Goal: Diagnose, service or replace the Throttle Position Sensor (TPS) on Toyota B‑series diesel engines (B, 3B, 11B, 13B). This is written for a beginner mechanic — clear component descriptions, how it works, why it matters, step‑by‑step checks and repair, and what can go wrong.

Quick overview / analogy
- The TPS is like a volume knob (position sensor) for the engine: it tells the ECU (or other controller) how far the throttle is open so the fuel delivery and other systems can respond. If the knob gives wrong info, the engine will behave wrong.
- On Toyota B‑series diesels the TPS may be mounted on the throttle body or on the injection pump throttle lever (depending on model/year). Function is the same: convert throttle lever angle into an electrical signal.

Main components (detailed)
1. TPS unit (sensor)
- Type: usually a three‑terminal potentiometer (mechanical wiper on resistive track) or a Hall‑effect sensor (solid‑state). Older B‑series typically use a potentiometer style.
- Housing: metal or plastic body that mounts to throttle body/pump.
- Shaft/wiper: mechanical spindle that follows the throttle lever.
- Mounting flange/screws: holds TPS to casting; alignment is critical.
2. Electrical connector/harness
- Typically 3 pins: Vref (reference voltage), Signal (wiper output), Ground. Pins may be labeled or color‑coded.
- Connector clip and wiring to ECU or governor box.
3. Throttle linkage and shaft
- Throttle lever (on TB or injection pump), return spring, throttle cable.
- Idle stop/fast idle cam and stop screw — these affect “closed” position.
4. ECU/governor (receiver)
- Receives TPS signal and adjusts fuel duration/timing, idle control, cruise, etc.
5. Tools & test gear
- Multimeter (DC volts and ohms), small screwdriver, hex/torx/ratchet set, contact cleaner, safety gloves, replacement TPS (if needed), small feeler gauge or alignment shim if required.

Why this repair may be needed (theory)
- TPS tells the control system the throttle opening, which allows: appropriate fuel delivery, smooth idle, acceleration enrichment, cruise control and emissions control. If TPS is faulty:
- ECU gets wrong throttle angle → wrong fuel control → hesitation, surging, stalling, poor idle, black smoke, reduced power.
- Some ECUs will enter limp mode or disable cruise.
- Wear or contamination on potentiometer contacts causes intermittent or erratic signal; wiring damage causes open/short; physical misalignment changes “closed” reference.

How the TPS system works (simple theory)
- With key on, ECU supplies a stable reference voltage to the TPS (commonly 5 V, but check manual).
- The TPS wiper outputs a voltage proportional to throttle angle: closed ≈ low voltage, open ≈ high voltage. The ECU reads this and adjusts fuel/time accordingly.
- Potentiometer behaves like a voltage divider; Hall sensors generate a corresponding voltage without contact wear.
- Smooth, monotonic voltage change is required — no jumps, dead spots, or noise.

Symptoms of a bad TPS
- Poor idle (too high, too low, hunting/surging)
- Hesitation or flat spot on acceleration
- Stalling at idle or during drive
- Excessive black smoke (overfueling) or reduced power
- Check/engine light or ECU fault codes (if present)
- Cruise control not engaging or dropping out
- Intermittent problems influenced by throttle movement or vibration

Safety first
- Work on level ground, parking brake on.
- Engine off for disassembly; disconnect negative battery terminal if you’ll be working on wiring.
- Wear gloves and eye protection.
- Avoid solvents on painted surfaces; use proper cleaners.

Step‑by‑step: locate, test, clean/adjust, replace

A. Locate TPS
- On throttle body versions: follow the throttle shaft; TPS screws to the side of the throttle body where the shaft exits.
- On pump‑mounted TPS: it will sit on the injection pump’s throttle lever area.
- Identify connector: usually plastic with 3 wires.

B. Visual inspection (first)
- Check wiring for chafing, corrosion, broken wires at the connector.
- Wiggle harness while watching for symptoms or test readings later.
- Check throttle return spring and linkage for free movement; ensure idle stop screw and throttle full‑open stop are not loose or wrong.

C. Static electrical checks (multimeter)
1. Voltage reference check
- With the ignition ON (engine OFF), back‑probe connector:
- Pin A = Vref; should be steady (commonly ~5 V; consult manual).
- Pin C = ground; check continuity to chassis ground (low ohms).
2. Signal output check
- With key ON, slowly open throttle by hand and measure voltage between signal pin and ground.
- Expect a smooth change from low to high (e.g., ~0.5 V closed to ~4.5 V wide open on a 5 V system). Numbers vary; the important part is smooth, monotonic change without jumps or dead spots.
3. Resistance check (potentiometer)
- With sensor disconnected, measure resistance between outer pins: that’s the full potentiometer resistance (e.g., 5 kΩ typical).
- Measure from outer pin to wiper while rotating; resistance should change smoothly and without abrupt jumps.
4. Wiggle test
- While monitoring signal, wiggle the TPS body and harness; intermittent changes indicate internal wear or loose connection.

D. Cleaning and minor repair (if marginal)
- If wiring is good but TPS dirty/noisy and sensor is potentiometer style, sometimes contact cleaner sprayed into sensor through connector area can temporarily reduce noise. This is a temporary fix — replacement is recommended if worn.
- Clean connector pins, apply small amount of dielectric grease after drying.
- Check and free any binding at throttle shaft; lubricate pivot points (not inside sensor).

E. Removal and replacement
1. Mark position
- With throttle closed, mark the relative position of the TPS body and throttle lever with a scribe or tape. This helps return to original alignment.
2. Disconnect connector and remove mounting screws.
3. Remove TPS gently — watch the gasket/seal.
4. Compare new TPS to old: hole pattern, shaft orientation, connector pins match.
5. Install new TPS in same orientation; don’t fully tighten screws until you verify alignment.
6. Adjust/align:
- For many TPS types you set closed throttle voltage or align the flat on shaft so closed throttle gives specified voltage. Procedure:
a) Reconnect sensor and back‑probe signal.
b) Hold throttle fully closed (throttle stop engaged).
c) Rotate TPS body slightly until closed throttle voltage equals spec (or within acceptable low range — e.g., ~0.4–1.0 V). On some applications you simply align index marks.
d) Tighten screws and recheck through full throttle sweep — confirm smooth rise to high voltage.
7. Final checks: secure harness, reassemble any covers, reconnect battery if removed.

F. Road test
- Check idle behavior, acceleration, engine light status, and for any stumble.
- If symptoms persist, check ECU codes, injectors, pump timing, and air intake/exhaust systems — TPS is only one possible cause.

Common failure modes and causes
- Worn resistive track or wiper (potentiometer): causes noisy or erratic voltage.
- Corrosion of connector pins: intermittent or open circuit.
- Broken wire at flex point: intermittent failures that come and go.
- Misalignment or loose mounting: incorrect closed/throttle reference.
- Contamination/soot ingress (diesels): accelerates wear.
- Mechanical binding in throttle linkage that places sensor out of range.
- ECU or reference voltage problem: bad Vref (not TPS) will give wrong signal — always check Vref and ground.

How to tell TPS vs other causes
- If Vref is missing or unstable, the sensor is not receiving the reference — check ECU/power harness first.
- If voltage is steady but engine misbehaves only under load, look at fuel pump, injection timing, boost, fuel supply.
- Use oscilloscope for better diagnostics: a clean TPS shows a clean, monotonic ramp; noise/jumps show problems.

Parts & tool checklist
- Replacement TPS (match OEM part number)
- Multimeter (and optionally oscilloscope)
- Small screwdriver/torx/allen sockets as required
- Contact cleaner and dielectric grease
- Thread locker (if specified) or torque wrench for mounting fasteners
- Marker or scribe to mark alignment

Practical tips
- Always compare old and new TPS electrical pinout and resistance before installing.
- If temporary contact cleaner fixes it, replace TPS as soon as practical.
- Keep throttle linkage free of slack; check return spring strength.
- Record pre‑replacement voltage/resistance values for comparison.
- If uncertain of exact voltage specs for your year/model, reference a workshop manual — the general behavior is more important than exact numbers.

Summary (in one line)
- TPS senses throttle angle and sends a voltage to the ECU; test by checking reference voltage, ground, and smooth signal voltage sweep; repair means clean/align or replace the unit and ensure good wiring and throttle linkage — worn pots, corroded connectors, bad grounds, or misalignment are the usual culprits.
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