All wheel drive not working?

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Feb 11, 2011
Hi All, took my 91 FJ80 in for a smog check for registration renewal in CA. Dropped the car off and walked across the street. When I returned, I found the smog place had done a "rolling" smog check and the report showed measurements at 15 mph and 25 mph. I looked at the shop floor and only saw rear rollers. I asked the smog tech how he did a rolling test on an all-wheel drive vehicle with only rollers for one axle. I thought he was bs'ing me about even running the test (which failed).

He stated he forgot it was all wheel drive but that when he put it on and tested it the front wheels didn't turn at all. I asked him to show me and he did - he pulled it back on and sure enough, the front wheels didn't turn at all and he ran it up to about 20 mph. I didn't get to try it in low range to see if that made any difference. I did a quick visual inspection after getting home and all looks fine. Thoughts on what I should look into? Think it's the transfer case? The front driveshaft is in place, I did verify that.

I've had the truck for about 2 years with no major issues but have never really had it off road (I know, I know)... Any thoughts on how best to diagnose this would be highly appreciated. FWIW, it has 195K miles on it and the previous (first) owner maintained meticulous records and maintenance. Thanks for your help all!

Could both drive flanges or stub shafts be stripped?
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In AWD mode, it is really one wheel drive, if any one wheel can spin, the others can be stopped. On the early rigs, without the viscus cup, what you explained is possible. On the viscus cup rigs, it will attempt to turn the other axle.
100% normal operation.
If it was in low or the center diff lock button was pressed the truck would have jumped forward.

So you're saying you could dyno an AWD 80 series on a single roller dyno, without having to remove the front driveshaft or otherwise disable the AWD?:confused:

That doesn't sound quite right.
Put only the front two wheels on rollers and see what happens. Probably will conclude to the same result.
So you're saying you could dyno an AWD 80 series on a single roller dyno, without having to remove the front driveshaft or otherwise disable the AWD?:confused:

Absolutely, as long as it's a 91-92 with no VC.

Still, a bonehead move by the tech and not good for the vehicle.

NEVER leave your rig alone in a situation like that until you're sure they know what they're dealing with.

One of the first things I taught my wife after we got our rig: If you ever need a tow, just keep repeating over and over, "It's AWD; I need a flatbed or dollies for the rear wheels. It's AWD; etc..."
Thanks all for the quick replies. I feel much better about things now. The damn smog machine is supposed to tell the tech it's an all wheel drive unit and disallow them from doing a rolling test. I think this just means I have to take it off-road and get some mud or loose dirt/rocks under the tires and try it out. I'll be installing a couple of oxygen sensors and bring it to a different smog place - will try it out with the center diff locked and see what happens. Thanks again, this forum has been a terrific resource and I appreciate all of your advice!
My lack of knowledge in this area is clearly showing - I had assumed it would simply drive off the dyno without causing any problems. I won't try that out now. Thanks!
Let me clarify - when he showed me it didn't turn the front wheels, he didn't chock them or tie the vehicle down at all. He simply put the rear tires on the rollers and slowly accelerated up to about 10 mph and showed me the front wheels didn't turn. I'm assuming if I were to do the same thing but enable the center diff lock it would be no different than if I backed the rear tires on a frozen lake or patch of ice and enabled the center diff lock - should just drive forward right? That's what I was intending to try out when I take it back. Shouldn't that work and be o-k? Thanks!
Driving in normal AWD mode, all 3 diffs are open. The power is sent to the wheel with the least traction, always. If he would have put the front wheels on the rollers and tried it, it would sit there and spin the fronts while the rears stay still. Later LCs have the viscous center diff that will send ~30% of the power to the end with the most traction. Locking the center diff will always send 50% of the power to the front, 50% to the back, no matter what. If you were to lock the center diff and put it on the rollers, front or rear, the thing will just drive right off.
It can not be good for the transfer case to run with the front wheels stopped for any significant amount of time.
There is no bearing or bushing on the pinion gears and the pinion shaft in the center differential, just metal on metal.
They are not designed to spin at high speed for long periods of time.
I have seen the pinion gear and shaft gall and sieze in rear differentials where the driver was spinning a tire for a long period of time in a futile effort to get out of a stick.
So you're saying you could dyno an AWD 80 series on a single roller dyno, without having to remove the front driveshaft or otherwise disable the AWD?:confused:

That doesn't sound quite right.

Might not sound correct, but it's the way it works. Open diffs are open diffs, regardless of where you put them.

When you add a viscous coupler or any other traction aid it changes the equation. A stock US spec 91 FJ80 has no such items.
Again, just wanted to thank you all for your input on this. I'm relieved and appreciate you all taking the time to educate me. Thanks again!

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