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I recently had the misfortune of having my 1997 4runner's rear diff lock up because of a leak and then no gear oil.

Unfortunately to add to that the frame and major members are very rusted/rotted. (I had a new rear end to put in, but would take A LOT of labor) to replace because nothing unbolts and bolts nicely.

So the question is - can I disable the rear end somehow, put it in 4wd and run it as a front wheel drive for a while?

If so, what needs to be done?

I assume I need to disengage the rear axles from the mangled mess in the rear diff and also disconnect the driveshaft from the mangled mess.

Unfortunately I think to do those two things requires as much work as swapping out the rear?

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Does the front differential lock when 4wd is engaged? If so, this won't work — engaging 4wd will prevent the front wheels from turning at different rates, which they must do if the car is to be driven on pavement. –  William Cline Mar 31 '11 at 0:43
    
Are you asking if it has lockers? I do not think it locks - the 4wd is meant to be used on pavement. –  Tim Mar 31 '11 at 3:40
    
+1 - Very interesting question! –  jmort253 Mar 31 '11 at 6:32
    
My question related to whether the car had a "part time" or "full time" 4wd system. Put more simply: does your owner's manual say it's okay to shift into 4wd mode while on dry pavement? –  William Cline Mar 31 '11 at 11:43
    
Yes, ok to shift while driving on pavement. –  Tim Mar 31 '11 at 15:06

5 Answers 5

up vote 1 down vote accepted

I have my 1990 Toyota V6 3.0 extra cab running on the front two wheels only, I had the same thing happen. I was told, a friend of mine, drove a similar truck all the way from the desert into San Diego.

I've been driving the truck all morning, the deal is, I have a "High" 4x4 and a "Low" 4x4, right now I have it engaged to be High 4x4, I am not using the vehicle under any heavy strain. It SEEMS like my RPMs are running much lower, under RWD, at 60 to 80 mph, my RPMs are about 4k. Using ONLY the front two wheels, I'm averaging at 2.5k RPMs.

I'm going to keep driving it like this, I'll post here, and let you know if it can take it. This particular Toyota is the most popular 4WD where I live, so I'll find a few owners and ask them about it too.

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Oh Yes, the reason I have to do this: I have a guy who will put a new rear differential in, but unfortunately I need to dredge 575 bucks for a rebuilt diff out of my own (Human) rear end. My front end is in top shape, so Im not reallty worried, I'm worried, about the back part, the round thing behind the differential, I'm no mechanic, don't know the words... I'll look it up in my program, the part name. I got it freshly lubed, and it rolls good coasting, but there is some rumbling. But my differential is froze solid. Thanx a lot you guys, good info in this post. –  Jonny Vee Oct 27 '11 at 23:22
    
thanks. glad to hear it is working for you! –  Tim Oct 28 '11 at 0:34
    
Tim, may I ask how you ultimately dealt with it? Im looking at the possibility of driving it like this for a short time. Will it wreck my front end? Or ruin tnf rest of my rear end? –  Jonny Vee Oct 28 '11 at 0:42
    
Unfortunately I ended up selling it to a mechanic for real cheap. It was in another state and I did not have the time or money to work on it or get it fixed up. From what I understand you can drive yours like that for a while but the front end may get a lot more wear than it was designed for. If I was able to drive mine like that I would have, but it was totally locked up. –  Tim Oct 28 '11 at 1:53
    
Thanx Tim - I am just learning about how my 4wd is set up now. I can shift into 4wd while moving, just have to have it in neutral and I slide the selector down to "4x HIgh" - Someone else who advised me thought that my 4wd was a low gear only. I think there are some setups that are more like all four wheels run off of the same power train. Mine is not like that, I have two separate power trains, one for back, one for front. There is a huge gearbox that connects the front power train to the tranny. With the driveshaft out, only the front train is working. –  Jonny Vee Oct 28 '11 at 2:34

It sounds like the vehicle is near the end of its life - I'd try removing the drive shaft from the transmission to the rear diff.

Note: I'm basing this advice on the Top Gear South America special, where they ended up turning a jeep into a front wheel drive vehicle. In theory, it should work, as long as there aren't a lot of fancy electronics & safety systems to get in the way.

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this is sort of the theory I was asking about –  Tim Mar 31 '11 at 3:40

I would really question this approach for these reasons:

  • The vehicle wasn't designed to run in 4WD full time. This may put extra strain on the frontend and wear out your front drivetrain and other components.
  • The vehicle wasn't designed for the front-wheel drive to handle all of the force of moving the vehicle. Again, this may put extra strain on the frontend and prematurely wear out the vehicle.
  • The transfer case generally spreads forces between front and rear drivetrains when in 4WD. It may not be designed for the front to handle 100% of the forces.

In short, if you're planning on scrapping the vehicle after this experiment, I'd be interested to hear how it goes. However, if you ever plan to repair the vehicle, you may discover that not only do you need a new rear end, but also you may discover you need a new frontend, as well as a transfer case and possibly other components.

Of course, just because the vehicle wasn't designed to be used in this manner doesn't mean it can't take the abuse.

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Yep - unless I can figure something out the vehicle will be scrapped. Your answer is basically the same as everyone else I have spoken to - which is: "That is interesting. Could work. Let me know how it goes because I am curious". –  Tim Mar 31 '11 at 15:06
    
@Tim - It could work, but in my experience "accidental features" rarely happen. In general, if a machine isn't designed to be used in a certain manner, using it in that manner will prematurely wear it out. –  jmort253 Apr 1 '11 at 5:43
    
I understand - the vehicle is already end of life - I am just trying to wring out a few more miles. –  Tim Apr 1 '11 at 13:41

I have seen an old series 3 Land-Rover used quite successfully in this fashion, however they are a lot more agricultural than a relatively modern Toyota and so are more capable of taking the abuse. A lot of it will be down to the design of the transfer box - in the case of the Land Rover it was a simple mechanical lock, which either drove just the rear axle, or both at the same speed. If the Toyota has a differential transfer box chances are it won't work at all, and if it does it may well ruin the transfer diff.

Disconnecting the rear propshaft should be fairly easy, as most 4x4s I've seen have traditional flanged UJs at either end of it. Removing the axle halfshafts may not be so easy though, depending on their design (in Land Rovers they slide out very easily, some other cars I have seen require a hydraulic press to separate them from the hubs).

You might find that you can remove the rear diff from the axle, which would make replacing it much easier, but from your description I am guessing that this isn't the case...

Like the other answers, I'm going to summarise with "yes, it is probably possible, but it is not recommended" - I suspect that the local authorities in your jurisdiction would probably take a dim view of it as well, especially given the state you suggest the rest of the vehicle is in!

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I have a 1991 Chevy Tracker on which I will be isolating the rear differential. The reason is that the pinion bearings have had extreme lube starvation do to a axle seal leak which lead to catastrophic failure. The vehicle value doesn't compare to the price of a new/used rear end, or the time it would take to pull each component and inspect for wear/damage. Time and money replacing damaged parts and ensuring proper backlash and end play spec is met. I'll just isolate the rear end.

Now a few things to think about. The difference in radius between the front drive shaft and rear drive shaft results in a very different torque load. Plus even if you isolate the rear axle you would still need to keep lubrication in it. (If it moves you need lube) So make sure you don't have any axle seals leaking.

The transfer case might be chain driven meaning that is your torque limit. The chain will break before your front drive shaft will more than likely. Or it has a planetary rear set for the ranges - Which you won't know till you're on the side of the road. Transfer case heat will be higher given the 100% torque flow to the front differential. For in town use I don't see problem with it. Hard acceleration will be a big no,no. Change the transfer case and front diff lube if you decide to proceed.

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