The problem I find is that no one seems to know where the ADD relay is. I've read several threads where someone describes it and then someone else corrects them. I've read it's on the drivers side kick panel, the passengers side kick panel, under the glove box, etc.

Where is this elusive ADD relay? Providing thats my problem at all. If it's not it then what might it be? FYI, the 4WD fuse is good. Any other thoughts?

Thanks
Dan

oldblue

i've seen the add motor on the front differential go bad numerous times. next time it won't go into 4wd, remove the front skid plate and gently tap on the add motor and see if it will engauge then. that thing is expensive.

tantacoma

BTW, ADD stands for Automatic Differential Disconnect. I believe that some models of ADD have a vacuum-controlled motor, and some have an electrical motor.

TheDurk

I have a standard post for this:

Go through this before you worry about the relay. It's rarely the problem, and the location does vary by year and type of transmission.

Dan Rudd

Thanks for the replies, I will start checking out the VSV's and see what I come up with.

The reason I was leaning towards the relay was the lack of "click" I'm used to hearing when I engage the 4WD. I was assuming that click was the relay.

I'll pass on what I find.

Thanks,
Dan

Dan Rudd

OK, I started working on the 4WD and so far here is what I've found.

I've checked the power at both VSV's. When the 4WD is off I have power to one of the VSV's and when the 4WD is turned on I have power to the other.

With the motor running I have good vacuum from the manifold to both VSV's. My vacuum gauge reads 19 in. going into both VSV's. With the engine running and the 4WD off I also have the same 19 in. coming out of both VSV's. Nothing seems to change whether the 4WD is on or off, 19 in. vacuum in and 19 in. vacuum out.

On a side note, when I initially found the VSV's one of them had the rubber cylinder had come off. I was able to slide it back on.

What do I make of this? Do I have 2 bad VSV's both at once?

Thanks,

TheDurk

That would appear to be the case. Ohm out the VSV solenoid coils, spec is 37-47Ω. You can also pull them and try to operate each one with an external 12V source. If it passes air flow with no voltage, it is bad.

Dan Rudd

OK Durk, since you seem to be very knowledgeable on this topic maybe you can help me fully understand the process of the 4WD engagement.

I hope I can type this so it makes sense and I can relay whats in my head.


If I understand this correctly the purpose of the VSV's are to move some type of mechanism to lock in the front differential. I assume my transfer case is engaging as I hear something that sounds like an electric motor cycling when I press the 4WD button. I can hear this with the engine off (allows me to hear over the sound of the engine) .
In the case of my situation, since I have vacuum traveling from both VSV's to the differential at the same time how does it know whether to be in 4WD or 2WD? And further more, if I were to simply pinch off the vacuum line which takes it out of 4WD wouldn't in then be in 4WD?

Lastly, What activates the 4WD light on the dash?

Thanks for all your knowledge and help.

Dan

TheDurk

Each VSV is a simple solenoid valve. When it gets power, air passes from one nipple to the other. No power, no flow. In normal operation, only one gets power from the 4wd ECU (or relay on a 5-speed). In the diff, the vacuum moves a fork that slides a collar that engages the diff to the axles for 4wd. When the vacuum moves to the other side, it moves back, disengages the axles and it's 2wd again. The light comes on from the detection switch in the diff plus one of the 4wd detection switches in the T/C. In a 5-speed it is ONLY the switch in the front, as the relay won't try to engage the front until the T/C switch is activated. This is for '96-'00. '01-'02 went electric in the front and it got more complicated.

In your case, it's a fail situation as only one side is supposed to get vacuum. I don't know what happens, but it won't be good. Yes, if you can force vacuum only to the correct hose, it will engage the ADD (assuming you are not leaking somewhere.) If the T/C has also engaged the front propeller shaft you will be in 4wd.

Dan Rudd

I took the VSV's off again tonight to confirm their condition. I ran 12v to them independently and and got the same results. In one of the VSV's air flows freely from the intake nipple to the diff nipple. In the other VSV air flows freely between both nipples and out the front where the rubber cylinder is.
Applying 12v gave me a faint click in both but no change in air flow.

I think it's fair to say they're both shot. It just seems odd to me that they both went bad at the same time.

I called the local parts store to order a couple VSV's. I like to shop local when I can but after they told me $57 ea I had to pass. I can order online via Amazon for $27 ea. $60 is too much to pay for local convenience.
The part they advised I needed was a Dorman 911-602 which when I look online appears to be what I've got. The online (or local shop) part doesn't have the "rubber cylinder" included. How do I know that mine are OK to use? They do offer a VSV without the rubber cylinder, 911-603 I think. Can that be used instead? Not sure the purpose of the rubber cylinder.

TheDurk

It seems to me any 12V air valve that fits will work. I always assumed the cylinder was some kind of filter/baffle to keep crud/moisture out of the system. Probably not needed unless you do extreme wheeling in dusty/muddy conditions, but that is just my internet guess.

[EDIT: New thought] Maybe the valve, when off, blocks the incoming vacuum, but leaves the diff hose open to the atmosphere as opposed to just blocking it off as I have always assumed. This would make it easier for the other side doing the pushing. That would be the only reason to need a filter/baffle, now that I think about it. So maybe these other valves that are built for the evap. emissions system just block when off, and might not work so well, if at all. I'm speculating, because no one ever asked the question to me before. Folks have just gone the junkyard route for real 4wd VSV units.

So my answer is: I don't know--but maybe not.

Dan Rudd

Update
Well I put on my new VSV's (with rubber cylinders) and they appear to work as you've described. But I still have no 4WD.
My next guess is that the actuator at the diff is not moving. Maybe it's stuck and the vacuum pressure is not enough to free it up? I'm not sure what I'm looking for but maybe when I'm looking at it it will seem obvious to me.