Power Steering Rack questions

Whizbang · 06-22-2012, 06:25 PM

so the rx7 pump has two pressure lines. and one return. The steering rack then has three ports on it. Two are pressure, one is out to the cooler then back to the pump.

With the swap i am doing, my pump has one pressure line out, like a normal car.

1.) Why are there two pressure lines in the first place.
2.) any thoughts on how to plumb this arrangement?.

RETed · 06-22-2012, 11:27 PM

It sounds like you've got mix-n-match parts between a speed-dependent and RPM-dependent power steering systems...

The RPM-dependent system is more common and cheaper of the two.
The higher the engine revs, the more fluid and pressure goes to the rack.
Pretty straight-forward and simple system to design and build, since the PS pump is directly connected to the engine (via pulleys).

The speed-dependent system requires a computer that bleeds off pressure as vehicle speed increases.
Additional parts include a VSS (vehicle speed sensor - duh), a computer calibrated to handle the bleed rate, and the extra hydraulic fittings and ports for the bleed hose channel.
This is why you see the extra, 3rd port.

-Ted

Pete_89T2 · 06-23-2012, 07:18 AM

Sounds like you have the vehicle speed sensitive rack (3 fittings), and need to use it with an engine speed sensitive PS pump. Ted's description is correct; the VSS type PS pump has an electronically controlled valve on it that takes that 2nd pressure line from the rack and bleeds off a % of the line pressure back to the PS fluid tank depending on vehicle speed.

What you'll want to do is figure out which of the 2 pressure fittings on the PS rack is for the pressure bleed/return, and just cap that fitting. There's a diagram in the FSM that illustrates the flow diagram to figure this out, but I don't have it in front of me now.

Only potential downside I see to this is you might be getting more PS assist out of this setup than you want. The VSS type rack is probably calibrated to give enough PS assist at idle/parking lot speeds. Without that VSS feedback loop controling the assist as the line pressure rises with RPMs, as your engine RPMs go up, so will your PS assist.

06-22-2012, 06:25 PM	#1
Whizbang Respecognize! Join Date: Jul 2008 Location: Δx = ħ/2Δp Posts: 3,190 Rep Power: 20	Power Steering Rack questions so the rx7 pump has two pressure lines. and one return. The steering rack then has three ports on it. Two are pressure, one is out to the cooler then back to the pump. With the swap i am doing, my pump has one pressure line out, like a normal car. 1.) Why are there two pressure lines in the first place. 2.) any thoughts on how to plumb this arrangement?. __________________ For current updates and event coverage check out Follow on Twitter! @WhizbangRally Whizbang Rally's Webpage \| Facebook

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06-23-2012, 07:18 AM	#3
Pete_89T2 Lifetime Rotorhead Join Date: Apr 2010 Location: Elkton, MD Posts: 874 Rep Power: 15	Sounds like you have the vehicle speed sensitive rack (3 fittings), and need to use it with an engine speed sensitive PS pump. Ted's description is correct; the VSS type PS pump has an electronically controlled valve on it that takes that 2nd pressure line from the rack and bleeds off a % of the line pressure back to the PS fluid tank depending on vehicle speed. What you'll want to do is figure out which of the 2 pressure fittings on the PS rack is for the pressure bleed/return, and just cap that fitting. There's a diagram in the FSM that illustrates the flow diagram to figure this out, but I don't have it in front of me now. Only potential downside I see to this is you might be getting more PS assist out of this setup than you want. The VSS type rack is probably calibrated to give enough PS assist at idle/parking lot speeds. Without that VSS feedback loop controling the assist as the line pressure rises with RPMs, as your engine RPMs go up, so will your PS assist.