INTRODUCTION
| CAUTION | When battery is disconnected, vehicle computer and memory systems may lose memory data. Driveability problems may exist until computer systems have completed a relearn cycle. See COMPUTER RELEARN PROCEDURES article in GENERAL INFORMATION section before disconnecting battery. |
Following diagnostic steps can help prevent overlooking a simple problem. Also use this article to begin diagnosis for a no-start condition. First step in diagnosing any driveability problem is verifying customer's complaint with a test drive under conditions during which problem reportedly occurs.
Before entering self-diagnostics, perform a careful and complete visual inspection. Most driveability or no-start problems are not related to computerized engine control systems, but are in fact simple mechanical, electrical, fuel or vacuum related faults. Most engine control problems are results of mechanical breakdowns, poor electrical connections or damaged or misrouted vacuum hoses. Before condemning computerized engine control system, perform each test listed in this article.
Note. All voltage tests should be performed with a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, unless stated otherwise in testing procedures.
VISUAL INSPECTION
Perform a visual inspection of all electrical wirings. Look for chafed, stretched, cut or pinched wiring. Inspect electrical connectors and connections for tight fit and corrosion. Repair as necessary. Inspect all vacuum hoses for proper routing, cuts or pinches. If necessary, see VACUUM DIAGRAMS article in this section to verify routing and connections. Repair as necessary. Inspect air induction system for possible vacuum leaks.
MECHANICAL INSPECTION
| CAUTION | Use a remote starter to crank engine during compression test; DO NOT use ignition switch. Fuel injectors on many models are triggered by ignition switch during engine cranking. This could create a fire hazard or cause engine flooding, crankcase contamination, hydrostatic lock or lubrication to be washed from cylinder walls. |
Compression
Check engine mechanical condition using a compression gauge, vacuum gauge, or an engine analyzer capable of performing a relative compression test. See engine analyzer instruction manual for availability and description of relative compression feature.
Check engine compression with engine at normal operating temperature at specified cranking speed, all spark plugs removed and throttle wide open.
| Application | Specification |
|---|---|
| Compression Ratio | 9.1:1 |
| Minimum Compression Pressure | (1) 100 psi (690 kPa) |
| (1) Maximum difference between each cylinder is 25 psi (1172 kPa). | |
| (1) | Maximum difference between each cylinder is 25 psi (1172 kPa). |
COMPRESSION SPECIFICATIONS
Exhaust System Backpressure
Check exhaust system using a vacuum gauge or pressure gauge. Remove O2 sensor or air injection check valve (if equipped). Connect a 1 - 5 psi (6.9 - 34.5 kPa) pressure gauge, and run engine at 2500 RPM. If exhaust system backpressure is greater than 1.75 - 2 psi (12.0 - 13.8 kPa), exhaust system or catalytic converter is plugged.
If using a vacuum gauge, connect vacuum gauge hose to intake manifold vacuum port and start engine. Observe vacuum gauge. Open throttle part way and hold steady. If vacuum gauge reading slowly drops after stabilizing, check exhaust system for restriction.
FUEL PRESSURE
| WARNING | Always relieve fuel pressure before trying to open system for testing or component replacement. High fuel pressure may be present in fuel lines and components. DO NOT allow fuel to contact engine or electrical parts while testing fuel system components. |
Fuel Pressure Release
- Disconnect negative battery cable. Slowly open fuel cap to release fuel tank pressure. Remove protective cap from pressure test port on fuel rail. On 5.2L, fuel pressure test port is located at rear of engine. CAUTION: To minimize fuel spillage, wrap a shop towel around and under pressure test port.
- Use Fuel Pressure Gauge/Hose (C-4799), with gauge removed, with gauge end of hose placed into approved gasoline container. To release pressure, screw other end of hose onto pressure test port. After pressure is released, remove hose from test port and continue fuel system service. When servicing is completed, install protective cap on fuel rail test port.
| WARNING | Before disconnecting any fuel line, or fuel system component during testing, repeat FUEL PRESSURE RELEASE procedure above. Perform FUEL PUMP PRESSURE TEST below with fuel tank at least half full. |
Fuel Pump Pressure Test
- Begin basic diagnosis of fuel system by determining fuel system pressure. Release fuel pressure. See FUEL PRESSURE RELEASE above.
- Remove fuel rail test port protective cap. Connect Fuel Pressure Gauge (C-4799) to fuel rail test port. On 5.2L, test port is located at rear of engine. Start engine and allow to idle. If pressure is more than 45 psi (310 kPa), fuel pump is okay. Replace fuel filter/fuel pressure regulator assembly located on top of fuel pump module. If pressure is zero psi, use DRB diagnostics. See «TESTS W/CODES - 5.2L»(/jeep/grand-cherokee/zj-1992-1996/remont/testing-diagnostics/#engine-controls-tests-wcodes-52l) article in this section.
| Application | Pressure - psi (kPa) |
|---|---|
| 5.2L | 35-45 (241-310) |
| (1) Fuel pressure taken at engine idle (no load). | |
| (1) | Fuel pressure taken at engine idle (no load). |
FUEL PUMP REGULATED PRESSURE (1)
AUTO SHUTDOWN (ASD) RELAY
Note. If ASD relay is suspected of causing a fuel or ignition system malfunction, use Diagnostic Readout Box (DRB) to diagnose ASD relay. See TESTS W/CODES - 5.2L article in this section.
Auto Shutdown (ASD) relay provides battery voltage to alternator field winding, electric fuel pump, fuel injectors, ignition coil and O2S sensor heating element. ASD relay is PCM controlled by switching ASD relay ground circuit on and off. ASD relay is located inside Power Distribution Center (PDC) which is located in engine compartment. See label on PDC cover for relay and fuse identification.
On 5.2L, PCM determines engine speed and synchronizes injectors from distributor pick-up signal. When distributor pick-up signal is not received when ignition switch is in RUN or START position, PCM turns ASD relay ground circuit off. ASD relay turns off battery voltage to alternator field winding, electric fuel pump, fuel injectors, ignition coil and O2S sensor heating element.
If ASD relay has a suspected malfunction, replace with a known good part and retest. If malfunction is still present, see TESTS W/CODES - 5.2L article in this section.
FUEL PUMP RELAY
Note. If fuel pump relay is suspected of causing a fuel or ignition system malfunction, use Diagnostic Readout Box (DRB) to diagnose pump relay. See TESTS W/CODES - 5.2L article in this section.
Fuel pump relay is controlled by PCM through the same ground circuit as ASD relay or through ASD relay. When PCM turns ASD relay ground circuit off, fuel pump relay ground circuit is also turned off. See AUTO SHUTDOWN (ASD) RELAY .
TESTING SPARK AT COIL
- Remove coil wire from distributor cap. Hold end of cable approximately 1/2" (13 mm) from engine ground. Crank engine and check for spark from coil wire. Spark should be present and constant.
- If spark is present and constant, continue cranking engine. Slowly move coil wire away from ground. As coil wire is moved away from ground, check for arching at coil tower. If arching at coil tower is present, replace coil.
- If spark is good and no arching is present, secondary ignition system is producing necessary voltage. Ensure spark reaches spark plugs by checking condition of rotor, cap, spark plug wires and spark plugs. Repair or replace secondary ignition components as necessary. If ignition system components are in good condition, ignition system is okay. If vehicle still fails to start, proceed to NO-START TEST. Also refer to «TESTS W/CODES - 5.2L»(/jeep/grand-cherokee/zj-1992-1996/remont/testing-diagnostics/#engine-controls-tests-wcodes-52l) article in this section.
Note. Ignition components may be damaged if test is performed with more than 1/4" clearance between wire and engine ground.
IGNITION COIL RESISTANCE TEST
- Inspect coil for arcing. Measure coil primary and secondary resistance. See «IGNITION COIL RESISTANCE»(/jeep/grand-cherokee/zj-1992-1996/remont/testing-diagnostics/#engine-controls-basic-testing-52l) table. Replace coil if resistance is not within specification. NOTE: Each coil tower is labeled with corresponding cylinder number.
- Remove spark plug wires from coil one at a time. Measure secondary resistance of ignition coil between grouped cylinders and coil towers. Replace coil if resistance is not within specification. See «IGNITION COIL RESISTANCE»(/jeep/grand-cherokee/zj-1992-1996/remont/testing-diagnostics/#engine-controls-basic-testing-52l) table. If ignition system components are in good condition, ignition system is okay. If vehicle still fails to start, proceed to NO-START TEST.
| Application | Primary | Secondary |
|---|---|---|
| Diamond | 0.97-1.18 | 11,300-15,300 |
| Toyodenso | 0.95-1.20 | 11,300-13,300 |
IGNITION COIL RESISTANCE
NO-START TEST
Note. Perform TESTING SPARK AT COIL before proceeding with NO-START TEST.
- Unplug coil connector and coil. Connect a pair of jumper wires between coil and coil connector. Ensure sufficient battery voltage (12.4 volts) is available to crank engine and operate ignition systems. Crank engine for 5 seconds while monitoring voltage at coil positive terminal.
- If voltage remains near zero during entire period of cranking, see «TESTS W/CODES - 5.2L»(/jeep/grand-cherokee/zj-1992-1996/remont/testing-diagnostics/#engine-controls-tests-wcodes-52l) article in this section to check PCM and ASD relay. If voltage is near battery voltage and drops to zero after 1-2 seconds of cranking, see «TESTS W/CODES - 5.2L»(/jeep/grand-cherokee/zj-1992-1996/remont/testing-diagnostics/#engine-controls-tests-wcodes-52l) article in this section to check PCM circuit.
- If voltage remains near battery voltage during entire 5 seconds, turn ignition switch to OFF position. Disconnect PCM 60-pin connector. Check 60-pin connector for loose terminals (push-outs). If connector is okay, go to next step.
- Remove positive lead from coil. Connect a jumper wire between positive battery terminal and coil positive terminal. Using a specially constructed jumper wire, momentarily ground PCM harness connector terminal No. 19. (Scheme 72)and (Scheme 73). Spark should be generated when ground connection is disconnected.
- If spark is generated, replace PCM. If no spark is generated, use special jumper wire directly on coil negative terminal. If spark is produced, repair wiring harness for an open circuit. If no spark is produced, replace ignition coil.
Scheme 72
Scheme 73
IDLE SPEED & IGNITION TIMING
Ensure idle speed and ignition timing are set to specification. Always refer to emission control information label in engine compartment before servicing. If manual and emission label specifications differ, always use specifications on emission label. For adjustment procedures, see ADJUSTMENTS article in this section.
SUMMARY
If no faults were found while performing BASIC TESTING procedures, proceed to TESTS W/CODES - 5.2L article in this section. If no hard codes are found in self-diagnostics, proceed to the TESTS W/O CODES article in this section for diagnosis by symptom (i.e., ROUGH IDLE, NO-START, etc.) or intermittent diagnostic procedures.