2.2 Brake Booster - Bosch iBooster Gen 2 + Wilwood MC¶

Overview¶

Electromechanical brake booster required for the Cummins R2.8 diesel (minimal manifold vacuum). The Honda Accord Hybrid donor delivers the same Bosch Gen 2 unit as the Tesla Model 3 with better DIY documentation and sourcing.

The Back Bay Customs adapter mates a Wilwood Tandem Compact master (1-1/8" bore; confirmed by the vendor against the 1.00" 260-15542) to the iBooster, solving the vertical-firewall reservoir-angle problem.

Vendor compatibility (Back Bay Customs / Adam, email 2026-05-30):

• The booster's 60×80mm firewall pattern¹² must be mounted 80mm vertical. Back Bay's 80mm-horizontal adapter is out of stock — design around 80mm-vertical.

Specifications¶

iBooster¶

• Current: 40A peak (braking), 0.25A idle, 12mA standby
• Mounting Bolt Torque: 13 Nm (115 in-lb) - 2× nyloc nuts, 13mm (M8)³

Master Cylinder¶

Wilwood Tandem Compact, 1-1/8" bore — re-sized 2026-09-16 for the Super Duty calipers on the 1-ton axles, for knockback margin rather than steady-state caliper volume. Wilwood lists the 1-1/8" Tandem Compact as 260-15541; confirm the part number and re-confirm the Back Bay adapter (vendor-confirmed against the 1.00" 260-15542) before ordering. Sizing rationale is on the Brakes page.⁴ The original 1.00" selection is kept for the record.⁵

• Bore: 1-1/8" — Wilwood Tandem Compact (260-15541, confirm)⁴
• Stroke: 1.10" on the 260-15542; confirm for the 1-1/8" unit⁶
• Outlets: tandem (independent front/rear); thread 1/2-20 IF⁷
• Reservoir/inlet ports: remote feed; thread is an in-hand check, not a datasheet lookup⁷

Brake Hardware (downstream — MC bore design inputs)¶

• Front calipers: Ford Super Duty factory caliper on the Dana 60 front axle; piston area to be confirmed⁸
• Rear: disc — Ford Super Duty factory disc with drum-in-hat park brake, on the Sterling 10.5 rear axle; caliper piston area to be confirmed⁸
• Pedal: 03-06 TJ/LJ auto pedal (ratio bench-measured on assembly)
• These set the MC bore selection — see the Master Cylinder section and Brakes.

Plumbing¶

Reservoir → MC feed (low pressure): -3 AN PTFE braided — Wilwood 220-12993 (see Master Cylinder section). Gravity feed only; not a pressure circuit.

MC → calipers (high pressure) — hybrid hardline + axle flex. Baseline design; switchable to a more standard (or full-PTFE) approach later without changing the MC or its 1/2-20 IF outlets.

• Chassis runs: 3/16" copper-nickel (NiCopp / Cunifer) hardline off the MC's 1/2-20 IF outlets, 45° double (bubble) flare with inverted-flare fittings. DOT-approved for hydraulic brakes, will not corrode, and hand-bends/flares without a hydraulic bender.⁹ Copper-nickel is softer than steel, so sleeve in stainless spring "gravel guard" armor at exposed points and route inside the frame rail where possible.¹⁰
• Axle ends: DOT-compliant (FMVSS 106) braided-stainless flex hose at each axle, extended length sized to full suspension droop. Lengths are blocked on axle + suspension selection (Section 10 TBDs) — do not cut to length until ride height/articulation is fixed.¹¹
• Hardline → flex transition: bulkhead fittings at each axle.
• Proportioning: the iBooster has no integral proportioning valve. Front disc + rear disc warrants an adjustable proportioning valve on the rear circuit (plus residual-pressure valves if needed); size on assembly. TBD #60

Alternative considered — full -3 AN PTFE braided throughout: field-repairable and common on non-street comp rigs, but not chosen for a street LJ — FMVSS 106 certification attaches to the finished assembly (DIY bulk-hose builds aren't road-certified) and the braid hides the liner from inspection.¹¹ The hybrid keeps inspectable hardline on the long runs and isolates flex to the axles.

Parts List¶

Wiring¶

No ABS or ESP module. There is no hydraulic modulator downstream and nothing on CAN; the three circuits above are the whole electrical interface. The booster is expected to run on pedal position alone. That standalone behaviour comes from builder reports rather than a Bosch document, so confirm it with the harness vendor before committing to a harness (TBD #105).

Relocated off the PMU: Main power now comes from the START+ Forward Distribution Bus (50A CB) instead of PMU OUT1+10, and the ignition enable from the Ignition Signal bus instead of OUT19 — removing the brake booster's dependency on the PMU module. The booster retains its mechanical push-through, so a power loss is a hard pedal, not zero brakes.

See START Battery Distribution for the forward-bus feed and breaker specs.

Firewall Backing Plate¶

3/16" A36 steel sandwich plate that sits on the cabin side of the firewall, opposite the iBooster's integral 4-stud flange. Spreads cantilevered load across a larger area of firewall sheet metal to prevent fatigue cracking at the new M8 holes.

Geometry:

• 152mm × 152mm (6"×6") square, 12mm corner radius
• 4× 9mm M8 clearance holes on a 60mm (horizontal) × 80mm (vertical) rectangular pattern (80mm dimension vertical, per vendor)
• 64mm center pass-through bore (clearance for iBooster body neck protrusion)
• 3/16" (4.76mm) thickness
• Material: A36 mild steel
• Finish: zinc yellow plating (corrosion resistance for the cabin-side mounting area; also lets the plate stay visible-looking-intentional behind the dash)

CAD file: lj-ibooster-backing-plate.dxf (Fusion 360 — needs update to 60×80mm pattern)

Fabrication workflow:

1. 3D-print a 1:1 PLA prototype first for trial fit against the LJ firewall + iBooster assembly. Use this to confirm hole spacing matches the donor unit's actual studs and that the 64mm bore clears the booster body neck.
2. Verify clearance for cabin-side hardware (pedal box, HVAC blower, dash brackets) before committing to steel.
3. If PLA mockup fits, upload the DXF to SendCutSend and order in 3/16" A36 steel with zinc yellow plating finish.

Brake Pedal Assembly¶

The factory LJ manual-trans pedal must be replaced with the 03-06 TJ/LJ automatic brake pedal to remove the clutch pedal and provide the correct pedal arm shape for an iBooster pushrod connection.

Signal/wiring impact: None new — the stop-lamp switch (Mopar 56045043AB) is identical on manual and auto brackets. The pedal swap inherits the existing brake-signal wiring already designed across three systems:

flowchart LR
    SUPPLY["Switched 12V<br/>(accessory feed)"]
    SWITCH["Stop-lamp switch<br/>Mopar 56045043AB<br/>(closes when pedal pressed)"]
    PMU["PMU In 2<br/>via Deutsch Pin 13"]
    LIGHTS["PMU OUT21 (7A)<br/>Brake lights"]
    PBSI["PBS-I Brake input<br/>(T-tapped with PMU In 2<br/>and Turbolamik TCU)"]
    TCU["Turbolamik TCU<br/>brake input"]

    SUPPLY --> SWITCH
    SWITCH -->|"Pole 1 tap"| PMU
    SWITCH -->|"Pole 1 tap"| PBSI
    SWITCH -->|"Pole 1 tap"| TCU
    PMU -->|"In 2 closes → OUT21 ON"| LIGHTS

    style SUPPLY fill:#ffd93d,color:#000
    style SWITCH fill:#d1d5db,color:#000
    style PMU fill:#a5d8ff,color:#000
    style LIGHTS fill:#d1d5db,color:#000
    style PBSI fill:#d1d5db,color:#000
    style TCU fill:#d1d5db,color:#000

• PMU is sensor + brain, not load path: PMU In 2 senses pedal-pressed; PMU OUT21 (7A) carries the actual brake-light current. Switch itself only handles logic-level (<1A).
• Single pole used: TJ/LJ 03-06 stop-lamp switches are 2-pole (4 pins). Pole 1 routes to PMU/PBS-I/TCU; Pole 2 was the factory cruise-deactivation pole — leave disconnected, not in this design.
• Splice location: Pole 1 output splices in the cabin into three branches — PMU In 2, PBS-I Brake input, and Turbolamik TCU brake input. Crank comes solely from the PBS-I PURPLE START output — brake switch Pole 1 does NOT connect to Firewall Pin 15.

See tail/brake (PMU lighting flow), starter (crank chain), and transmission (TCU brake input) for the downstream details.

Mechanical impact:

• Pushrod interface: Same on both pedal styles - iBooster clevis attaches at factory pushrod hole. Pivot-to-pushrod distance is identical; pedal arm shape differs (straight on auto vs. offset on manual) but does not affect the pedal ratio. Bench-measure before final assembly to confirm.
• BTSI provision: Auto pedal has a second boss for the factory floor-shifter BTSI solenoid - leave empty; Kilduff/Turbolamik handles unlock-from-Park electronically.
• Clutch removal: Clutch pedal, clutch master cylinder, and CMC firewall pass-through are no longer used. Plug the ~1.25" CMC hole in the firewall with a block-off plate or weld closed.
• Pedal pad: Auto pedal pad is wider - cosmetic only.

Donor checks:

• Stop-lamp switch + retaining clip present (or buy new Mopar 56045043AB)
• Pushrod hole bushing in good shape
• Pedal arm not bent/cracked at pivot

Installation Notes¶

• Firewall mounting strategy: Factory LJ vacuum booster holes are abandoned. The Honda Gen 2 iBooster bolts directly to the firewall via its own integral 4-stud flange (60×80mm M8 pattern, 80mm dimension vertical per Back Bay Customs; 62mm body neck through the firewall) — no separate steel bracket. Drill matching holes in the LJ firewall, then sandwich a SendCutSend 3/16" steel backing plate on the cabin side to distribute the cantilevered load and prevent fatigue cracking at the new holes. If an 80mm-horizontal orientation is ever required, Back Bay sells a re-orientation adapter (currently out of stock).
• Fallback paths: If integral flange has engine-bay packaging conflicts, fall back to (a) SendCutSend custom one-off firewall plate that re-patterns mounting holes, or (b) Back Bay Customs one-off commission — only worthwhile if they have access to an LJ for trial fit; without one, customer-measurement-blind design carries the same risk as SendCutSend at higher cost.
• Reservoir height: Mount reservoirs 4-6" above MC flange on firewall standoff for proper gravity feed. More than 6" risks hood clearance issues.
• Bench test before fab: Apply 12V ignition signal to the iBooster and verify motor cycles + pedal rod assists. Used iBoosters fail frequently.
• Cantilever support: Gen 2 is lighter than Gen 1 but on a vertical firewall through-hole the cantilevered load + Jeep vibration warrants a secondary front support point (inner fender or frame brace).
• Pushrod length (MC side): Back Bay's adapter pre-sets MC pushrod length - no adjustment required.
• Pushrod length (pedal side): Re-measure after auto-pedal swap; adjust per iBooster install kit instructions (Tulay's or EVcreate) before bleeding.

Installation Resources¶

• Wiring the iBooster - EVcreate
• Installing the iBooster - EVcreate
• iBooster Donor Vehicles - EVcreate
• Back Bay Customs Shop

Outstanding Items¶

Build Tasks¶

Waiting on external action:

MC bore — resolved:

• Bench-measure TJ auto-pedal ratio on assembly — geometry/travel check only: confirm the ratio leaves MC stroke headroom (no long/sinking pedal) and the iBooster input rod gets full travel + full return. Does not affect the bore; pedal feel is drive-and-decide, not bench-tuned.

Backing plate redesign (vendor revised the firewall pattern):

• 🔴 Redesign lj-ibooster-backing-plate.dxf from the 72×72mm square pattern to the confirmed 60×80mm rectangular pattern (holes at ±30mm H / ±40mm V, 80mm vertical). Center bore (Ø64), plate size (152×152mm), and corner radius unchanged. Blocks the SendCutSend steel order.
• Update firewall drilling jig/template to the 60×80mm pattern before mocking up

In progress:

• 3D-print PLA prototype of the revised backing plate (DXF) for trial fit; verify the 60×80mm hole spacing against donor studs and clearance for cabin-side hardware

Pending donor arrival (purchased 2026-05-30, listing #397546491129):

• Confirm casting part #s on the actual unit match the listing (46680-T3Z-A00 booster / 01469-TWA-A58 MC kit)
• Measure body-neck Ø against the 64mm backing-plate bore
• Bench test donor iBooster before any fab work (12V to ignition signal → motor cycles + pedal rod assists)
• Verify 4 mounting studs intact and body neck undamaged
• Photograph back face — finalize backing plate alignment + harness pigtail condition
• Select wiring harness: Tulay's Gen 2 universal (plug-and-play) vs EVcreate Gen 2 connector kit (DIY)

Pending mockup:

• Mock up iBooster + Wilwood MC assembly against LJ firewall (cardboard/3D-print) to verify engine-bay clearance before drilling
• Confirm flexline length after firewall mockup (220-12993 = 8", longer SKUs available if needed)
• Design / select reservoir standoff bracket location and height (4-6" above MC flange)

Ordering:

• Order master cylinder — Wilwood Tandem Compact 1-1/8" bore (260-15541, confirm part number) — re-sized for the Super Duty calipers
• Order plumbing: 2× 260-16392 reservoirs + 250-16393 dual bracket + 2× 220-12993 flexlines (vendor-confirmed)
• Order Back Bay Customs Wilwood MC adapter (confirmed vs 260-15542; re-confirm against the 1-1/8" unit)
• Confirm outlet threads on the 1-1/8" unit (plumbing assumes 1/2-20 IF at the MC)
• Verify the master cylinder reservoir-feed port thread on the actual MC in hand before buying any extra inlet fittings — Wilwood's datasheet leaves the inlet field blank (it's an integral-reservoir unit), so this is an in-hand check, not a datasheet lookup. Back Bay confirmed the remote reservoirs work with the adapter (Adam, 2026-05-30) and the 220-12993 is the flexline kit described to them.

Brake plumbing — caliper side (hybrid: copper-nickel hardline + axle flex):

• Order 3/16" copper-nickel hardline + inverted-flare fittings (1/2-20 IF at MC outlets) + stainless spring gravel-guard armor
• 🔵 Select extended-length DOT (FMVSS 106) braided flex hoses — blocked on axle + suspension selection (Section 10); size to full droop, do not cut until ride height is fixed
• Select adjustable proportioning valve for the rear circuit (iBooster has no integral proportioning) + any residual-pressure valves; size on assembly
• Confirm Super Duty front and rear caliper piston areas and check 1-1/8" MC stroke headroom (both axles changed)
• Plan routing: chassis runs inside the frame rail, bulkhead fittings at each axle for the hardline→flex transition

Fab + install:

• After PLA fit confirms: order SendCutSend backing plate in 3/16" A36 steel with zinc yellow plating
• Drill LJ firewall: 4× M8 clearance holes (60×80mm pattern, 80mm vertical) + 62mm center bore for booster shaft pass-through
• Plug clutch master cylinder firewall hole (~1.25" block-off plate or weld closure)

Related Documentation¶

• START+ Forward Distribution Bus - Main power feed + 50A breaker
• Ignition Signal Distribution - Ignition enable source
• Engine Bay Ground Bus - Stud 7 ground connection
• Firewall Ingress - Mounting and wire routing