1) System theory — what air suspension does and how it fails
- Components: air compressor (pump), pressure reservoir (tank) if present, air dryer/desiccant, pressure switch/regulator, valve block/solenoids (supply/isolate corners), air springs/airbags or struts, height sensors/linkages, wiring/ECU or control module.
- Normal operation: compressor builds pressure → valve block routes pressurized air to individual air springs to raise a corner; exhaust valves dump air to lower; height sensors tell the controller to open/close valves to maintain set ride height. Pressure switch protects compressor from overrun and signals when to run.
- Common faults and root causes: leaks in hoses/fittings or damaged air springs cause slow sink and continuous compressor run; failed compressor (wear, valve plate, piston rings, stuck inlet/exhaust) causes no/low pressure; clogged dryer or water causes valve sticking and corrosion; faulty valve block/solenoids cause a corner not to fill or to leak back; bad height sensor or wiring causes incorrect height control.

2) Safety and prep (brief)
- De-pressurize system before disconnecting fittings. Chock wheels, support vehicle on stands if working under it. Isolate battery before electrical repairs. Use eye/hand protection.

3) Symptom confirmation and system overview
- Record symptoms (continuous compressor run, slow sink, sagging corner, no raise). Note any fault codes. Locate service ports, pressure switch, valve block, compressor and height sensors so you know what to exercise/test.

4) Static leak detection (first, because leaks are the most common cause)
- Theory: a leak is the only fault that forces the compressor to run repeatedly to maintain pressure.
- How to test: pressurize system to operating pressure, isolate (close valve block or tank drain if possible), apply soapy water to joints, fittings, air spring seams and around valve block and height sensor linkages; use an ultrasonic/electronic leak detector for small leaks.
- How repair fixes fault: sealing or replacing the leaking hose/connector/airbag removes the air path loss so the system can hold pressure and the compressor will stop running when set height/pressure is reached.

5) Compressor functional testing
- Theory: compressor must produce sufficient flow and pressure and not draw excessive current. Mechanical wear (rings/valve plate), seized inlet/exhaust valves, or electrical faults reduce capacity.
- Tests: measure compressor run time/cycle behavior, listen for unusually loud operation, measure current draw with clamp meter (compare to spec), measure pressure rise at service port with gauge. Check for vacuum or suction at inlet filter indicating blockage.
- How repair fixes fault: rebuilding or replacing the compressor restores pumping capacity and correct flow/pressure so the system can reach/maintain set pressure; correcting electrical faults prevents intermittent failure.

6) Pressure switch/regulator and dryer check
- Theory: pressure switch signals compressor to cut in/out; dryer removes moisture that otherwise forms ice or corrodes valves and leads to leak paths or stuck solenoids. A failed switch can stop compressor or prevent it from stopping. Wet desiccant reduces pressure stability and accelerates valve block corrosion.
- Tests/inspect: verify switch cut-in/out pressures using gauge and observe compressor control; inspect/replace dryer cartridge if saturated or old.
- How repair fixes fault: replacing switch/dryer assures correct compressor control and prevents moisture-caused valve sticking/leaks.

7) Valve block / solenoid isolation and testing
- Theory: valve block isolates and routes air to each air spring; leaking solenoids or internal seals allow air to bypass and cause corner sag or continuous loss.
- Tests: energize/de-energize each solenoid and observe air flow and pressure change at the corresponding air spring; use soapy water to detect leakage at valve block ports; bench-test solenoids for proper resistance and movement.
- How repair fixes fault: rebuilding or replacing faulty valve block/solenoids eliminates unintended bleed paths and restores independent control over each corner.

8) Air spring / strut inspection and replacement
- Theory: air springs provide the springing and sealing surfaces; failure (cracks, delamination, seam failure) leads to rapid or slow air loss. Aging changes spring rate.
- Repair: remove and replace damaged air springs or struts, use OEM-spec parts and check mounting hardware and alignment.
- How repair fixes fault: replaces the failed pressure vessel; restores sealing and correct load-carrying characteristics so the system can hold and control ride height.

9) Height sensors, linkages and electrical checks
- Theory: height sensors provide feedback to the controller; incorrect sensor output or stuck linkages cause wrong valve commands (overfill, underfill, no action). Wiring/connectors can degrade and cause intermittent signals.
- Tests: measure sensor voltages/resistance and check full travel corresponds to voltage change; move linkages by hand and observe controller response; scan for control module codes.
- How repair fixes fault: repairing/replacing sensor/linkage or wiring restores correct feedback so the controller opens/closes valves properly and maintains set height.

10) Order of repairs (recommended sequence)
- a) Fix leaks first (hoses, fittings, air springs). Rationale: if leaks remain, other repairs will be masked by ongoing air loss.
- b) Replace dryer and check/replace pressure switch. Rationale: moisture and wrong cut-in/out behavior will affect subsequent tests.
- c) Verify compressor performance; repair or replace as necessary. Rationale: once system is sealed and dryer is good, the compressor must be able to build system pressure.
- d) Test and repair/replace valve block/solenoids. Rationale: with good pressure source and no leaks, valve block operation can be confirmed and fixed.
- e) Replace faulty air springs/struts and check mount hardware.
- f) Repair/replace sensors and correct electrical faults.
- g) Re-calibrate height control/ECU per manufacturer procedure.

11) Functional verification and road test
- Theory: the system must hold pressure over time and behave correctly under dynamic load changes.
- Tests: with vehicle at normal load, cycle raise/lower several times, measure compressor run cycles (should be infrequent at steady-state), road test to check dynamic height control and ride quality, re-scan for fault codes. Confirm no leaks with a final soapy-water sweep.

12) Why each major repair step fixes the fault (summary)
- Sealing leaks: stops unwanted air loss so pressure and height are maintained.
- Replacing dryer: removes moisture that causes valve corrosion/sticking and freeze-related faults.
- Fixing/replacing compressor: restores ability to generate required pressure and flow.
- Servicing valve block/solenoids: restores correct routing/isolation of air so corners do not bleed and can be controlled independently.
- Replacing air springs: removes failed pressure vessels so the springing function and seal are restored.
- Correcting sensors/electrical faults: restores proper feedback and commanded responses so the control logic works.

13) Common test values and signs (quick reference)
- Leaks: audible hiss, soapy-bubble reaction, continuous compressor run.
- Compressor: excessive current draw or no pressure rise = internal failure; short cycling = leak or failed pressure switch.
- Valve block: a corner that won’t hold pressure while others do indicates internal leakage in that circuit.
- Sensors: no change in sensor voltage with travel or erratic readings indicate sensor/linkage/wiring fault.

End — follow factory torque, sealing and calibration procedures when replacing components.
rteeqp73

Land Cruiser 12HT testi Testing rebuilt Toyota Land Cruiser HJ61 12H-T engine.

Engine Bay, Wiring & Installing the 12HT - HJ75 Troopy Build (EP8) What is going on legends, Super stoked with today's episode, jam packed with a lot of good work. Follow along as we get stuck ...

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