IRS Diff fluid question

[slpro1207]

I have just filled my IRS differential that came from FFR complete kit. The book calls for 3.5 oz of friction modifier and 3.3 pints of diff fluid. I put in the modifier but the unit would only that a little less than 2 pints before running out. The car is on jacks and level. Any thoughts?

[edwardb]

I have just filled my IRS differential that came from FFR complete kit. The book calls for 3.5 oz of friction modifier and 3.3 pints of diff fluid. I put in the modifier but the unit would only that a little less than 2 pints before running out. The car is on jacks and level. Any thoughts?

I assume you're talking about the newer 2015+ Mustang based center section. There's only the one fill port, on the back of the cover off center to the right about axle height. Mine took the recommended 3.2 pt (1.5L) lube plus 4.0 fl oz (118 ml) friction modifier before running out slightly. I can only guess yours must have had some fluid in it already.

[slpro1207]

It is the 2015+ Mustang center section. Should I wonder what fluid was in it. Should I drain it and start over so I know what is in it and the levels are correct?

[wareaglescott]

It is the 2015+ Mustang center section. Should I wonder what fluid was in it. Should I drain it and start over so I know what is in it and the levels are correct?

That option is cheap and easy at this point vs the potential worst case. Probably you would be fine but do you really want to chance it?

[edwardb]

I agree the safe bet is to drain it and start fresh. That fluid lasts a long time. Get it right the first time.

[slpro1207]

That option is cheap and easy at this point vs the potential worst case. Probably you would be fine but do you really want to chance it?

Good advise. Tomorrow I'm popping the drain plug and starting over.

[JRL16]

Mine came with some fluid in it so I drained it out. The recommended capacity in the manual seems to be a little off. When I put in what FFR recommended it wasn't full. My understanding is the fluid level should be at the bottom of the fill hole. Took a little more than FFR recommended.

[CraigS]

Mine came from the salvage yard full. But I am old, and it's heavy, so I suspected that somewhere along the line it would get tipped over and I'd have a big puddle under one of the axle holes. Once in the car I filled it. I used 80-90 Mobil1 gear oil like I always have in Ford clutch type lim slips. No additive. It works great and no noise. My theory is that what the additive does is make it slip and I don't want that.