- Important first note: most original Massey Ferguson MF35 tractors (1950s–1960s) were built without electronic fuel injection or an oxygen (O2) sensor. If your MF35 has been converted to electronic fuel injection or has an aftermarket O2 sensor fitted to the exhaust, follow the instructions below. If it has no sensor or bung in the exhaust, you will need exhaust modification or a different tuning approach (explained near the end).

- Symptoms that indicate O2 sensor testing/replacement may be needed:
- Rough idle, surging, or poor throttle response on an EFI-converted engine.
- Increased fuel consumption after EFI conversion.
- ECU trouble codes showing O2 sensor faults (on tractors with an ECU).
- Visible damage, heavy soot/oil contamination, or an old sensor (older than ~5–10 years).

- Safety first:
- Disconnect the battery before working on electrical connectors.
- Work on a cool exhaust; hot exhaust components cause severe burns.
- Use eye protection and gloves when removing rusted hardware.
- Park on level ground, set parking brake, and use blocks under wheels if needed.

Tools (with detailed descriptions and how to use each)
- Multimeter (digital, 0–20 V DC range, and continuity function)
- Use to measure O2 sensor voltage (narrowband ~0.1–0.9 V switching; heated sensors also have heater circuit resistance).
- Set to DC volts for signal testing. Backprobe connector (or wiggle wires with the harness disconnected and sensor still hot) to read live voltage.
- Use resistance (ohms) range to check heater element resistance (heated 3– or 4‑wire sensors typically ~3–16 ohms depending on type).

- Oxygen sensor socket (22 mm or 7/8" O2 socket) or deep 22 mm hex socket with cutout
- Special socket fits over the sensor harness and grips the sensor hex to loosen/tighten without cutting wires.
- Use with a ratchet and possibly an extension. Turn counterclockwise to remove, clockwise to install. Apply steady force; use penetrating oil if seized.

- Ratchet (3/8" drive) and breaker bar
- Ratchet for normal turning; breaker bar helpful for initial break loose on seized sensors. Use breaker bar carefully to avoid rounding the sensor hex.

- Penetrating oil (e.g., PB Blaster or WD-40 Specialist Penetrant)
- Spray on sensor threads and let sit 10–30 minutes to free seized threads. Repeat if needed. Avoid getting heavy amounts onto oxygen sensor tip.

- Wire strippers/crimpers and insulated butt connectors or soldering iron and heat-shrink tubing
- Use to repair or replace sensor wiring or connectors. Strip only what you need, crimp or solder cleanly, and insulate with heat-shrink to avoid corrosion and shorts.

- Small wire brushes and contact cleaner
- Use to clean connector pins and mating surfaces. Do not scrub the sensor element.

- Anti-seize compound (sensor-specific; do not contaminate sensor tip)
- Put a small amount on the sensor thread (not on the tip) to ease future removal. Many replacement sensors come with anti-seize pre-applied.

- Torque wrench (range including ~22–40 ft·lb / ~30–55 N·m)
- Tighten sensor to manufacturer-recommended torque (if unknown, tighten to ~22–30 ft·lb / ~30–40 N·m). Over-torquing can damage threads.

- Jack and jack stands or ramps (if sensor is hard to access from above)
- Use to safely lift and support the tractor to access the exhaust underbody. Ensure stability.

- Electrical tape and zip-ties
- Use to secure wiring away from hot or moving parts after installation.

- Replacement oxygen sensor (universal or vehicle-specific)
- Choose a sensor that matches connector type and thread size (commonly M18 x 1.5 for many universal sensors). For engines with ECU expecting a heated sensor, use a 4‑wire heated O2 sensor (two wires for heater, one for signal, one ground/reference). If unsure, take the old sensor to an auto parts store to match.
- Replacement reasons: sensor failure, poor switching (stuck high/low), heater circuit open, contamination.

- Exhaust bung (weld-in) and welding gear or an exhaust repair shop (if your MF35 has no bung)
- Required only if exhaust must be modified to accept a sensor. Welding tools and fittings are needed to add a threaded bung; this is a fabrication job often better done by an exhaust shop.

- Optional: scan tool or ECU interface (for EFI systems)
- Use to read trouble codes and live O2 sensor data if your EFI system uses a diagnostic interface.

Step-by-step — locate and test an O2 sensor (assume one is fitted)
- Locate the O2 sensor in the exhaust system: typically threaded into an exhaust pipe or manifold downstream of the engine or catalytic converter on EFI systems.
- Inspect the sensor and wiring for obvious damage, melted insulation, heavy soot, or contamination. If wires are broken or the connector is corroded, replacement or connector repair is required.
- For sensors on an EFI system with an ECU and the engine running:
- Reconnect battery, start engine and let it reach operating temperature (O2 sensors work when hot).
- Backprobe the signal wire with the multimeter set to DC volts. Expect rapid switching between ~0.1 V (lean) and ~0.9 V (rich) for a narrowband sensor. If the reading is fixed near 0 V, 0.45 V, or ~1 V, the sensor may be bad or the system may be forcing a condition via the ECU.
- For heated sensors, turn ignition off and measure the heater resistance with the sensor unplugged: typical low-ohm values (check replacement spec). Infinite resistance means heater open and sensor needs replacement.
- For bench testing if no ECU or to verify a sensor element:
- Heat the sensor tip gently with a propane torch (do not overheat) while monitoring voltage output – it should produce voltage as it heats in air/fuel mixtures (do this carefully, outside, and avoid contamination). This is advanced; if uncomfortable, replace sensor instead of bench testing.

Step-by-step — remove old O2 sensor
- Let exhaust cool completely or work with heat-protection gloves.
- Spray penetrating oil on the sensor threads; allow time to soak.
- Disconnect the electrical connector or cut the wires close to the connector if the connector is corroded and you plan to replace it.
- Use the oxygen sensor socket and ratchet or breaker bar to turn the sensor counterclockwise. Apply steady force; if it won’t move, apply more penetrating oil and wait or use a breaker bar carefully.
- Once loose, unscrew by hand and remove. Inspect threads in the exhaust for damage or cross-threading.

Step-by-step — install new O2 sensor
- Clean the thread area in the exhaust bung with wire brush; remove heavy rust but avoid removing metal that forms the thread.
- Apply a small dab of anti-seize to the sensor threads (avoid the sensor tip). If the new sensor already has anti-seize, do not add more.
- Thread the new sensor into the bung by hand to avoid cross-threading. Tighten hand‑tight, then use torque wrench to final torque (~22–30 ft·lb / ~30–40 N·m) unless the sensor manufacturer specifies otherwise.
- Reconnect the electrical connector or splice the wires using crimp connectors or solder and heat-shrink. Match heater wires and signal/ground exactly—install the proper connector type for your harness.
- Secure wiring away from hot surfaces with zip-ties and ensure no chafing.
- Reconnect battery, start engine, and confirm system operation. If equipped with ECU, clear codes if present and verify O2 sensor readings with a scan tool or by monitoring engine performance.

When replacement is required and why
- Replace sensor when:
- Sensor signal is dead (no switching), heater circuit is open, or resistance out of spec.
- Sensor is physically damaged, contaminated with oil or silicone, or covered in heavy carbon.
- Sensor is very old (lifespan varies; typically recommended replacement after several years or 60k–100k miles for narrowband, sooner for some conditions).
- Wiring/connectors are broken beyond simple repair.
- Recommended replacement parts:
- A matching-thread O2 sensor (common universal thread M18 x 1.5, but verify by measuring old sensor). Prefer a 4‑wire heated sensor if your ECU expects a heated type.
- Matching electrical connector or pigtail harness if original connector is corroded.
- Anti-seize compound, new gaskets if the bung uses a gasket, and possibly a replacement exhaust bung if the flange is damaged.

Special case — no sensor bung on MF35 exhaust
- If your MF35 exhaust has no threaded bung, you have two choices:
- Fit a weld-in bung into the exhaust pipe/manifold: requires cutting, welding, and a threaded bung. Tools/skills: angle grinder, pipe cutter, welder, welding PPE. Recommended to have an exhaust shop do this to ensure correct placement and avoid damaging the exhaust.
- Use an inline sensor adapter (if available) that clamps to the exhaust and provides a bung without welding — these are not ideal long-term but can work for testing.

Extra tools (why they may be required)
- Welding equipment or exhaust shop service:
- Required if no bung exists and you want a permanent, threaded mounting point for the sensor.
- Scan tool/ECU reader:
- Required to read sensor data and codes on EFI systems. Without it, diagnosis is limited to voltage readings and symptoms.
- Heater circuit power probe:
- Useful to test heater supply voltage if ECU pinouts or harness wiring is unclear.

Quick troubleshooting summary (no yapping)
- No sensor present: either fit a bung (weld/shop) or you cannot use an O2 sensor for tuning.
- Sensor present but no voltage switching: check wiring/connectors, heater resistance, then replace sensor.
- Sensor voltage fixed but ECU commanding rich/lean: check fuel system, vacuum leaks, ignition timing, and ECU codes.
- Wiring damaged: repair or replace harness and connectors; do not run sensor wires near hot or moving parts.

- Final practical note: if your MF35 is largely stock and carbureted/pure-mechanical, there is no factory O2 sensor; tuning is done mechanically (carburetor, ignition). Retrofitting an O2 sensor requires exhaust modification and an EFI system or aftermarket controller. If you have an EFI conversion or an existing sensor, follow the removal/test/install steps above.
rteeqp73

Restored 1958 Massey Ferguson 35. 4 cylinder This is a beautifully restored tractor of a high quality.

1955 Ferguson 35 - Dual Stage Clutch - Hard Lessons Learned Thought this would be easy... I was mistaken!!!

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