O&K MH-4 City — Diagnosing and Fixing Intermittent Hydraulic Over-Temp and Power Loss

[MikePhua]

Overview
A 1996 O&K MH-4 City was experiencing an odd, repeatable fault: with the engine at low idle everything looks normal; when throttle is raised the hydraulic “High Temp” lamp sometimes illuminates even though the oil is cold (≈20 °C), and when that lamp comes on the machine immediately derates — boom and swing get sluggish and drive drops into a slow/creep state. The machine can run fine for 20–30 minutes, then lose power; after a 15–20 minute cool-down it will restart and behave normally again. Sensors and the tank-pressure switch have been checked, an oil cooler was replaced, and the fan (driven by the hydraulic system) and hoses were inspected — but the symptom persists. Below is a systematic, technician-focused write-up that turns that operating story into a step-by-step diagnostic plan, likely causes, practical fixes, plus background, terminology and preventive advice.
What the symptom pattern tells us (high-level diagnostic clues)

• Fault is intermittent and time/thermal-related: machine runs, then after a while the lamp lights and power is removed; cooldown restores normal operation. That points at either a genuine thermal event (local heat at a sensor or component), a thermal threshold inside an electronic controller, or an electrical fault that changes state as components heat up.
  
• The lamp can come on when oil is cold → strong hint the lamp might be driven by an electric/logic fault (short, grounding, bad sensor wiring, or control module) rather than only true oil over-temperature.
  
• Reset by power-cycle suggests an electronic module, relay, or thermal protector that latches until it cools or is reset.
  
• Sudden loss of hydraulic power across all functions when the lamp lights suggests system-level derate (engine/ECU limiting or pump bypassing) rather than a single actuator failure.
  

Likely root causes (ranked by probability for this symptom set)

1. Faulty temperature/pressure sensors or wiring harness (intermittent short to ground, intermittent open, corroded connector, or internal sensor failure that changes with temperature).
   
2. Electronic control module / diagnosis board under the seat that is overheating, producing a false over-temp warning and commanding derate.
   
3. Pilot/priority circuit or proportional control valve (electrically controlled) glitch — pilot pressure loss or electro-valve driveline fault that collapses main spool pressure when electronics signal it.
   
4. Hydraulic pump cavitation or internal slip under load (air ingestion or suction restriction) — initially provides flow, then as pump or oil heats/conditions change it loses flow; control system detects low pressure or high temp and derates.
   
5. Thermostatic bypass/cooler circuit malfunction (oil routed incorrectly, cooler bypass stuck, fan drive slipping) causing local hotspots and a sensor seeing high temp even if bulk oil seems cool.
   
6. Relief valve or main pump mechanical problem that depends on temperature/viscosity — e.g., relief valve springs sticking or spool sticking when hot.
   

Measurements & data you should collect first (tools needed: multimeter, thermocouple/infrared thermometer, hydraulic pressure gauges, clamp-on flowmeter if available, scope/logging if possible)

• Log or note the exact sequence — engine rpm, when lamp shows, which functions weaken, time from start to fault, how long until recovery after shutdown.
  
• Measure hydraulic tank oil temperature at the sensor location and at other spots (near pump suction, cooler inlet/outlet) with a thermocouple or infrared pyrometer. Do this during normal operation and when the lamp illuminates. Don’t rely on just the reservoir average.
  
• Measure system pressures: main pump outlet, pilot pressure, and at steering/priority ports while machine is healthy and again when it derates. Note if pilot pressure collapses. Typical pilot pressure on similar O&K machines: ~60–80 bar (confirm for this model) and main work circuits up to ~250–320 bar depending on function — capture actual readings.
  
• Check for air/aeration: inspect return lines and tank suction for froth or foam in the reservoir; aerated oil will cause intermittent loss of flow under load.
  
• Record electrical voltages & continuity on the temp sensor, pressure sensor and to the controller; warm the wiring harness with a heat gun to try to reproduce intermittent behavior.
  
• Check for stored fault codes in the onboard diagnostics module (PMS/ECU) and read any event logs — the O&K diagnostic system often records engine/hydraulic faults and the condition that triggered derate.
  
• Physical inspection: wiring connectors under the seat, any ECM heatsinks, relays/solenoids for discoloration or corrosion, coolant/cooler routing and thermoswitches.
  

Step-by-step diagnostic plan

1. Replicate with logging
   • Start the machine and run it until the lamp appears while logging oil temps and pressures. If you can’t wait for the fault naturally, run sustained hydraulic cycles to induce the condition faster. Record exact timestamps.
     
2. Sensor sanity checks
   • Unplug the temperature sensor and substitute a calibrated thermocouple at the same physical location (if possible). If thermocouple shows normal temps while the system still flags high temp, fault is electrical/logic side.
     
   • Check sensor wiring for chafing, exposed conductors, water ingress or corroded pins — wiggle tests while running may reveal intermittent opens/shorts.
     
3. Control board / ECU thermal behavior
   • With the machine running, monitor voltages at the ECU, and feel (or measure) the temperature of the control board housing under the seat. If the module gets hot and you can correlate module temp rise to the lamp and derate, suspect module failure or poor mounting/ventilation.
     
   • Try powering the machine and supplying forced air to the control box (fan or shop blower) and see if problem goes away — a quick diagnostic.
     
4. Pilot pressure & proportional valve check
   • Attach a gauge to pilot supply. If pilot pressure drops when the lamp appears, the pilot circuit or its electronic control is likely at fault. A failing proportional solenoid or pilot relief valve might stick when it warms.
     
   • If pilot pressure holds but main flow collapses, suspect main pump or main relief/priority valve.
     
5. Main pump & cavitation checks
   • Inspect suction strainer and tank breather for clogging. Replace or clean as necessary. Measure pump inlet vacuum (if you have the tool) — excessive vacuum indicates airflow blockage.
     
   • Observe return oil for foaming (air ingestion). Replace hoses that have internal collapse (old suction hose can collapse under suction and cause intermittent aeration that varies with temperature).
     
6. Thermostatic bypass / cooler circuit checks
   • Check that the oil cooler bypass (thermostatic valve) is not jammed open/closed. On machines with oil-driven fan couplings, inspect coupling for slip when warm. If cooler piping was altered during prior repairs, restore proper routing.
     
   • Check flow through the cooler when hot and when cold (use temp differential across cooler).
     
7. Relief valve and main spool operation
   • If pressures are present but functions lack power, bench-test or remove and inspect main relief valves and priority spools for seizure, wear or contamination.
     
8. Eliminate EMS/logic causes
   • With sensor bypassed (careful: only for test) — force ECU to see normal temps — see whether ECU still commands derate. If derate persists with the ECU convinced temps are normal, the problem might be a limp mode triggered by another parameter (engine management/air intake/boost) or a safety interlock.
     

Common fixes that have proven useful in similar O&K cases

• Replace suspect sensors and connectors (even if they previously tested “OK” at room temp). Sensors can fail at temperature. Use OEM sensors where possible.
  
• Repair/replace the ECU or its cooling/ventilation if the controller is overheating or its solder joints are failing. Thermal cycling cracks solder joints and causes intermittent logic errors. Program/reflash ECU if a software glitch is suspected.
  
• Repair suction side (clean/replace suction strainer, check breather cap, replace old suction hoses). Fixing aeration often resolves intermittent pump loss after warm-up.
  
• Service or rebuild the main pump if flow drops under steady mechanical load—look for internal wear or heat-caused pressure loss.
  
• Replace or service pilot/proportional valves and solenoids if pilot pressure collapses or proportional valves behave erratically when hot.
  
• Correct thermostatic bypass/cooler problems — installing a working thermostat valve often stabilizes oil temp distribution and prevents local hotspots that trip sensors.
  
• Address wiring harness and grounds — clean ground points, repair corroded connectors, apply dielectric grease, and secure harness to prevent chafe and water ingress.
  

Practical checks you can do today (quick wins)

• Install an inexpensive data logger or handheld gauge to capture temp/pressure trends — even a simple IR gun and analogue pressure gauges will give insight.
  
• Swap the temperature sensor with a known good unit if you can borrow one.
  
• Clean and re-seat every connector and fuse under the seat and at the tank.
  
• Replace the tank breather and check for foamy returns in the reservoir.
  
• Monitor oil color and check for coolant in oil (unlikely here but always sanity check).
  

Parameters & reference numbers (typical for machines of this era — verify in a proper manual for exact spec)

• Hydraulic oil nominal operating temp: ~40–80 °C (derate thresholds often set between 90–110 °C depending on OEM).
  
• Pilot pressure (approx): 50–80 bar (check exact model spec).
  
• Main system working pressure: 200–320 bar depending on function.
  
• Tank volumes and cooler surface should provide a temp delta across the cooler of at least 5–15 °C under load when functioning.
  

Case comparisons / anecdotal examples

• On a different O&K RH-8.5, users reported identical behavior — machine ran fine cold, after ~30 minutes the hydraulics slowed drastically and a restart fixed it. In that case the failure was traced to inconsistent pilot pressure due to an intermittent electrical fault to a proportional valve. Replacing the proportional valve driver solved the issue.
  
• Another field story: a mini-excavator lost lift power after 10–20 minutes; diagnosis showed the suction strainer was partially collapsed (internal fabric failure). After replacement, the aeration stopped and the machine ran continuously without derates.
  

Maintenance & preventive checklist (convert into your shop card)

• Replace hydraulic filters and suction strainer per schedule (or sooner if symptoms).
  
• Inspect and replace tank breather and vents yearly.
  
• Check wiring harnesses and ECU mounting for secure, dry, ventilated placement.
  
• Replace temperature and pressure sensors every few years or on first sign of intermittent issues.
  
• Periodic thermal imaging of control electronics and pump during run tests to spot hotspots.
  
• Maintain a log of fault events with timestamps, ambient temp, runtime and actions taken.
  

Glossary — short technical definitions

• Pilot pressure: low-pressure hydraulic circuit that supplies control valves; if it collapses the main functions can be disabled or limited.
  
• Cavitation / aeration: air in the hydraulic fluid caused by suction leaks or foaming; causes loss of pump performance and erratic behavior.
  
• Thermostatic bypass (oil): a valve that routes oil around the cooler until oil reaches operating temperature — if it fails it can cause incorrect cooling.
  
• Derate: deliberate reduction of engine/hydraulic output by the control system to protect equipment when a fault or unsafe condition is detected.
  
• ECU (Electronic Control Unit): the onboard computer that monitors sensors and enforces protections; can command derate.
  

Practical parts & tools you may want on hand

• Spare hydraulic temp sensor and pressure sender for tank and pilot.
  
• Pressure gauge set capable to 350 bar with quick connectors for pump and pilot test ports.
  
• Thermocouple/IR thermometer for quick temp mapping.
  
• Harness repair kit (pins, seals) and dielectric grease.
  
• Spare suction strainer and tank breather.
  
• Portable data logger (or laptop with diagnostics if O&K diagnostic protocol accessible).
  

When to consider professional or component rebuild

• If pump inlet vacuum is excessive or pump produces foam/air even after suction fixes → pump rebuild/replacement likely.
  
• If ECU reboots/derates and you can reproduce by heating the control module → replace ECU or repair board solder joints with professional rework.
  
• If pilot pressure is inconsistent and valves test electrically but stick mechanically → valve body overhaul.
  

Closing recommendation / action plan for an O&K MH-4 City with your symptoms

1. Fit gauges and thermocouple; reproduce the fault and log pressures/temps.
   
2. Bypass or swap the hydraulic tank temp sensor with a calibrated sensor to rule out false sensor/ wiring.
   
3. Monitor/measure pilot pressure when the lamp comes on. If pilot drops, focus on pilot circuit valves and wiring; if pilot holds, focus on main pump and relief valves.
   
4. Inspect & renew suction strainer, breather and any suspect hoses.
   
5. If no hydraulic cause is found, concentrate on the ECU/control board under the seat — test with forced cooling and consider replacement/reflash.