Friction Modifier used with Limited Slip Differential: What does it do?

Does it increase the friction, or decrease the friction in the differential clutches? I think if I had a little more friction in the clutches, that might help me solve a problem with my 40 year old Ford 4x4 truck equipped w/a Dana limited slip differential.

The problem is a “clunk” coming from the rear axel, almost certainly the diff slipping, when engaging “drive” or “reversed” from neutral. This truck has always done this and I just ignore as it doesn’t seem to cause any other problems. Other than being annoying. About 15 years ago I took a stab at solving it by replacing the diff clutches, but that provided only a minor improvement. I did add in the Ford recommended friction modifier. But I’m wondering if I could experiment by either putting in more, or less, or used something entirely different for the axel fluid and/or friction modifier, something recently invented. What do you think? And what does that friction modifier do, increase the friction, or decrease the friction?

The friction modifier does the same thing as it does with friction modifiers in transmission fluids. Allows the clutches to engage properly.

That’s why you can’t use Dexron tranny fluid in a transmission that calls for ATF+4. Wrong friction modifiers.


If I knew it increased the clutch friction, I’d put in some more and see if that helped. Likewise if I knew it decreased the friction, I’d mix up a new batch with less friction modifier and try that.

The clunking sound could be a variety of things from the U joints to taking up the slack in the gears. The truck is 40 years old. And, it still runs is pretty great. Don’t expect it to run perfectly as it may be just the nature of the beast. Lots of older rwd vehicles clunk when the gears are engaged. I would use nothing that wasn’t recommended for it.

There’s no play at all in the driveline from the xfer case to the diff input. I don’t think this is a u-joint problem. And the sound is clearly coming from the rear axel. If this had come on gradually I’d probably not attempt to fix it, like you say it is an old truck. But this truck did this since I bought it, and it was only 1 year old when I bought it.

Rear end clunk is usually the slack between the ring gear and pinion.
Unless you have old ujoints that can also mimic the clunk.
The age of the truck says a lot and normal wear leaves a little slack in the gears.
This slack is what is adjusted with the shims when they assemble the rear differential parts. some have side bearing shims and some have pinion shims but either is used to set the pattern so the gears mesh at a maximum.

There’s a really engine knowledgeable fellow where I used to work that race cars in his spare time. You know, he liked to participate in local stock car races. He’d rebuild an engine on Friday night and Saturday to get the car ready to race on Sunday kind of thing. When I installed new clutches, while the diff was out I brought it to work and had him feel the amount of play in the ring gear/pinion interface. He said it was already correct and didn’t need to be re-shimmed. I don’t think that is the problem.

Since this was a favor to me, I was then forced to hear what is probably a standard mechanic’s joke about the dimension of my ring gear … lol …

I don’t think a clunk at standstill would be caused by the clutches or the fluid. There’s no differential movement going on. More likely accumulated wear in the various parts, as others have said.

It could be from play at where the rear axels spline to the diff I suppose. I don’t think they spline at the wheel hub end; the axel as I recall just ends in a round thing that the wheel somehow either directly or indirectly bolts to.

The goal of the gear oil additive in limited slip differentials was to allow smooth cornering when the vehicle was being accelerated…Without it, as medium to heavy power was applied with the right foot, the differential would “lock up” as designed but this could be annoying and dangerous in curves and corners…The “friction modifier” allowed a certain amount of slippage but it degraded the Posi-Traction effect somewhat…In cars used exclusively for drag racing, ATF type “F” was the usual lubricant used. Not suitable for everyday street use…

Aha, good info @Caddyman . It sounds like it makes it more slippery, reduces the friction in other words. You are right there is a sort of annoying sound the diff makes when going around corners. I notice it more at low speeds, the sound of the clutches slipping. When I haven’t driven the truck in a couple weeks it is very apparent, but diminishes if I drive the truck every day like I am now.

From the comments here though it doesn’t sound like experimenting with the concentration of friction modifier isn’t the answer to my clunking problem. I’m starting to suspect maybe the fellow who judged the amount of play in the out-of-case differential (technically I think that part is called the “third member” for some reason), maybe he was just incorrect, and there is too much play in the pinion/ring gear interface. I think my poor truck had a difficult life in its first year of life, before I bought it, and its quite possible the differential was damaged by an overaggressive off-road technique and needs to be re-adjusted. Back to the drawing board. Thanks everybody for some good ideas.

I stuff I had for the Olds posi was called whale oil at the dealer. It was pretty thick stuff and you had to squeeze the bottle to get it in. I’m pretty sure it would have decreased friction to get rid of the chatter on tight turns.

With the truck off (and chocked), transmission in neutral, grab the driveshaft and twist back and forth as hard as you can. Can you make it clunk? How much rotation can you get?

Fords of that era often make a clunk when shifting into drive from neutral. There is a good amount of slop in that drive train that just gets more so with age. U joints are often the source, but it could be internal in either the transmission, or the differential. If your limited slip rear end isn’t chattering when you make turns, then I doubt the clunk has anything to do with the limited slip clutches - therefore will not change with more or less friction modifiers.

When was the last time the diff fluid was changed? Perhaps fresh fluid with the proper dash of friction modifiers is due anyway? What is the rpm of the motor at idle? Perhaps it is a bit high? Most old Fords would make the clunk more pronounced due to the high rpm of a cold motor before the choke opened up. If the clunk is worse at a fast idle, that leads me to suspect the u-joints more.

just out of curiosity what truck do you have exactly? my truck(which I may retire soon) is a '75 ranger supercab, split shaft, carrier bearing, plenty of u-joint failure and carrier bearing has gone bad once. I have learned thru experience to change all my u-joints at once. if I only change one I ll be back under it in a month to change another. previously I had a '75 explorer custom supercab, much nicer and the previous owner had put in a 390 galaxy motor. I loved that truck but the rust defeated me

In order to really identify where the sound is coming from, it’s important to get under the vehicle and replicate it while feeling around and/or using a stethoscope. Based on your description, the first things I would look at are the mounts for the leaf springs, their u-bolts, and the pinion depth/gear lash there.

There’s also another thing that can happen. The mounting shafts for the gears in the transmission, if it’s an automatic, can develop too much end play and, because the gears they use result in side thrust, the whole shaft can clunk forward and back when you shift from drive to reverse and vice versa.