4x4 (ADD) engaging itself at hwy speed, not good!

[Jon]

Noticed a heavy jarring that effected the whole truck on the drive to Fordyce yesterday. We were on rock slide prone roads and I dodged a few fist sized ones. I blamed the jarring on me missing some of them. It hit three times during the 45min drive out of Grass Valley. Cruising along at 55 and BOOM! WTF?

Hit Fordyce for the rest of the day and was back on the HWY after dark. As soon as I hit the HWY 20 exit it happened again BOOM! and then some more lighter bangs, possibly do to me decreasing in speed. But since it was dark I my eyes were drawn to the coinciding flash of the green 4x4 indication light. That means that for some reason the ADD is sliding over on it's own NOT GOOD to engage a heavy static diff to a shaft that is spinning at 60mph. I pulled over and played with the t-case and the 2-4 button. No problem staying engaged in 4wd. Rather than have it slam anymore, I elected to just leave it engaged and put home where I could do better trouble shooting. It was about 20deg at this point:smow:

This is an electrical, vacuum, mechanical system. Electric 2-4 select switch tells the 4WD ECU it wants or does not want 4x4. The ECU engages one or the other Vacuum Switching Valves. VSV2 for disengage and VSV4 to engage the front diff. The VSV will route vacuum to the ADD actuator which is a simple diaphragm connected to a fork that slides a sleeve that engages the front diff to the passenger side extension. And the knee bone is connected to the leg bone....

My truck was originally an A/T so I'm the bottom diagram. Still using the 2-4 switch on the stick but the 4WD switch is spliced to ground due to the 5spd swap. Never been an issue in the past 3yrs.

Today's test... visual inspection didn't show anything out of place or broken. The vacuum at the intake is at 20hg at idle. Both lines at the ADD are pulling vac individually when required. Amount of vac is unknown, I don't have another gauge, but it's pretty light. I drove it at HWY speed for a few miles and had no issues. Every stop light I'd engage and at the next light disengage the ADD. No problems.

Any ideas? I'd hate for this to happen again and break something.

Thanks

 

[TheDurk]

I´ve never seen this symptom set on here and I´m in almost all the ADD threads. My first guess is a vacuum issue due to the intermittent nature of the fail. By any chance, did the 4x4 pop in when you took your foot off the gas to slow for the exit? That would be a dead giveaway as that is when vacuum at the manifold goes almost to zero. Thus the system may not be maintaining vacuum during decel.

Check the one-way valve in the hose coming from the manifold (where my money is). (The basic blow in both ends test.) Check all hoses. Blow into the hose going to the reservoir can in the fender to see if it leaks. Verify that 4wd VSV is not leaking vacuum when no power is applied.

 

[Jon]

I'll check that one way valve. I don't remember if I was off the throttle or not for every occurrence but I definitely was off it taking the exit.

 

[Jon]

Check valve is good. If vacuum is good to the ADD, i guess I'd have to have an internal leak in the actuator diaphragm. Ans by doing a quick search... those thing's aren't cheap!

 

[TheDurk]

Check valve is good. If vacuum is good to the ADD, i guess I'd have to have an internal leak in the actuator diaphragm. Ans by doing a quick search... those thing's aren't cheap!

True, but I don´t think you have ruled out all the things that could be wrong with the vacuum system: leaky hose, leaky canister, leaky VSV--either side. You have a vacuum guage--get inventive with a t-fitting and some golf tees and test everything. Also, you may be able to pull the actuator and find an issue you can fix.

Like you said, actuators ain´t cheap.

 

[KidVermicious]

Manual hub conversion FTW!






Sorry, I'm no help.

 

[Brique]

I'm pretty clueless at this pay grade, but I've heard plenty of stories about odd behavior that end in "turns out it was a bad ground on the..."

Any chance that ground you mentioned from the 5spd swap might have gotten wanky? Or another ground for that matter?

 

[Jon]

I'm definitely down for doing some further TS. Both Suggestions are good, testing the vac system and making sure the ground is good.

One thing bothers me though. I am almost SURE that as smart as Toyota engineers are that they wouldn't design a system where, without engine power or vacuum it would engage. That's scary. So every time a 4Runner runs out of gas going down the hwy the front diff explodes?

I still need to drop the skids and drain the fluid....maybe tomorrow. Maybe even pull the CVs while I'm at it. Wouldn't be the first time I broke a shaft off inside the diff. What a pain in the A$$! Hope that's not the case.

 

[TheDurk]

I'm definitely down for doing some further TS. Both Suggestions are good, testing the vac system and making sure the ground is good.

One thing bothers me though. I am almost SURE that as smart as Toyota engineers are that they wouldn't design a system where, without engine power or vacuum it would engage. That's scary. So every time a 4Runner runs out of gas going down the hwy the front diff explodes?

I still need to drop the skids and drain the fluid....maybe tomorrow. Maybe even pull the CVs while I'm at it. Wouldn't be the first time I broke a shaft off inside the diff. What a pain in the A$$! Hope that's not the case.

Well, if you run out of gas, the engineers expect you to stop. A tight system holds more than enough reserve vacuum in the canister for you to stop safely. This truck is not designed for extended coasting with engine off. Steering and brakes become very funky as well.

 

[Jon]

So I did a bunch of vac testing today with the front tires off the ground. Using my 2-4 select switch, everything was working fine. Engage the 4WD and the VSV got full 13.8V and opened up pulling vac at the bottom line on the ADD. Switch it off and VSV kicks off and vac died. I did this several times and spun tires to make sure the drive shaft was turning and vice versa.

Test drive showed different results. Right at 30 MPH I could hear the gears start grinding and trying to mesh. Once slowed under 10mph it would fully engage. Jumped under the truck real quick and found that the bottom line had full vac, pulling the ADD to 4WD. The vac would come and go. I could hear the VSV clicking on and off with the pull of vac. I left the line disconnected and parked in the garage w/o turning it off.

Now the vac was solid. Pulled the line at the VSV that pulls on the ADD. VSV was open and sucking away. Pulled the electrical connector at the VSV and it closed. VSV4 was getting constant voltage no matter how many times I cycled my 2-4 switch, multimeter set on the hood so that I could read it while operating the switch. Now from underneath I started to fiddle with my splices. I bent one and the VSV closed. Perhaps it was a coincidence but it's a start. Time to dig up my old thread on my 5spd swap and see what that splice is for. Sounds too easy, fingers crossed!

 

[Jon]

FROM MY 5SPD SWAP THREAD

So now it start and runs and drives. Awesome, now time to get 4wd working again. Of course the Limited has the ADD front diff which has to lock in electronically. Putting the t-case shifter into 4H and 4LO only goes as far as engaging the front drive shaft. No good since the front diff isn't locked in. Another simple fix of splicing a wire in. I stuck the auto's diff stick knob in there since it has the factory "on the fly" 4WD button. Just find the connector that went to the t-case, gray two wire. Dike off the connector and run a wire from the green/red stripe up to the shifter pink wire. Boom, 4WD works. If you just jump from wire to wire at that plug, 4WD will engage BUT the front diff will not disengage without restarting the truck or removing the jumper.

So I think that wire and splices are the culprit. The wire was slightly pulled from the loom running down the transmission and the loom rubbed through the wire insulation on a small part. Wasn't grounding out on anything but never good to have bare wire showing. Since I pinched the splice, the issue hasn't come back. I'll do a proper fix with new wire and more secure routing. This was all quickly done as a test of concept during the swap and I decided not to fix what wasn't broken. Well now it's showing it's frailty and needs to be done right.

 

[Qyota]

Very interesting!

I've been watching this thread, hoping I'd learn something about diagnosing vacuum issues in the ADD/transfer system.

Jon - by chance, does/did the grinding you hear sound like this: https://www.youtube.com/watch?v=szN8B1cJjyw