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Replacing Hydraulic Fluid Filter (Power Steering)


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2 hours ago, jacwjames said:

X2,

I have the larger reservoir that supplies oil for my side radiator fan motor and steering.

Hydraulics are pretty complicated, lots to take into consideration.  Just throwing this out there but I don't believe my fan motor & power steering circuit have any type of cooler.  So I think the cooler might serve two purposes, cooling and as an expansion tank.  The tank has a dipstick and sight glass which allows for some void at the top of the tank.  It takes ~4 gallons of oil.  This amount of oil does expand quite a bit when it gets hot so the tank allows for this.  My guess is if the tank was replaced with a spin on filter the system could pressurize and seals in the motors could fail.  Not a chance worth taking.  If you don't have a side radiator and only the power steering circuit it might be OK but even still there is usually a small reservoir on most systems. 

I know my manual says to drain only enough oil to expose the last filter by loosening a fitting at the bottom.  But I decided to just go ahead and drain all the oil to get a good look at the tank.  IMHO I believe this is better in that the way the system is designed.  The oil is drawn through the filters via the suction line.  The oil is returned to the reservoir by two return lines located outside of the filter base.  If you only drain enough oil to remove the last filter there is potential for contamination to be disturbed and migrate into the large suction line.  By completely draining the tank this reduces the chance of contamination. 

Thinking back over the years and I can't remember any type of hydraulic system/circuit that did not have some sort of reservoir.  Allowing for expansion due to heat is pretty much standard engineering best practice.  Fluid Dynamics in college has formula's to calculate expansion. 

You have the earlier jumbo size.  The later models, including some Dynasties had the smaller single filter like mine.  The trick is to check the fluid at ambient or “COLD”.  Then when you remove the cover, you measure down and write down the level.  Then, after you change it….fill to the mark,  prefill the external. Then run the engine to get air out.  Maybe 4 minutes.  Then you check.  Maybe add an ounce or two, but I doubt it.  In the later years, the larger one was used on the 4000 Allison …..in the high ends….mine took about 2 gallons maybe a quart less…

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