Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
SYS.OK|MANUAL.DB● LIVE
HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 86 (ENGINE CONTROLS - SELF-DIAGNOSTICS - 3.0L)DIAGNOSTIC TESTSDTC P0638: THROTTLE ACTUATOR CONTROL COMMAND PERFORMANCEDIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic
Diagnostic Procedures
1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2002 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.
- Place transmission in Park. Start engine. Rev engine up and down the RPM band a couple of times while using scan tool to monitor status of DTC P0638. If DTC P0638 has run and failed LAST TEST, go to next step. If DTC P0638 has not run and failed LAST TEST, problem is intermittent. See DIAGNOSTIC AIDS .
- Turn ignition on, engine off. Remove air induction tubing. Disconnect TAC module harness connector. Wait 1 minute. Using DVOM and a spare male terminal, measure voltage on TAC module harness connector terminal No. 1 (Yellow wire), and then on terminal No. 4 (Brown wire). If 2.6-4.6 volts are present on each circuit, go to next step. If 2.6-4.6 volts are not present on each circuit, go to step 4 .
- Connect Noid Light (J-34730-405) between TAC module harness connector terminals No. 1 (Yellow wire) and No. 4 (Brown wire). Turn ignition on (for about 5 seconds) and off while monitoring noid light. If noid light turns on for a couple seconds, then off each time ignition is cycled, replace throttle body assembly. If noid light does not turn on for a couple seconds, then off each time ignition is cycled, check for short between Yellow wire and Brown wire. If wires are okay, replace ECM.
- If a circuit has more than 4.6 volts, repair short to voltage in that circuit. If a circuit has less than 2.6 volts, go to next step.
- Turn ignition off. Disconnect ECM connector J1. Check continuity between circuit that had less than 2.6 volts and ground at TAC module harness connector. If continuity exists, repair short to ground in circuit that had less than 2.6 volts. If continuity does not exist, check circuit that had less than 2.6 volts for open. If circuit is okay, replace ECM.
RENDER: 1.0x
NO RELATED
Recommended Tools & Savings
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Affiliate disclosure: We may earn a commission at no extra cost to you.
When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.