StevieC

Guys,

Snow is here and I've put off fixing my 4WD long enough! My 91 V6 Auto with ADD will not drive the front wheels when shifted into 4 HI or 4 low. It will shift into low gear when shifted into 4 Low but only in 2 wheel. So the hi low clutch is functioning. The 4WD dash light does come on when shifted into 4, so the ADD is locking.

Things that check out ok:

ADD
Transfer Chain
Transfer Case oil pump
Transfer clutch looks ok

Since the ADD is working, the ADD transfer pressure switch must be seeing hyd. press. So does this mean hyd. press. is getting to the transfer clutch?

I'm going to put some low press. compressed air into the hyd. vlave control box to trace the flow and see if it is obstructed somehow.

Is the #4 Solenoid only a high speed safety lockout for the 4 low?

Anything else I should check?

Great Thanks to anyone who can help!

Steve

Swissdrums

Hey man,
dont know about ur situation, sorry...but i gotta tell ya, park city is AMAZING
I go up there every year to hit the slopes during sundance, but this year i couldnt get a spot in our timeshare. hows the snow? Anyway, sorry i couldnt help.
-mike

hektikwon

Does your 4wd light stay on or blink? When in 4wd try to turn your front driveshaft, does it turn or not? If it doesn't then your transfer is fine and you may have a vacuum leak or blockage. Try hooking the hose from you ADD vsv that engages the front diff directly to the intake manifold and see if that helps. I can't remember if it's the red or blue but you can do a search and find that out.

Flamedx4

Nothing about ADD uses hydraulic anything.
Here's how it works. You shift into 4wd. The transfer case goes into 4wd and a switch on the Tcase sends power to the add solenoids and to the switch on the front diff ADD mechanism. When the solenoids release vacuum on the "hold it in 2wd" side of the ADD mechanism and instead apply vacuum to the "hold it in 4wd" side, then the ADD mechanism slides over the splines to the locked position and the switch on the ADD mechanism closes and turns on the light. So, If the light comes on, the transfer case is supposed to be in 4wd and the front axle is supposed to be engaged. You can easily test both. First by disconnecting the vac lines from the ADD mechanism, with the TCase in 2wd, you should be able to turn the front driveshaft by hand. Put the Tcase in 4wd and if the Tcase actually goes into 4wd you should not be able to turn the front driveshaft by hand. You can test the ADD itself with a Mighty VAc, but you need to remove the mechanism to do that. I would assume it works, because the switch on the mechanism is actually mechanically pressed by the slider within the mechanism when it slides - it cannot turn on the light unless it engaged.

Of course, you have an auto, so I might be totally wrong about how yours works. I don't think there's any difference in the ADD operation with an auto, but I dunno fer shur.

Based on your description I'd say you have a transfer case problem...

StevieC

Swissdrums

Yeah, I love it here. We just got some snow this past weekend and we now have some decent skiing. Next time your out shoot me an e-mail and we'll go make some turns.

hektikwon

The 4wd indicator light stays on and does not blink. My understanding is that the light only comes on when the ADD is physically engaged. Is this correct?

The auto trans transfer case engages with two hydraulic clutches. One for the drive to the front axel and one to the low/hi gear shift. They need hydraulic pressure to engage. Therefore the front drive axle will turn when the truck is not running even if the t-case is working properly. You need to get the wheels of the ground, run the truck and see if it spins. It's a crazy unneccesarily complicated system, that's a real PITA to work on.


Flamedx4

Thanks for the info on the ADD. Sound like my thinking on the 4wd indicator light is correct. Since the auto trans t-case is hydraulically switched the electric signal that goes to the ADD is a hydraulic pressure sensor that lives inside the hydraulic switch box. When it is put in 4wd it sees the hydraulic pressure and send the electric signal to the ADD.

Sorry I wasn't clear enough. I didn't mean that the ADD is directly actuated with hydraulic pressure. The manual trans is all mechanically actuated with two shift forks since it has no ATF throughout the system. Trying to turn the drive shafts on tells us something. Not so with an auto.

I found out when i took apart the switch box that the pressure switch can see pressure without the transfer clutch seeing pressure. Seems like the transfer switch body may have been clogged. The thing looks like a complicated rat maze of hydraulic channels inside. I ran some air through the transfer port that runs to the transfer clutch and moved the plunger that swithes the path of the hydraulic pressure. It didn't switch. Took it apart and didn't see any debris but did find a small amount of sludge. The little filters were clean. Put it back together and tried it again and it switched. So i'm hoping this was the problem.

We'll see if it works. I'm going to leave the front drive axle disconnected and put the rear end on stands. this way I can check to see if the front drive flange turns when in four wheel. I can also try to turn the front diff flange by hand to make sure it's locking.

This truck is the biggest PITA to work on. Nothing is easy! Come spring she's getting a GM 6.2 Diesel and a 700R4. I'm tired of the thing not working. I'm tried of difficult repairs. I'm tired of not having any power and bad fuel economy when it is working.

Thanks for the help guys! I'll let you know if it works.

Steve

nakman

Sounds like this isn't much different in principle from an FJ62 Land Cruiser. Sorry if it's different, that's where my experience lies. Is it different? What you do is locate the solenoids, then just switch the vacuum lines around that run towards the T-Case to get you in 4wd. The test is feeling the front driveshaft, like Flamedx4 said.. HTH.

StevieC

Seems to be working, although i'm not sure the front drive grabs as well as it should. I might not have put in enough ATF or I'm just paranoid.

Don't plan on touching the front drive shaft when the thing is running. However, with the front and rear drive shafts off, the front drive companion flange did turn faster when shifted into 4wd.

lrosenquest

I can't find anyone with my exact problem. I have a 2000 4runner limited 4x4 that is stuck in 4hi. The light stays on and does not blink. If I shift the transfer case knob to the middle position to take it out of 4x4, the light remains on, but then no gears will engage....not reverse, D, etc..
Please advise..