changing rear end gears

[danimal]

Thanks for the info. I bought a 3.73 locker carrier from a bone yard, doner vehicle 2006 w/60,000 miles for $200 and having installed for $300 and changing the axle seals. Found out it was a 9.75" ring gear. There is also a difference for the 2005 year whether it was manufactured before/after 11/29/2004. Mine was manufactured after that date. Dealer quoted $105 to flash computor, knew he could change tire size but wasn't sure about changing axle ratio. Another dealer said the speedo should be fine, no need to flash. Is any of this correct? Planning to upgrade the radiator and the trans cooler. Should I replace the existing cooler or just add another to the in/out transmission lines on the radiator. Would it be a problem to have 2 transmission coolers (i.e. restriction of fluid flow). Thanks again.

 

[Thermo]

danimal, there should be no need to reflash the computer as the speedo is sensed off of the wheel speed sensors, not off of the drive shaft speed. Take the truck out for a drive and all should be good. If it isn't, then go to the dealership and have them connect up their computer just to tell the truck that it has a different gear ratio in it.

If you are going to add a different cooler to your truck, I would pull the factory external cooler out and put in a bigger unit. Keep in mind that you already have 2 tranny coolers in the truck (assuming you have the tow package). You have the tranny cooler in the bottom of the radiator and then you have a cooler that sits out front of the radiator. IF you think you need more cooling, then I would remove the one that is outside of the radiator, find one with more surface area and then install that. Sure, you can add a 3rd cooler to the truck and it is not going to affect anything as long as you are getting one with decent size lines running through it.

 

[toms89]

Trac-loc: While this is a brand name, the concept needs to be explained. This is a differential that automatically goes between an open differential and a locked differential. It is essentially a limited slip differential on steroids. It allows for a small amount of slip (like a limited slip). But, when it senses too much slip, it engages and becomes a locked differential. This engagement can be rather harsh if done under load. This gives a bigger one time bump than the limited slip vibrations, but once locked, it is locked. This bump transfers a lot of force to the non-slipping wheel and can lead to it breaking loose during engagement.

The Trac-loc as how ford identifies their limited slip differential. It is essentially a limited slip design.

The ford trac-loc uses a series of clutches and steel plates preloaded via a heavy z spring. The clutches are installed on the side gears and are sandwiched between the carrier case and side gear and preloaded via the large z spring between both side gears. With this set up as you drive straight the wheels are going the same speed and there is no speed differential. But as soon as you begin to turn or one wheel slips the differential in speed forces the side gears to spin in relation to the carrier case. The clutches resist this spinning or slipping at all times. It really does not engage or disengage it simply resist the clutches slipping. They are designed to have a certain percentage of torque required to slip. Saying it acts as an open differential is misleading and incorrect.

They can be repacked to provide more torque required to slip which will make it "lock" better by manipulating the sequences of clutches/steels and overall thickness. I did this with my mustang.

The following shows the clutches/ side gears. Ford 8.8" rear end Traction-Lok clutch setups - YouTube

The downside to the trac-loc is the clutches do wear and eventually need to be replaced. As they wear the preload becomes less and less not to mention the loss of friction material reducing the torque to slip the clutches in effect reducing the traction "lock" capacity.

 

[toms89]

I will concede that there are different pcm's used through the years but with my 2000 changing gear ratios requires a change to the programming as it uses the output shaft to determine speed.

When my tuner originally programmed my set up he had input 3.73's and I quickly found out my speedo was off as I have 3.55's. There is an input for gear ratio in the software.

 

[danimal]

Thank you Thermo. I do not have the towing package but there is a small factory trans cooler in front of the radiator, I believe 7 row plate and fin. I think that may be a little small for pulling a travel trailer mostly due to the aerodynamics of the trailer frontal area. I am looking at trailers in the 3500-4500 lbs range which I could tow with the factory package. Of course, once everything is loaded it will probably be 1000 lbs heavier. I might also change out the radiator for the 1-3/8" core HD.

 

[Thermo]

danimal, having a bigger cooler doesn't hurt anything other than initially making your wallet lighter. But then, the coolers normally are not that expensive. So, it is cheap insurance. Go for it and don't worry about pulling the trailer.

 

[danimal]

Thanks again. Got the new-to-me 3.73 LS install, works great. Just wanted to let everyone know that the speedometer is off by about 10 MPH, reading too fast. Luckily, I was able to pick up a Hypertech programmer III from Ebay for $75 which is for an 05-06 Expedition and will allow me to reprogram the computer for the new gears. Supposedly, I can also reprogram many other things as well which may be fun to tinker with.

 

[Thermo]

toms89, when I was referring to Trac-Lok, I was referring to the system that GM uses in their trucks. In short, it is a gear setup that spools when the tires are spinning at different speeds. The spooling then engages a locking device which then locks the two axle shafts together, making both tires turn at the same speed. As long as load is maintained on the rear end, the differential remains locked. You left off of the gas to allow the axle to free spin and then spooling undoes itself and returns the axle to an open differential. What you described is what I have always heard of a being a true "limited slip" differential. Ford has just adapted the "Trak-Loc" name to their set up.