How to: Replace HBMC Strut and Charge the System

[SouthMNinfiniti]

I had the front passenger Hydraulic Body Motion Control (HBMC) strut break part of the spring off. The springs are not a separate servicable item. Being that i now have 187k in the truck and one new strut is $1200 ($800 from discount sites), i found a used one on eBay with about 120k on it for $169.

The interesting part is how to deal with the HBMC system which "requires" a super expensive dealer hydraulic pump (like $8k?), Nissan hydraulic fluid (appears to be discontinued?), and a whole bleeding and pressurization process.

Cost: $900 for pump, hose, connectors, gauge, and especially the Pentosin CHF 11S fluid. $190 for used strut +tax

Warning: Do this at your own risk. I am not a certified Nissan tech nor even ASCE certified. You may cause damage to yourself, your vehicle, and others, up to and including death.

With that said...
After reading through the Suspension Control System (SCS) service manual several times, searching the web for any videos or info (i did find a few videos from overseas using the dealer pump and fluid), i ended up building the pump and hose assembly myself and deciding on Pentosin CHF 11S for the fluid as it has similar viscosity and heat props and is used for other manufacturer hydraulic suspensions. I purchased three 1.3 gal jugs to be safe. (i only needed about two of them for this job)

The strut replacement:
- Disconnect battery negative
- Break loose all 4 wheels' lug nuts
- Put all 4 corners on jackstands
- Remove all 4 wheels

- Wear safety goggles and nitrile gloves

- Mark current HBMC left rear Passive Pressure Maint Unit (PPMU) needle valves' A, C, E rotation position with a paint marker or similar
- Depressurize the HBMC via its PPMU using a custom hose assembly (info below) routed to an empty bottle/container. See SCS manual for which needle valves to open (A, C, then E) and to loosen and only turn them no more than 2 full rotations.
-Close needle valve E when no pressure remains (takes like a second or two).

Marked PPMU:
https://drive.google.com/file/d/1gL6-js ... xgtbt/view

Depressurizing:
https://drive.google.com/file/d/1T3m2t1 ... 8wRu_/view


- Attach small hose to top HBMC bleed screw of the broken strut and other hose end in a container and loosen the bleed screw to drain fluid then close bleeder. A clear 3/16" hose about 3.5 ft works well.
- Drain the fluid from the bottom bleed screw on the strut in same manner.

Draining strut:
https://drive.google.com/file/d/18qY-1b ... tFIG2/view

-Disconnect the top and bottom HBMC hydraulic lines from the strut (these will still leak a bit of fluid but not much). Be careful to not scratch, smash, break, get dirty the o-rings (unless you have new ones)
- Disconnect front right steering tie rod end
- Unbolt the strut bottom from lower control arm.
- Take top strut 3 nuts off while holding the strut so it doesn't fall on you
- Pull the strut down thru upper mount and dump out remaining fluid in a container

Disconnected tie rod:
https://drive.google.com/file/d/1uTMdAT ... xALV8/view

Pulled strut:
https://drive.google.com/file/d/1wyu0jt ... luzPV/view

- Reverse those steps to mount the replacement strut (be sure the bottom spring tang faces out) but leave the hydraulic connectors off until after torquing top strut nuts
- Torque steering tie rod nut per ST service manuel
- Torque strut top nuts and then lower bolt per FSU service manual
- Carefully connect and torque the hydraulic lines per SCS service manual page 26.

Replacement strut:
https://drive.google.com/file/d/1SIOtEy ... 5ktP5/view

Lines reattached:
https://drive.google.com/file/d/1zi2KTl ... dYyLh/view

Pump w/hose attached:
https://drive.google.com/file/d/1Z-z_xy ... cxeKi/view

Quick connects and gauge (this gauge broke after 2 pumps. I replaced it with the Apache one listed in parts section):
https://drive.google.com/file/d/1ldOTg3 ... LkQKo/view

Pump and hose assembly attached to PPMU:
https://drive.google.com/file/d/14f7ofq ... FAm9d/view

Pump position lever at middle/90 deg:
https://drive.google.com/file/d/1NPHKvt ... EoJMb/view

- Connect the custom hose assembly to the hydraulic pump
- Fill pump with the new Pentosin fluid (pump takes a full gallon at a time. Do not let it run out or you will add more air to the system)
- Disconnect hose from PPMU and prime the hose into a container using the hydraulic pump
- Connect hose back to PPMU
- Open needle valve E (only 2 rotations)
- Pump new fluid into PPMU up to about 150 psi (very roughly half of 2.3 MPa). Use SCS service manual chart on page 19 to determine the right system pressure for your climate, then use roughly half of that for now.
- Quickly Move hydraulic pump selector output handle to middle/90 deg (straight out position) to close it
- Using the clear bleeder hose and container, bleed all air out of strut bottom hydraulic connector. This may take a bit since the replacement strut was drained/dry.
- Close bleeder screw
- Go back to pump, move selector to the right output position and pump the pressure back up to half of desired system pressure.
- Quickly move pump selector back to middle/90 deg position
- Move hose to top strut hydraulic bleeder screw. Bleed all air out and close bleeder screw.
- Top off the new fluid in the pump.

- Now, follow the SCS service manual system bleed process, pages 18-19. Using the custom hydraulic pump, selector lever, and gauge alike the dealer ones.
- Note that if you're a one person show like myself, follow this slight change to the manual steps:
-- Pump to the desired full system pressure, then only bleed the top of RH rear.
-- Pump fluid again to desired full system pressure, bleed bottom of RH rear.
Then do that for each corner, pump, bleed top, pump bleed bottom, next corner the same, and so on.
- Do not let the pump run out of new fluid. Top off after 2 corners are bled.

- Bleed all four corners in a 2nd round, again following the service manual exact order.
- When done bleeding, pump the system pressure back up to desired from the chart.
- Ensure pump handle is all the way down with piston fully compressed.
- Move pump selector handle to middle/90 deg position.
- Close PPMU needle valves A and C, then E
- Disconnect hose from PPMU and cap off its quivk connector or keep upright so the prime is maintained. You're not done with it yet.

- Reattach PPMU cover
- Put the wheels back on and torque to spec
- Jack up and remove jack stands.
- Connect the battery negative

- Take for a careful test drive and ensure the ride and handling/cornering feel ok. If you hear new clunking sounds when hitting bumps, potholes, train tracks, or the like, it's most likely the spring being loose on your replacement strut if the hydraulic shock still has air in it or not enough oil or pressure.
-if it drives safe, drive it like normal for a day.

- After a day, put back on jack stands, disconnect the battery negative, and do the same all four corners bleeding and pressurization process. Again, keep the exact order and don't let the pump run out of new fluid.
- Put everything back on, torque your wheels, take off jack stands, connect the battery negative.

- You should be done

Parts from amazon (higher pressure than needed for this job but just in case i want to later use it for something else):

- ‎Bailey Hydraulics, Chief Double Acting Hydraulic Pump - Hand Operated with 1.3 Gallon Reservoir, 3625 PSI, 25 Gpm, 3/8" NPT Ports, 220996

- Apache 98398226 3/8" x 24" 2-Wire Hydraulic Hose Male x Male Assembly (can use cheaper 1/4" with 3:8 npt adapter too)

- Apache 3,000 PSI Glycerine Filled Pressure Gauge, 2-1/2 IN, 99019109. I purchased this from Bomgaars in store. Likely other fleet/farm type places would have it.

- Metalwork 316 Stainless Steel Pipe Fitting Forged Tee 3/8" NPT Female x 3/8" NPT Female x 3/8" NPT Female 2000psi

- (2) TAISHER 304 Stainless Steel Reducer Hex Bushing, 3/8" Male NPT to 1/4" Female NPT

- (1 pair) Zinger 3/8" Ag ISO 5675 Hydraulic Quick Connect Couplers with Dust Caps,Pioneer Style Hydraulic Coupler,3/8” NPT Thread and 3/8” ISO Body

- Brass 1/4" NPT to AC hose screw-on flare adapter.

- An old school brass Interdynamics R134 car AC quick connect low pressure hose assembly with a low-to-high pressure adapter. (it's good enough for this approx 333 psi job. hose is rated 2500 psi).
-- I'm trying to find a source for this but you can find similar stuff on amazon. I had this in my tool box for many years.
-- The PPMU quick connect mates with no leaks to my R134 AC high pressure connector.

[rolocr]

hello. thanks for the write up.The links are down and i want to build a similar set up, do you still have the pictures? thanks

[Rogue One]

hello. thanks for the write up.The links are down and i want to build a similar set up, do you still have the pictures? thanks

The OP hasn't been an active member for over two years now, so he may not respond.