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Electric power steering pumps and hydroboost, will it work?

5742 Views 56 Replies 8 Participants Last post by  SafariMike
Making this thread to either be a monument to my failure or a how-to on my success

Hydroboost is a system which uses hydraulic pressure from the power steering system, as the pressure to press the brake master cylinder, instead of vacuum or electrics

I'm doing this instead of electric power steering + electric brake booster because this will give significantly better breaking and steering feedback and feel, also this has much higher packaging efficiency, and comes with a lot less fabrication requirements for bolting stuff to the firewall. It's also way cheaper and uses stuff the car came with.

The pump is a volvo electric power steering pump, from an 04-13 volvo,
04-13 Volvo Electric Power Steering Pump S40 V50 OEM Bracket/Pigtails Included | eBay
it uses up to 80 amps at peak when the steering is at lock, has a beefy main power input plug and a little canbus input plug as well.

it'll be paired with a hydratech brake booster and a flaming river steering rack
Hydratech Braking Systems :: 1968-1982 C3 Corvette Hydraulic Brake Assist System Listings
1967-82 Corvette Power Rack & Pinion Cradle Kit - No Column
in a 1971 corvette

big unknowns as of now:
  • unclear if backpressure is a problem, most hydroboost systems have two separate return lines because the brake booster HATES backpressure. the volvo pump has only a single return inlet...
  • unclear if the "failsafe mode" of the pump, which runs it at 50% speed, is sufficient to drive the system, getting good steering assist and good brake pressure, but not overboosted steering
  • unclear if adding variable speed to the pump, using a control board like this: Universal PSC/NMS-PSC – NMStec , will cause the braking force to be significantly reduced at low pump RPM. the idea being that outside of a parking lot, you don't need that much steering assist so why not turn it down pretty low
  • unclear what is a good way to silence the pump, since its pretty loud and annoying at low speeds, i'll need to put it into some kind of sound-resistant box, and still keep it somewhat cool in there (possibly running the PS hose around the motor a few times, and putting the fluid through a little heat exchanger somewhere)

I'm about to purchase the pump, here goes nothing

some videos on the pump from gas car builds

here's how it sounds when running, very forklift esque
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My motorhome has a Hydro-Max system (apparently from Bosch), which is hydraulically boosted braking with a backup electric pump mounted on the booster assembly to provide boost if the engine-driven pump is not providing flow (as indicated by a flow switch)... which happens if the driver pushes on the brake pedal before starting the engine, and is really loud. Fortunately, the GM system to be used apparently doesn't have that feature.

I note in online remarks about the Hydro-Max system that in at least some cases the booster does not tolerate any back pressure, and apparently separate pumps are used for steering and braking... although I have not confirmed that on my motorhome. So it's just a heads-up: you might need two pumps, as already mentioned. But you might not, because apparently some systems (both no-backup Hydro-Boost and Hydro-Max) does tolerate routing the flow from one pump in series through two loads (steering and brakes).
Motor vehicle Rectangle Font Parallel Auto part

Power steering hydraulics are weird: the pump is controlled as a pressure source; instead, the pump speed determines volumetric flow rate, and the control valve (at the steering rack of box) controls a bypass to determine effective pressure. Having both steering and braking assists working in parallel should work (since both systems tolerate widely varying flow rate due to engine speed changes), but the interactions could be a challenge.

The backup pump on the Hydro-Max system might initially seem tempting to use full-time, instead of a separate pump. Unfortunately the backup only produces a fraction of the full assist force... and it is so loud that the first time I heard it I was seriously concerned about what might be broken. :(
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I seem to recall some trucks having a dedicated hydroboost electric pump....

Too busy to do an extensive search: BENDIX BOSCH HYDROMAX ELECTRIC PUMP MOTOR 2771544
That is labelled as "HYDROMAX BOOSTER EMERGENCY PUMP MOTOR ASSY." in the description, and is presumably the backup pump which mounted to the bottom of the Hydro-Max booster (#29 in the following illustration from a Bosch manual), rather than a regular service pump dedicated to the brakes.

Font Line Auto part Parallel Motor vehicle
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... On the other hand, the pump itself isn't the part that creates the pressure, only the flow.
The pump obviously is the part which creates the pressure - that's what pumps do. The pump does not control the pressure - it is controlled on a flow basis, and the pressure that it must produce to push that flow is determined by the valve in the powered device (steering rack or brake booster).

Higher-than-required pressure capability of the pump isn't a problem by itself. That high pressure will only occur if the steering rack or brake booster excessively restricts flow.
called up hydratech this morning and got a tech on the line, i took some notes:
  • ...
  • pressure doesn't control control quickness on steering, flow flow rate does
  • ...
It's not entirely clear to me what they mean by "quickness" here.
  • If they mean that maximum rate that the rack can be moved while maintaining assistance, then yes, that's limited by the flow rate (since obviously the assist cylinder displaces fluid).
  • If they mean level of assistance, then that assistance is the result of pressure output from the control valve in the rack, and that is determined by torque applied by the steering wheel and flow rate of the fluid, limited at the extreme by the pressure that the pump can deliver.
The odd way that these assist systems work, in which flow rate is determined by pump speed and pressure (and thus assistance force) is determined by control valves which bypass flow, will make the steering and brakes interact if connected to the same pump.
If they are plumbed in series (flow through one then the other) they'll get the same flow - like electrical devices wired in series getting the same current - then both will have suitable flow but they may be messed up by pressure to ambient (analogous to voltage to ground) being affected by the resistance of the other device. This is the "backpressure" which has been mentioned as a potential issue.
If they are plumbed in parallel (flow splits then re-combines at the return to the pump) they'll get the same inlet pressure - like electrical devices wired in parallel getting the same voltage - then the flow rate through each will be affected by the resistance of the other device.
From the way the guy said it, too high gpm results in the rack being over boosted and very little steering input resulting in a lot of steering "sneeze and you fly off the highway", etc. This is from experimenting with foxbody racks
So level of assistance (or boost gain), rather than rack motion speed. (y) That makes sense, hydraulically.
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