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 appropriate VACUUM DIAGRAMS article below to verify routing and connections. Repair as necessary. Inspect air induction system for possible vacuum leaks.
- «VACUUM DIAGRAMS»(ref-11120) (for Dakota)
- «VACUUM DIAGRAMS»(ref-11129) (for Ram Pickup)
- «VACUUM DIAGRAMS»(ref-11130) (for Ram Van/Wagon)
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 and 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 oxygen 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 service port on fuel rail. fuel service 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. Place 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. Perform FUEL PUMP TEST 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 under «FUEL PRESSURE»(/dodge/ram-van-b2500/b-pre-1995-1995/remont/testing-diagnostics/#engine-controls-basic-testing-39l__fuel-pressure) .
- Remove fuel rail test port protective cap. Connect a fuel pressure gauge with a minimum 60-psi (414-kPa) capacity to fuel rail test port. 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, perform diagnostics using DRB. See «TESTS W/CODES - 3.9L»(/dodge/ram-van-b2500/b-pre-1995-1995/remont/testing-diagnostics/#engine-controls-tests-wcodes-39l) article in the ENGINE PERFORMANCE section.
| Application | Psi (kPa) |
|---|---|
| 3.9L | (1) 135-45 (241-310) |
| (1) Fuel pressure taken at engine idle (no load). | |
| (1) | Fuel pressure taken at engine idle (no load). |
FUEL PUMP REGULATED PRESSURE
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 - 3.9L article in the ENGINE PERFORMANCE section.
ASD relay provides battery voltage to alternator field winding, electric fuel pump, fuel injectors, ignition coil and oxygen sensor heating element. PCM controls ASD relay 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.
PCM determines engine speed and synchronizes injectors from distributor pick-up signal. If Crankshaft Position (CKP) signal is not received when ignition switch is in RUN or START position, PCM opens ASD relay ground circuit. ASD relay turns off battery voltage to alternator field winding, electric fuel pump, fuel injectors, ignition coil and O2S 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 - 3.9L article in the ENGINE PERFORMANCE 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 - 3.9L article in the ENGINE PERFORMANCE section.
Fuel pump relay is controlled by PCM through the same ground circuit as ASD relay or through ASD relay. When PCM opens ASD relay ground circuit, 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 - 3.9L»(/dodge/ram-van-b2500/b-pre-1995-1995/remont/testing-diagnostics/#engine-controls-tests-wcodes-39l) article in the ENGINE PERFORMANCE section.
IGNITION COIL RESISTANCE TEST
Inspect coil for signs of arcing. Measure coil primary and secondary resistance. See IGNITION COIL RESISTANCE table. Replace coil if resistance is not within specification. No further information is available.
| Application | Primary | Secondary |
|---|---|---|
| Diamond | 0.97-1.18 | 11,300-15,300 |
| Toyodenso | 0.95-1.20 | 11,300-13,300 |
IGNITION COIL RESISTANCE - Ohms @ 70-80°F (21-27°C)
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 measuring voltage at coil positive terminal.
- If voltage remains near zero during entire period of cranking, see «TESTS W/CODES - 3.9L»(/dodge/ram-van-b2500/b-pre-1995-1995/remont/testing-diagnostics/#engine-controls-tests-wcodes-39l) article in the ENGINE PERFORMANCE 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 - 3.9L»(/dodge/ram-van-b2500/b-pre-1995-1995/remont/testing-diagnostics/#engine-controls-tests-wcodes-39l) article in the ENGINE PERFORMANCE 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 128)and (Scheme 129). Spark should be generated when ground connection is removed.
- 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 128
Scheme 129
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 - 3.9L article in the ENGINE PERFORMANCE section.
SUMMARY
If no faults were found while performing BASIC DIAGNOSTIC PROCEDURES, proceed to TESTS W/CODES - 3.9L article in the ENGINE PERFORMANCE section. If no hard codes are found in self-diagnostics, proceed to appropriate H - TROUBLE SHOOTING - NO CODES article in the ENGINE PERFORMANCE section below for diagnosis by symptom (i.e., ROUGH IDLE, NO-START, etc.) or intermittent diagnostic procedures.
- «TESTS W/O CODES - GASOLINE»(ref-11123) (for Dakota)
- «TESTS W/O CODES - GASOLINE»(ref-11131) (for Ram Pickup)
- «TESTS W/O CODES - GASOLINE»(/dodge/ram-van-b2500/b-pre-1995-1995/remont/testing-diagnostics/#engine-controls-tests-wo-codes-gasoline) (for Ram Van/Wagon)