trac-lok

Can anyone tell me how to figure out whether my 1997 XJ with the 8.25 has trac-lok? without pulling the cover that is. I have heard that if you jack up the rear and spin the rear tires depending on how they spin in relationship to each other that will tell me. Is that true?

Nope, that is basically false. The Trac-Lok has no preload on the clutches at all, it only locks up when torque is applied to the pinion, such as when you accelerate.
On many axle tags (not sure of yours), there will be an "L" after the numbers indicating the axle gear ratio. The "L" stands for Limited Slip. So, if you have 3.55:1 axle gears, the tag would have the following:

3 55 L

Good luck!

DeadEye is wrong. The Trac-Lok does have a preload.

If you jack up the rear axle and spin one wheel, if the opposite wheel spins in the same direction you have a functional trac-lok. If the opposite wheel spins in the reverse direction, either you don't have a trac-lok or it's worn out.

DeadEye is right.

I own and have recently rebuilt a trac-lok. It is in the rear of a 1987 Bronco II with a Ford Corp 7.5" rear end. The only thing that loads the clutches is the spider gears trying to force the side gears away from eachother. There are no springs or any other tensioning device in the carrier. The only preload that COULD exist in this system is just shimming the clearances too tight with extra shims.

The clutches are located between the carrier and the side gears, they "lock" when the side gears are pressed outward by the spider gears by the crosspin. With no load, such as when the vehicle is jacked up and wheels off the ground, they function essentially as an open diff. Spin one wheel forward and the other spins backward with vehicle in gear.

Some other posi units, such as Eaton, have springs which apply constant pressure to the plates so that the clutches are engaged at all times. This is not the case with the factory Trac-Lok.

Jared

Sorry --

I currently own four Cherokees with trac-lok. If you jack up the rear axle and spin a wheel, the opposite wheel turns in the same direction.

The prescribed field check, direct from the 1994 XJ/YJ FSM:

The differential can be tested without removing the differential case by measuring rotating torque. Make sure brakes are not dragging during this measurement.
(1) Engine off, transmission in neutral, and parking brake off.
(2) Place blocks in front and rear of both front wheels.
(3) Jack up one rear wheel until it is completely off the ground.
(4) Remove wheel and bolt special tool (6790) to studs.
(5) Use torque wrench on special tool 6790 to rotate wheel and read torque.
(6) If rotating torque is less than 22 N-m (30 fit-lbs) or more than 271 N-m (200 ft-lbs) on either wheel the unit should be serviced.

Click to expand...

The Trac-Lok doesn't use springs for the pre-load. It uses what the FSM refers to as a "forcing screw" to compress the clutch packs. If the unit didn't have a pre-load, the FSM method os testing wouldn't work.

Then perhaps the Jeeps differ from the Fords. On mine there is nothing that could be described as a "forcing screw". The unit works great with both wheels on the ground but quickly disables if you lose traction with one wheel. The only way to engage it at that point is to apply some e-brake to load up the clutches.

Have you ever seen this "forcing screw" while working in the diff, Eagle? I have not, but if I am overlooking something in my own rear diff, it would be nice to know. Having the clutches preloaded would be awesome.

It just made sense that factory trac-loks would be similar in function and design. Aren't they all made by Dana?

You can add shims to make a trac-lok so tight it won't ever disengage. What I think you are describing is the effect of torque transfer.

The clutches attempt to route power equally to both wheels. However, if you have one wheel on a surface with good traction and the other wheel in the air, if you give it enough power the clucthes will be overcome and the airborne wheel will spin. Applying some brake slows it down enough to let the clutches hook up again.

"Trac-Lok" is a Dana trade name -- sort of, I think. Reider Racing sells Dana-Spicer Trac-Loks, but they also sell something called a PG Trac-Lok, which I believe is a Precision Gear knock-off of the Dana unit, for diffs Dana doesn't service. I've never seen a PG Trac-LOk and don't know if it operates the same as a Dana.

Note that the FSM allows for a wide range of acceptable torque bias. 30 ft-lbs to break loose a LSD diff is rather anemic, IMHO. In the old days of autocrossing AMC stuff we considered a LSD to be worn out if it torqued less than 50 ft-lbs.

I rebuilt my Trac-Lok a couple months back with clutches from the dealer. I purchased a shim pack also and shimmed the thing so darn tight I couldn't put the thing together without a dead blow hammer. The thing still is basically open without some torque on the driveline. Perhaps the 33's just overwhelm the puny little axle, I dunno.

Being a stock part out of an '87, I would bet that it is a Dana unit. It seems that it should at least operate similarly to one in a Jeep. Do you know where this "forcing screw" is? Perhaps mine is adjusted only with shims.

It seems like shimming it as tightly as I have, it would be constantly engaged. I would prefer for this to be the case as it is no longer street legal. I was planning to weld the diff but realized that the Trac-Lok would make welding the side gears a big hassle, so I did the rebuild. Any tips on tightening this thing up?