CODE 55 - ECM PROBLEM (2.8L/4.3L/5.0L/7.4L)
Be sure ECM grounds are okay and that MEM-CAL, PROM, and/or CALPAK are properly latched. If okay, replace ECM. Clear codes, confirm "Closed Loop" operation, and no "SERVICE ENGINE SOON" light.
The MAP sensor measures manifold pressure (vacuum) and sends that signal to the ECM. The MAP sensor is mainly used for fuel calculation, when ECM is running in the throttle body back-up mode.
The MAP sensor is also used to determine barometric pressure and to help calculate fuel delivery.
Note. Test numbers refer to test numbers on diagnostic chart.
- Check MAP sensor output voltage to ECM. This voltage, without engine running, represents a barometric reading to the ECM.
- Apply 10 in. Hg vacuum to MAP sensor should cause voltage to be about 1.2 volts less than voltage in step 1). Upon applying vacuum to sensor, the change in voltage should be immediate. A slow voltage change indicates a faulty sensor.
- Check vacuum hose to sensor for leaks or restriction. Be sure no other vacuum devices are connected to MAP hose.
MAP Output Check Flow Chart & Schematic. Scheme 107
MAP Output Check. Scheme 108
The P/N switch contacts are closed to ground in Park or Neutral, and open in all Drive ranges. The ECM supplies ignition voltage, through a current limiting resistor, to circuit No. 434. The ECM senses a closed switch when voltage on circuit No. 434 drops to less than one volt. The ECM uses P/N signal as one of the inputs to control idle air, VSS diagnostics, and EGR flow.
Note. Test numbers refer to test numbers on diagnostic chart.
- Checks for closed switch to ground in Park position. Different makes of "SCAN" testers will read P/N signal differently. Refer to tester's instruction manual for type of display used for specific tester.
- Checks for an open switch in Drive or Reverse.
- Be sure "SCAN" tester indicates Drive, even while wiggling shifter, to test for an intermittent or misadjusted switch in Drive.
If circuit No. 434 indicates P/N switch grounded while in Drive, EGR would be inoperative, resulting in possible detonation.
If circuit No. 434 always indicates Drive (open), a drop in idle may exist when gear selector is moved into Drive.
Park/Neutral Switch Diagnosis - Flow Chart & Schematic. Scheme 109
Park/Neutral Switch Diagnosis. Scheme 110
The crank signal is a 12-volt signal to the ECM during cranking to allow enrichment, and to cancel diagnostics until engine is running, or 12 volts on circuit is no longer present.
Note. Test numbers refer to test numbers on diagnostic chart.
- Checks for normal (cranking) voltage to terminal "C9" of ECM. Test light should be on during cranking and then go off when engine is running.
- Checks to determine if source of open fuse or fuse link was a faulty ECM.
Crank Signal Diagnosis - Flow Chart & Schematic. Scheme 111
Crank Signal Diagnosis. Scheme 112
The power steering pressure switch is normally open to ground, and circuit No. 495 will be near battery voltage. Turning steering wheel increases power steering oil pressure, and its load on an idling engine. The pressure switch will close before the load can cause an idle problem. Closing switch causes circuit No. 495 to read less than one volt. The ECM will increase idle air rate and retard timing.
Note. Test numbers refer to test numbers on diagnostic chart.
- Checks for ECM signal voltage on circuit No. 495, and confirms ground circuit No. 450 is okay.
- Maximum resistance, or infinity, indicates an open switch.
- Less than one ohm indicates that switch is closed when power steering pressure is high. Switch is okay.
A pressure switch that will not close, or an open circuit No. 495 or circuit No. 450, may cause the engine to stop when power steering loads are high.
A switch that will not open, or a circuit No. 450 or circuit No. 495 shorted to ground, will cause timing to retard at idle, and may affect idle quality.
Power Steering Pressure Switch (2.5L) - Flow Chart & Schematic. Scheme 113
Power Steering Pressure Switch (2.5L). Scheme 114
The ECM control of the A/C clutch improves idle quality and performance by delaying clutch application until idle air rate is increased, releasing clutch when idle speed is too low, releasing clutch at wide open throttle, and smoothing cycling of compressor by providing additional fuel at instant clutch is applied.
Turning on A/C supplies circuit No. 459 battery voltage to the clutch control relay and terminal "B8" of ECM connector. After a time delay of about .5 second, the ECM will ground terminal "A4" (circuit No. 458), and close the control relay. A/C compressor clutch will engage.
Note. Test numbers refer to test numbers on diagnostic chart.
- Checks for low refrigerant as cause for no A/C.
- This and following tests check for faulty A/C control relay.
A/C Clutch Control Diagnosis Flow Chart (2.5L) (1 of 3). Scheme 115
A/C Clutch Control Diagnosis. Scheme 116
A/C Clutch Control Diagnosis Schematic (2.5L) (2 of 3). Scheme 117
The ECM control of the A/C clutch improves idle quality and performance by delaying clutch application until idle air rate is increased, releasing clutch when idle speed is too low, releasing clutch at wide open throttle, and smoothing cycling of compressor by providing additional fuel at instant clutch is applied.
Turning on A/C supplies circuit No. 459 battery voltage to the clutch control relay and terminal "B8" of ECM connector. After a time delay of about .5 second, the ECM will ground terminal "A4" (circuit No. 458), and close the control relay. A/C compressor clutch will engage.
Note. Test numbers refer to test numbers on diagnostic chart.
- Checks for faulty cycling switch. Solenoids and relays are turned on and off by the ECM, using internal electronic switches (drivers). Each driver is part of a group of 4, called"Quad-Drivers". Failure of one driver can damage any other driver in set.
- Solenoid and relay coil resistance must measure more than 20 ohms. Less resistance will cause early failure of ECM driver. Using an ohmmeter, check coil resistance of A/C relay before replacing ECM.
Before replacing ECM, use ohmmeter to check resistance of each ECM-controlled relay or solenoid. Replace any relay or solenoid that measures less than 20 ohms.
A/C Clutch Control Diagnosis Flow Chart (2.5L) (3 Of 3). Scheme 118
From A/C Clutch Control Diagnosis Chart (1 Of 2). Scheme 119
The ECM control of the A/C clutch improves idle quality and performance by delaying clutch application until idle air rate is increased, releasing clutch when idle speed is too low, releasing clutch at wide open throttle, and smoothing cycling of compressor by providing additional fuel at instant clutch is applied.
Turning on A/C supplies circuit No. 59 battery voltage to the clutch control relay and terminal "B8" of ECM connector. After a time delay of about .5 second, the ECM will ground terminal "A2" (circuit No. 459), and close the control relay. A/C compressor clutch will engage.
Note. Test numbers refer to test numbers on diagnostic chart.
- Checks for low refrigerant as cause for no A/C.
- This and following tests check for faulty A/C control relay.
A/C Clutch Control Diagnosis Flow Chart (2.8L) (1 Of 3). Scheme 120
A/C Clutch Control Diagnosis. Scheme 121
A/C Clutch Control Diagnosis Schematic (2.8L) (2 of 3). Scheme 122
The ECM control of the A/C clutch improves idle quality and performance by delaying clutch application until idle air rate is increased, releasing clutch when idle speed is too low, releasing clutch at wide open throttle, and smoothing cycling of compressor by providing additional fuel at instant clutch is applied.
Turning on A/C supplies circuit No. 59 battery voltage to the clutch control relay and terminal "B8" of ECM connector. After a time delay of about .5 second, the ECM will ground terminal "A2" (circuit No. 459), and close the control relay. A/C compressor clutch will engage.
Note. Test numbers refer to test numbers on diagnostic chart.
Checks for faulty cycling switch. Solenoids and relays are turned on and off by the ECM, using internal electronic switches (drivers). Each driver is part of a group of 4, called "Quad-Drivers". Failure of one driver can damage any other driver in set.
Solenoid and relay coil resistance must measure more than 20 ohms. Less resistance will cause early failure of ECM driver. Using an ohmmeter, check coil resistance of A/C relay before replacing ECM.
Before replacing ECM, use ohmmeter to check resistance of each ECM-controlled relay or solenoid. Replace any relay or solenoid that measures less than 20 ohms.
