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.
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HOMESERVICE MANUALSFORD1999MUSTANG GT, 2D CONVERTIBLE, STANDARDREPAIR AND DIAGNOSIS (SINGLE PAGE)EXTERNAL PAGESDIFFERENT CARSECTION 2556 (ENGINE CONTROLS - PINPOINT TEST A: NO START THROUGH PINPOINT TEST DK: ACCELERATOR PEDAL POSITION (APP) SENSOR - (EXCEPT DIESEL & HYBRID))PINPOINT TEST B: POWERTRAIN CONTROL MODULE (PCM) POWER RELAYTEST PROCEDURE
1999 Ford Mustang GT, 2D Convertible, Standard
Test Procedure
1999 Ford Mustang GT, 2D Convertible, StandardSECTION Test Procedure
WARNING: This page does not describe the selected car, but rather 31 other vehicles, including the 2008 Mercury Sable, 2008 Mercury Mountaineer, 2008 Mercury Milan, 2008 Mercury Mariner, and 2008 Mercury Grand Marquis. However, it is still accessible from the selected car via links, so may be relevant.
- B1 CHECK FOR DIAGNOSTIC TROUBLE CODES (DTCS)
- Is DTC P0685, P0689, or P0690 present?
Yes No For DTCs P0685 or P0690, GO to B8.
For DTC P0689, GO to B11.GO to B2. - B2 CHECK THE B+ AND IGN START/RUN VOLTAGE TO PCM POWER RELAY
- Key in OFF position.
- PCM Power Relay connector disconnected.
- Key ON, engine OFF.
- Measure the voltage between:
( + ) PCM Power Relay Connector, Harness Side ( - ) B+ Ground IGN START/RUN Ground - Are the voltages greater than 10 V?
Yes No For Crown Victoria,
Grand Marquis,
E-Series,
Escape/Mariner,
Ranger, and
Town Car, GO to B6.
For all others, GO to B3.REPAIR the open circuit. CLEAR the DTCs. REPEAT the self-test. - B3 CHECK THE PCMRC CIRCUIT FOR AN OPEN IN THE HARNESS
- Key in OFF position.
- PCM connector disconnected.
- Measure the resistance between:
( + ) PCM Power Relay Connector, Harness Side ( - ) PCM Connector, Harness Side PCMRC PCMRC - Is the resistance less than 5 ohms?
Yes No GO to B4. REPAIR the open circuit. CLEAR the DTCs. REPEAT the self-test. - B4 CHECK THE PCMRC CIRCUIT FOR A SHORT TO VOLTAGE IN THE HARNESS
- Key ON, engine OFF.
- Measure the voltage between:
( + ) PCM Connector, Harness Side ( - ) PCMRC Ground - Is any voltage present?
Yes No REPAIR the short circuit. CLEAR the DTCs. REPEAT the self-test. GO to B5. - B5 CHECK THE ISP-R VOLTAGE AT THE PCM HARNESS CONNECTOR
- Key in OFF position.
- PCM Power Relay connector connected.
- Key ON, engine OFF.
- Measure the voltage between:
( + ) PCM Connector, Harness Side ( - ) ISP-R Ground - Is the voltage greater than 10 V?
Yes No GO to B7. REPAIR the open circuit. CLEAR the DTCs. REPEAT the self-test. - B6 CHECK THE PCM POWER RELAY GROUND CIRCUIT FOR AN OPEN
- Measure the voltage between:
( + ) PCM Power Relay Connector, Harness Side ( - ) PCM Power Relay Connector, Harness Side B+ GND - Is the voltage greater than 10 V?
Yes No GO to B7. REPAIR the open circuit. CLEAR the DTCs. REPEAT the self-test. - Measure the voltage between:
- B7 CHECK FOR AN OPEN VPWR CIRCUIT BETWEEN THE PCM AND POWER RELAY
- Key in OFF position.
- PCM connector disconnected.
- Measure the resistance between:
( + ) PCM Power Relay Connector, Harness Side ( - ) PCM Connector, Harness Side VPWR VPWR - Is the resistance less than 5 ohms?
Yes No INSTALL a new PCM Power relay.
CLEAR the DTCs. REPEAT the self-test.REPAIR the open circuit. CLEAR the DTCs. REPEAT the self-test. - B8 DTCS P0685 OR P0690: CHECK THE PCMRC CIRCUIT FOR A SHORT TO GROUND IN THE HARNESS
- Key in OFF position.
- PCM Power Relay connector disconnected.
- PCM connector disconnected.
- Measure the resistance between:
( + ) PCM Connector, Harness Side ( - ) PCMRC Ground - Is the resistance greater than 10K ohms?
Yes No GO to B9. REPAIR the short circuit. CLEAR the DTCs. REPEAT the self-test. - B9 CHECK THE ISP-R CIRCUIT FOR A SHORT TO VOLTAGE IN THE HARNESS
- Key in OFF position.
- Measure the voltage between:
( + ) PCM Connector, Harness Side ( - ) ISP-R Ground - Is any voltage present?
Yes No REPAIR the short circuit. CLEAR the DTCs. REPEAT the self-test. GO to B10. - B10 CHECK THE INJPWRM CIRCUIT FOR AN OPEN IN THE HARNESS
- INJ connector disconnected.
- Measure the resistance between:
( + ) PCM Connector, Harness Side ( - ) INJ Connector, Harness Side INJPWRM VPWR - Is the resistance less than 5 ohms?
Yes No GO to B12. REPAIR the open circuit. CLEAR the DTCs. REPEAT the self-test. - B11 DTC P0689: CHECK THE ISP-R VOLTAGE AT THE PCM HARNESS CONNECTOR
- Key in OFF position.
- PCM connector disconnected.
- Key ON, engine OFF.
- Measure the voltage between:
( + ) PCM Connector, Harness Side ( - ) ISP-R Ground - Is the voltage greater than 10 V?
Yes No GO to B12. REPAIR the open circuit. CLEAR the DTCs. REPEAT the self-test. - B12 CHECK FOR CORRECT PCM OPERATION
- Key in OFF position.
- Disconnect all the PCM connectors.
- Visually inspect for:
- pushed out pins
- corrosion
- Connect all the PCM connectors and make sure they seat correctly.
- Carry out the PCM self-test and verify the concern is still present.
- Is the concern still present?
Yes No INSTALL a new PCM. REFER toFLASH ELECTRICALLY ERASABLE PROGRAMMABLE READ ONLY MEMORY (EEPROM) , Programming the VID Block for a Replacement PCM. The system is operating correctly at this time. The concern may have been caused by a loose or corroded connector.
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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.