Newb to the site and wrenching. Having trouble with engaging 4wd. Here's the details:

• 1990 4Runner 3.slow V6 manual 4wd
• ADD system
• When switching to 4H or 4L, the 4wd dash light comes on
• When starting to drive, it pulls hard to the right and the tires squeal
• 4wd used to work just fine

It sounds like the tcase is engaging the front diff, but the ADD isn't engaging, so I essentially have 3 wheel drive. Is that correct or another issue like axle shafts?

If so, do I just check for vacuum leaks (though I don't hear the classic sucking sound and the majority of the lines are newer and pliable)? Or is there something else to check?

 

Are you engaging 4WD in the dirt or on hard surface .

Possible that something is hanging up actuator.

When did things appear to work correct last ??

Up to anything that may have broke something

 

Does your front differential have a locker? Without a front locker, I would assume that if the 4WD vacuum actuator is not working, you'd have NO power to the front wheels because the differential would send all power to the non-connected axle. If you have a front locker, then yeah maybe I could see this operation. Are your front and rear gear ratios the same? You say it pulls hard to the right, which would indicate the left front wheel is getting some power...I can't remember exactly, but I think the driver side wheel is the one that the vacuum actuator controls...I really don't understand the symptoms, a failed actuator would tend to mean you'd never get power to the front wheels, if by some miracle it failed in the actuated position and was stuck there, seems like you'd get normal 4WD operation with the lever in the 4WD position. I think we need more info.

 

Thanks for the replies!

coryc85:

There are no lockers. Also, since it used to work, I don't think different ratios is a problem.

The way I see understand the ADD diagram, the right wheel can be engaged with just the tcase engaged, while the left wheel is engaged with the ADD. So, with the ADD not working, only the right wheel would be engaged, but that would mean pulling to the left. Unless I'm understanding it wrong...

wyoming9:

I've put it in 4wd on on both dirt and hard surface, but most recently on hard surface to keep things lubed.

 

swobo - thanks for linking the diagram. Your statement is not quite correct. The passenger axle stub is always connected to the differential carrier and the passenger axle has to get coupled to the differential carrier through that ADD actuator. But here's the thing with the ADD actuator not doing its thing to couple the driver side axle to the carrier...since passenger side wheel is on the ground with good traction, and the drivers wheel is essentially disconnected, you don't have 4WD and you don't even have 3WD at that point. Because the front driveshaft will be spinning and the front differential will send 100% of the power to the driver side axle which is completely de-coupled from the wheel. Your passenger wheel will just be sitting there doing nothing.

So if your actuator is not working because of vacuum issues or whatever, your front wheels would be doing nothing, no tire squealing, no pulling to the right, nada, just like having the tcase lever in 2WD. This is why I don't understand your symptoms.

 

coryc85, what you say makes complete sense, which makes the symptoms not make sense. I'll have to put it in the dirt next week and pay better attn to the details and maybe get all 4 wheels off the ground and see which tires spin.

For now, assuming the ADD isn't an issue, would that point to CV joints/axles, wheel bearings, or something wrong with the differential or tcase? Or could it just be low on fluid??

 

I've done some more digging. Apparently, the 4WD dash light comes on only if both the tcase and ADD switches are in the closed position, which are based on physical movements of the tcase's shift lever and the ADD's sleeve. My dash light comes on, so both appear to be at least closing their respective switches, which is a good indication they're both working.

So, it would appear it's something else. I'll inspect my CV boots and wheel bearings tonight. Not sure what to inspect for on the front diff other than the fluid...