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P1587: Throttle Actuator Control System – Underload Condition – Complete Technical Analysis
This comprehensive technical guide provides detailed diagnostic procedures, wiring specifications, voltage parameters, and manufacturer-specific troubleshooting for OBD-II code P1587. Designed for professional technicians and advanced DIY enthusiasts seeking in-depth understanding of throttle actuator control system failures.
Technical Definition
P1587 – Throttle Actuator Control System – Underload indicates that the Powertrain Control Module (PCM) has detected insufficient mechanical resistance during throttle plate actuation. The system monitors the current draw of the throttle actuator motor; when the plate moves with less torque than the calibrated threshold (typically 20-30% below expected), this fault is set. This is fundamentally different from “overload” conditions and often points to mechanical rather than electrical failures.
Technical Specifications & Parameters
System Voltage Specifications
| Parameter | Normal Range | P1587 Threshold | Measurement Point | Tolerance |
|---|---|---|---|---|
| Actuator Motor Current (Idle) | 0.8 – 1.2A | < 0.6A | PCM Driver Circuit | ±0.1A |
| Actuator Motor Resistance (20°C) | 1.2 – 3.5Ω | > 4.0Ω or < 1.0Ω | Motor Terminals | ±0.2Ω |
| TPS Signal Voltage (Closed) | 0.45 – 0.85V | Any value with underload | TPS Signal Wire | ±0.05V |
| Reference Voltage | 5.0V ± 0.1V | 4.7V or 5.3V | TPS Ref Pin | ±0.1V |
| Actuator PWM Frequency | 500 – 2000Hz | Frequency Irregular | PCM Output | ±100Hz |
Component Specifications
| Component | Manufacturer P/N | Torque Specification | Resistance Value | Backdrive Force |
|---|---|---|---|---|
| Throttle Actuator Motor (Bosch) | 0 280 750 558 | 0.8 – 1.2 N·m | 2.1Ω @ 20°C | 45 – 55N |
| Throttle Position Sensor | 0 281 002 537 | 2.0 – 3.0 N·m | Variable | N/A |
| Return Spring Assembly | Integral to TB | Spring Rate: 8-12 N/mm | N/A | 18 – 22N @ closed |
| Throttle Body Bolts | N 910 647 01 | 8 – 10 N·m + 90° | N/A | N/A |
Complete Diagnostic Procedure
⚠️ Safety Precautions
Before beginning diagnosis: 1) Disconnect battery negative terminal, 2) Allow PCM to power down (5 minutes minimum), 3) Use proper insulated tools, 4) Never force throttle plate open, 5) Wear safety glasses when cleaning throttle body, 6) Ensure work area is well-ventilated when using chemical cleaners.
Pre-Diagnostic Checks
- Scan for Additional Codes: Retrieve all stored and pending codes. P1587 often appears with P0121 (TPS Range/Performance), P0222 (TPS Low Input), or P2101 (TAC Motor Circuit Range/Performance).
- Check Freeze Frame Data: Record engine RPM, load, temperature, and vehicle speed when code set.
- Visual Inspection: Examine throttle body area for vacuum leaks, damaged wiring harness, oil contamination, or obvious mechanical damage.
- Battery Voltage Check: Verify battery voltage >12.4V with engine off. Low voltage can cause false underload detection.
Throttle Body Removal & Mechanical Inspection
- Removal Procedure: Disconnect battery, remove air intake assembly, disconnect electrical connector (depress locking tab), remove 4 mounting bolts (8mm).
- Plate Movement Test: Manually rotate throttle plate. Should have smooth, consistent resistance throughout travel. Sudden “easy” spots indicate worn bushings.
- Return Spring Inspection: Plate should return crisply from 90° open position. Lazy return indicates spring fatigue.
- Shaft Play Check: Attempt to move plate laterally and axially. More than 0.5mm play indicates worn shaft/bushings.
- Carbon Deposit Assessment: Examine plate edges and bore. Deposits >1mm thick can reduce friction, causing underload.
Electrical System Diagnosis
Voltage Testing Procedure
| Test | Expected Value | Fault Condition | Possible Cause |
|---|---|---|---|
| Motor Resistance (Pins 2-3) | 1.2 – 3.5Ω @ 20°C | <1.0Ω or >4.0Ω | Shorted/Open windings |
| Insulation Resistance (Motor to Case) | >10MΩ | <1MΩ | Internal short to ground |
| TPS Reference (Pin 4 to 5) | 5.0V ± 0.1V | <4.7V or >5.3V | PCM or wiring fault |
| Signal Voltage Correlation | TPS1 + TPS2 = 5.0V ± 0.2V | Sum ≠ 5.0V | Sensor fault |
Actuator Performance Test
- Current Draw Test: Using amp clamp on motor feed wire. Start engine, observe current: Should be 0.8-1.2A at idle, increasing with throttle command.
- PWM Duty Cycle: Connect oscilloscope to motor control circuit. Should see 500-2000Hz square wave with duty cycle varying with throttle position.
- Backdrive Force Measurement: Using force gauge on throttle lever. Should require 45-55N to open plate from closed position against actuator.
- Adaptation Reset Procedure: After repair, perform throttle adaptation: Ignition ON 30 seconds (no start), then start and idle for 5 minutes.
Related Error Codes & Companion Faults
P1587 rarely occurs in isolation. Understanding companion codes is essential for accurate diagnosis. Below is a comprehensive list of related codes with their diagnostic relationships:
Code Setting Criteria & Enable Conditions
| Code | Enable Conditions | Time to Set | MIL Illumination | Freeze Frame |
|---|---|---|---|---|
| P1587 | Engine running, battery voltage >10.5V, no TPS faults | 2 consecutive trips | 2nd trip | Yes |
| P0121 | Throttle angle change >5% with TPS change <2% | 500ms | Immediate | Yes |
| P2101 | Actuator current outside 0.5-1.5A range for 2 seconds | 2 seconds | Immediate | Yes |
Manufacturer-Specific Information
BMW (N52, N54 Engines)
BMW Technical Service Bulletin SI B12 07 08
Issue: P1587 with drivetrain malfunction message. Cause: Throttle actuator gear wear. Solution: Replace throttle actuator (P/N 13 54 7 585 261) and perform adaptation via ISTA. Note: Requires battery registration after replacement.
Chrysler/Dodge/Jeep (3.6L Pentastar)
- Common Issue: Carbon buildup on throttle plate edges reduces friction.
- Diagnostic Tip: Monitor “Throttle Plate Angle Learned” vs “Throttle Plate Angle Actual” using WiTECH.
- Repair Procedure: Clean throttle body with Mopar Carb Cleaner (P/N 05182101AA).
- Adaptation: Requires PCM reset: Disconnect battery 15 minutes, reconnect, idle 5 minutes.
Volkswagen/Audi (2.0T TSI)
- Software Update: Many P1587 faults resolved with ECU software update (TPI 2035174).
- Special Tool: Requires VAS 5054A for adaptation procedure.
- Common Fault: Intake carbon deposits on both throttle plate and intake valves.
- TSB Reference: Group 24, TSB 24-13-01 addresses P1587 with EPC light.
General Motors (LS/LT Engines)
- Resistance Specification: Throttle actuator motor should measure 1.8-2.2Ω at 20°C.
- Scan Tool Requirement: Use GM GDS2 for bidirectional throttle control test.
- Fastener Torque: Throttle body bolts: 8 N·m + 90° turn.
- Service Procedure: Documented in SI 2014-02-06-001A.
Frequently Asked Questions
A healthy throttle actuator motor typically shows resistance between 1.2-3.5Ω at room temperature (20°C/68°F). Readings outside this range indicate problems:
- <1.0Ω: Shorted windings within motor. Current draw will be excessive, often tripping overload protection.
- >4.0Ω: High resistance due to worn brushes, commutator damage, or internal opens.
- 0Ω (dead short): Complete winding failure, usually accompanied by blown fuse.
- Infinite resistance: Open circuit in windings or brush assembly.
Important: Measure resistance at the motor terminals with connector disconnected. Temperature affects readings: resistance increases approximately 0.4% per °C rise.
| Symptom | Carbon Buildup | Mechanical Wear | Test Method |
|---|---|---|---|
| Throttle Plate Movement Feel | Sticky spots, then easy movement | Consistently easy throughout travel | Manual rotation test |
| Current Draw Pattern | Erratic spikes, then drops | Consistently low current | Oscilloscope monitoring |
| Visual Inspection | Black deposits >1mm thick | Shiny wear marks on shaft | Borescope examination |
| Onset Pattern | Gradual worsening over months | Sudden onset or progressive | Service history review |
| Cleaning Result | Often resolves issue | No improvement | Pre/post cleaning test |
Diagnostic Tip: Perform current draw test before and after cleaning. If current remains below 0.6A after thorough cleaning, mechanical wear is likely.
General Adaptation Procedure
- Battery Preparation: Ensure battery voltage >12.4V, connect maintainer if necessary.
- Ignition Cycle: Key ON, engine OFF for 30 seconds (don’t touch accelerator).
- Initial Start: Start engine, allow to idle until coolant reaches normal temperature.
- Idle Learning: With A/C OFF, let idle stabilize for 5-10 minutes.
- Drive Cycle: Perform specific drive pattern (varies by manufacturer).
Manufacturer-Specific Variations
- BMW: Requires ISTA/D for “Throttle Valve Adaptation.” Process takes 15 minutes.
- Mercedes: Use Star Diagnosis for “ETC Learn Procedure.” Must be at 68-86°F ambient.
- Toyota: Techstream required: “Throttle Position Learn” under Powertrain/Engine.
- Honda: HDS procedure: “ETCS Learn” with engine at operating temperature.
- Ford: IDS or equivalent: “Throttle Body Relearn” with specific pedal sequence.
⚠️ Critical Warning
A shorted throttle actuator motor CAN damage the PCM driver circuits. Always test actuator resistance BEFORE reconnecting to PCM after replacement.
PCM Driver Circuit Testing Procedure
- Disconnect PCM: Remove PCM connectors, identify throttle control pins.
- Resistance Check: Measure between driver output pin and ground. Should be >100Ω.
- Diode Test: Using multimeter diode function: Place red lead on ground, black on driver pin. Should read 0.4-0.7V (diode forward voltage).
- Short Test: Check continuity between driver pins and power/ground. Should be infinite resistance.
- Oscilloscope Test: With engine running (actuator connected), monitor PWM signal. Should be clean square wave without ringing.
PCM Damage Indicators
- Blown fuse for PCM power (typically 10A or 15A)
- Burnt smell from PCM
- Multiple unrelated codes appearing
- No communication with scan tool
- Visible damage on PCM board (requires opening)
