2nd gen transfer case and/or transmission problem... HELP!

Hi everyone,

I have a 1991 4runner (22RE motor) with manual transmission + 4WD and and am having a problem with the transmission and/or transfer case.

The symptom: When driving at highway speeds (60+ MPH) in 2WD, a cyclic bumping/pulsing sound emanates from the transmission and/or transfer case. I immediately break to slow down/pull over, and I can feel the bump/pulse as I step on the break. The bump/pulse continues regardless of whether I'm in gear or in neutral, until the car is stopped. The sound slightly resembles driving on uneven pavement or when you have a tire that's about to blow, but that's not what's going on. Sometimes I can drive for hours on the highway and the problem never comes up, sometimes it happens quite often... I've driven long-distance (LA to Phoenix) with the car like this, just pulling over and doing the "quick fix" below if/when the problem comes up until it stops.

The quick fix: after pulling over, I put the truck in 4WD and drive a few feet, then back in 2WD, also in reverse, to "reset" the gearing because it seems that this is where the problem is. I accelerate back to highway speeds and the problem resumes sometimes, less often if I accelerate slowly to 60+ MPH. The problem may or may not come up again. I've also experimented with driving on the highway in 4WD (yes, I know this is bad) and the symptoms never occur, but I don't do it for very long for obvious reasons. It seems that the transmission/transfer case somehow gets "stuck" between 2WD and 4WD, but I don't know enough about automobiles to talk intelligently about it...

What I've done: I explained these symptoms to a mechanic and after he listened to the transfer case with a stethoscope, he was convinced that the drivetrain was bent and that I needed a new one. I had him install a "new" (used/spare/functional) drivetrain for a couple hundred bucks but this didn't fix the problem. Perhaps the symptoms occur less frequently but I'm not sure...

Help!!! Does anyone have any ideas of what might be going on? The truck is great otherwise (even with 250,000+ miles) and I want to fix this!!!

Thanks,

James

 

Ok, so if you crawl under the truck and shake the rear driveshaft hard with both hands at both ends, is there any play at all? A very small amount of rotational slop is ok, but there should be no wiggle at the ujoints or the slip joint. If there is, pull the driveshaft (4 bolts on each end, not hard at all) and take it to a driveline shop to get checked out. Not just any mechanic (and not the guy who had you buy a new one for him to install) take it to a professional driveshaft shop. Tell them what you found and ask them to make sure the shaft is balanced and the joints are tight, and have them repair it if not. If the driveshaft checks out, then your trouble can probably be traced to the tcase output bearing or the pinion bearing in the 3rd member, depending on which end was loose. It's possible that both ends could be shot, if your original driveshaft really was bent it could have beat the snot out of those bearings. If so, you'll want to drop the tcase and/or 3rd member, the shop that looked at your driveshaft can probably look at them for you as well, if not, ask if they'll recommend somebody. If they're bad, it'll cost you less to buy used/junkyard parts than to have your parts fixed, but that's your call.

Now assuming the driveshaft and output/input bearings are good, what you want to do next time you get the vibration is wait for a straight section of road where it's safe to coast for a bit (don't do this in traffic), then put first the transmission and then the transfer case in neutral. It's important that you put them both in N, and I don't know if your truck is shift on the fly so do it in that order. If your vibration doesn't go away, then your problem is almost definitely not the transmission or (most of) the transfer case. If the vibration does go away when you drop the tcase into N while rolling, but not when you just shift the trans into N, then that's almost certainly where your trouble is. Again, it'll be cheaper to source used parts on Craigslist or your local classifieds than to have your existing parts rebuilt.

So check out the driveshaft for play and try that test, then come back and we can go from there.

 

OK, thanks for the detailed advice! I'll follow up on this and get back to you.