Sometimes 4WD engages, sometimes it doesnÆt. I assume the light on the dash is simply connected to the lever and is not a good indicator if everything is working or not.

IÆve noticed that sometimes when I shift into 4Hi while moving, I can feel a drag on the rear end of my truck. This morning when I was supposedly in 4WD, I ended us spinning one of my rear wheels wildly on ice while my front tires were not being driven. I then heard a loud bang as 4WD seemed to engage.

I remember reading somewhere that the transfer case uses a vacuum to stay in 4WD. Is it possible there is a leak somewhere and only when the engine revved, was there enough vacuum to engage 4WD with a subsequent bang?

What tests/diagnostics can I do to try and narrow down the problem?

Thanks!

Read all about it:
http://www.off-road.com/toyota/tech/add/

Thanks, but I don't have ADD. I have manual hubs.

You mentioned having vacuum involved. Were the manual hubs stock or added on later? If you have just a regular old t-case, then it shifts totally from the stick, no vacuum or anything. If it is not shifting right, likely cause is that the shifter is "stabbed" into the shift rails wrong. Pull the shifter out and take a look. There are a couple of ways it can go in and only one of those ways is right. If that does not fix it, must be a problem with the t-case. The dash light is operated off of the switch in the t-case.

Manual hubs were stock, but the engine needs to be running for the transfer case to be engaged. There is definately no ADD stuff on the axle.

So, does the 4WD light being on indicate that the lever is "stabbing" correctly?

Not terribly familiar with the automatic t-case setup, but if it is directly shifted with a shifter into the top of the case, then it should shift in and out of 4WD any time. If there is some vacuum actuated part of the shift linkage, then inspect that for leaks. If directly shifted, the 4WD light says that the 2WD/4WD shift rail has moved far enough to actuate the switch and turn the light on. If Hi/Lo range also works then the shifter is in properly, if not it may be just stabbed into the 2/4 shift rail.

I have an 88 with auto trans also. The light is activated by a simple switch on the transfercase, so I wouldn't use that as any indicator of whether or not your transfer case is turning the front shaft. The linkage that connects the lever in the cab and the transfer case is all over the place under the truck. Check to make sure it's tight and a moving correctly. The lower pivot for this thing is easily bent/mangled if you catch it on something. My lower pivot got bashed while wheeling and it would not go into 4x mode at all until I repaired it. There is no vaccuum stuff on the transfer case it is all mechanical.

Did a quick check over lunch. It shifts from high to low range fine, but still power to only two wheels. I did notice it was a bit tougher shifting into 4lo than normal(2Lo in my case) which hopefully means it's only the linkage. Right? Or is it a bad sign that when I shift into low-range it's only 2wd?

And yes, I triple-checked to make sure the hubs were properly locked. <img src="/forums/images/graemlins/smile.gif" alt="" />

IT may be play in the shifter linkage (assuming it is not a otop shift case). If so, that could mean that the linkage tries to push into 4WD but can't quite make it (enough for the light to come on but not enough for the gears to mesh). Check the linkage under the truck:

my 86 had trouble shifting in to 4wd ,I changed the trans case oil , added lucas addative,seem to clear it up.