friction modifier -- can there be too much?

[CCI]

1996 Dodge B3500 9-1/4" Axle 76,000 miles -- no idea if friction modifier was added at what appears to be a recent service, but oil is clean.

At 75 to 80 degrees ambient and 75 mph for a couple of hours the center section of the differential is uncomfortably warm to the touch and it seems a little loud. Is this normal?

Dodge manual calls for the addition of friction modifier at each service.

Rather than drain clean oil and refill, would it hurt anything to just add 4 ounces of friction modifier?

 

[SwampSurvivor]

Differentials make a lot of heat just by the nature of how they operate.

I bought my Jeep with the smaller 8.25 Chrysler with 95000 miles. It always howled. When I took the differential apart at 215,000 miles it looked brand new. Lash and pinion preload were still in spec. No weird wear patterns or anything.

Isn't the friction modifier just for clutch packs?

 

[D60]

Unless someone got crazy with FM it's not gonna cause overheating....even if you ran straight FM I'm not sure what would happen.

I replaced a 9.25 in a 2nd Gen that growled pretty good, but the diff appeared to have water in it. Make sure the breather isn't clogged. This one was and I wonder if condensation could just not escape.

IOW I wonder if it was recently serviced because of something like this.

 

[Asterix]

If you have a clutch LSD, lots of FM will drastically reduce the LSD action, if you care about that.

Adding lots of FM won't hurt anything, so it's worth a try. If it does reduce your clutch LSD action unacceptably, the fix is changing the oil and using less FM, which isn't hard.

If it's not a clutch LSD, it seems to me that lots of FM may reduce the temperature, but I've never tried. Uncomfortably warm to the touch could be only 120F, which is certainly not too hot. Point a pyrometer at it to get a better idea of temperature. I wouldn't worry unless it's running over 220F.