Isuzu NPR NQR NPS NKR NHR N SERIES TRUCK Workshop Manual Digital Download

1) Purpose & basic theory
- The TPS (throttle position sensor) is usually a 3‑wire potentiometer mounted on the throttle shaft. It converts throttle plate angle into a voltage signal (signal pin) referenced to a stable 5 V supply and ground from the ECU.
- ECU uses TPS voltage to determine idle control, fueling, ignition timing, torque management and limp mode decisions. A smooth, monotonic voltage rise from closed throttle to wide open throttle (WOT) is required. Abrupt steps, dropouts, wrong offset, or noise = wrong engine control.
- Common failure modes: worn carbon track (potentiometer wear), contaminated internals, broken/oxidised connector pins, intermittent wiring, or internal electronic failure.

2) Symptoms and common fault codes
- Symptoms: unstable idle, surging, hesitation, poor throttle response, high idle, stalling, limp mode, check engine light with TPS-related codes (e.g., P0120–P0124 family or manufacturer equivalents).
- Codes indicate open/short, out-of-range voltage, or inconsistent signal.

3) Tools & safety
- Multimeter (DC volts, ohms), backprobe pins or quality breakout adapter, oscilloscope recommended for best diagnosis, basic hand tools, cleaning spray (electrical contact cleaner), replacement TPS if needed. Disconnect battery when removing sensor (if instructed by manual). Work with engine off for wiring checks.

4) Preliminary visual checks (in order)
1. Park, set handbrake, disconnect negative battery if you will remove sensor. Inspect throttle body area, harness and connector for corrosion, bent pins, torn insulation.
2. Wiggle harness and connector with ignition ON (engine off) while watching codes or voltage to detect intermittent faults.

5) Electrical verification — step by step (engine OFF for static tests, ON/idle for dynamic)
1. Locate connector: identify three pins: 5 V reference (Vref), ground, signal. If unsure, use wiring diagram; most Isuzu TPS uses 5 V reference.
2. Turn ignition ON (engine off).
3. Measure Vref (connector pin to ground): should be stable ~5 V (4.8–5.2 V). If Vref missing, trace/repair reference supply.
4. Measure sensor ground: continuity from sensor ground pin to chassis ground should be near 0 Ω (≤1 Ω). Poor ground causes erroneous readings.
5. Backprobe signal pin with multimeter. With throttle fully closed, note voltage — typical closed throttle ~0.2–1.0 V (commonly ~0.5 V). With throttle slowly opened to WOT, voltage should increase smoothly to about 4–4.8 V at full open. Record closed and WOT voltages.
6. Watch for jagged jumps, sudden drops or dead spots while moving throttle; those indicate worn pot or intermittent contact.
7. If you see no change or stuck at rail (0 V or ~Vref), check wiring for short/open or replace sensor.

6) Dynamic / oscilloscope check (recommended)
- Use scope on the signal wire while slowly opening/closing throttle. A good TPS shows a clean, monotonic ramp without noise spikes, flat sections or sudden voltage steps. Noise/erratic pattern = worn internals or electrical interference.

7) Resistance check (bench or removed)
- Remove sensor. Measure total potentiometer resistance between end terminals. Typical range varies by design (commonly 1–10 kΩ; many ~2–5 kΩ). The resistance should be stable and smooth as you rotate shaft. Use ohm meter and slowly rotate throttle: resistance should change smoothly; steps or dead spots = bad.

8) Adjustment (if sensor is adjustable)
- Some TPS units are adjustable: loosen mounting screws, set throttle to closed stop (but not pressing throttle stop), set signal voltage to specified closed-throttle value (commonly ~0.45–0.6 V) while ignition ON, then tighten screws and recheck full range. If sensor is non-adjustable, replace if out of spec.
- Important: do not force the throttle plate while adjusting; set at true closed stop position.

9) Repair/replace & final checks (in order)
1. If connector pins corroded, clean or replace connector. Repair damaged wiring harness (splice with solder and heat shrink or proper terminal replacement).
2. If cleaning contact surfaces helps (temporary), use electrical contact cleaner and operate throttle to work cleaner in — this may restore contact briefly but is not permanent if pot is worn.
3. If signal is out of spec, intermittent, noisy or shows dead spots, replace TPS with a correct OE or specified part.
4. Reinstall connector, clear diagnostic trouble codes (ECU stored faults).
5. Start engine, allow idle to stabilize. Some ECUs require a simple relearn: let idle sit 1–3 minutes, blip throttle a few times to full open then back to idle to let ECU adapt. Confirm closed-throttle voltage remains at setpoint with ignition ON and engine off after installation.
6. Road test under load, check drivability, and re-scan for codes.

10) How the repair fixes the fault (mechanism)
- A worn/dirty TPS produces incorrect, noisy or intermittent voltage. The ECU receives a wrong throttle angle signal and miscalculates fuel/air, ignition timing and idle control, triggering limp mode or rough running.
- Repair actions restore a correct, stable voltage waveform:
- Cleaning/repair of connector restores reliable electrical paths (fixed intermittent connections).
- Adjusting sets the correct closed-throttle reference voltage so the ECU interprets idle as closed throttle.
- Replacing a worn TPS restores a continuous, linear potentiometer or proper electronic sensor output, removing noise and dead spots.
- With correct TPS signal, the ECU can properly calculate fueling, idle air control commands and throttle response maps, eliminating surging, hesitation, stalls, and diagnostic faults.

11) Verification criteria
- Vref ~5 V, ground near 0 Ω.
- Signal at closed throttle ~0.2–1.0 V (typical ~0.5 V); at WOT ~4–4.8 V.
- Smooth monotonic voltage ramp from closed to WOT (no jumps/dropouts).
- No TPS-related DTCs after clear and road test; drivability normal.

12) Notes / cautions (brief)
- Always confirm exact model values and relearn procedure from the vehicle’s workshop data where available; different years/engines can vary slightly.
- Do not force throttle beyond its stop. Replace sensor rather than rely on temporary fixes for worn pots.

End.
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