If your car cranks and cranks but refuses to start, it’s a frustrating situation many drivers encounter. While your first instinct might be to reach for a free OBDII trouble shooting code reader to diagnose the problem, sometimes the issue lies deeper than what these devices can immediately detect. One very common culprit for this no-start condition is a failing Crankshaft Position Sensor (CPS).
The CPS is a critical component that tells your engine’s computer the position and speed of the crankshaft. Without this information, the computer can’t properly time the spark and fuel injection, leaving your engine unable to ignite and run. What’s particularly tricky about CPS failures is they can often occur without any prior warning signs. Your car might be running perfectly one minute and then simply refuse to start the next.
Here are some common symptoms that might indicate a failing CPS:
- Engine cranks but doesn’t start.
- Fuel and voltage gauges may behave erratically or not display correctly.
- “No Bus” message may appear on the odometer after a short period (30-60 seconds).
- Absence of spark at the spark plugs.
- Fuel pressure is normal.
Crucially, a failed CPS can sometimes prevent your free OBDII trouble shooting code reader from even connecting to the car’s computer. In other cases, while a connection might be established, the reader may not display any relevant diagnostic trouble codes. This is because the CPS failure itself can disrupt the communication needed for the OBDII system to function correctly.
So, how can you diagnose a potential CPS issue if your free OBDII trouble shooting code reader is not providing answers? Here are some diagnostic steps you can take:
Initial Checks:
- Voltage and Fuel Gauge Test: A simple preliminary check involves unplugging the CPS connector and turning the ignition key to the “on” position (without starting). If the voltage and fuel gauges now display correctly, this strongly suggests a faulty CPS.
- Relay Swap: Exchange the fuel pump relay and the ASD (Automatic Shutdown) relay with other identical relays in your car’s Power Distribution Center (PDC). This eliminates the relays as a potential cause of the no-start.
- Neutral Safety Switch (NSS) Check: Try starting the car while wiggling the gear shift lever, or attempt to start in Neutral. Check if the reverse lights come on when the shifter is in Reverse. This helps rule out a faulty NSS.
- Basic System Troubleshooting: Inspect and clean all battery terminals and connections to the starter and alternator. Look for any signs of corrosion or damage to cables and replace if necessary. Also, check the engine and battery ground connections to the car’s frame.
CPS Specific Testing:
If the initial checks don’t pinpoint the problem, you’ll need to test the CPS directly. The testing procedure varies slightly depending on your engine year.
For 1991 – 2001 4.0L H.O. engines:
- Locate the CPS connector near the rear of the intake manifold and disconnect it from the main wiring harness.
- Set an ohmmeter to the 1K-to-10K scale.
- Place the ohmmeter probes across terminals B and C of the CPS connector.
Crankshaft Position Sensor Connector Diagram for engine testing.
- The meter should read “open” (infinite resistance). If you get a low resistance reading, the CPS is likely faulty and needs replacement.
For 1987 – 1990 4.0 L engines:
Test #1 (Resistance Test):
- Set your multimeter to read resistance (Ohms) in the 0-500 Ohm range.
- Disconnect the CPS connector.
- Measure the resistance across terminals A and B of the CPS connector.
- The reading should be between 125 – 275 Ohms. If the reading is outside this range, replace the CPS.
Test #2 (AC Voltage Output Test):
- Set your multimeter to read AC voltage in the 0-5 AC Volts range.
- Connect the multimeter probes across the CPS connector terminals.
- Have a helper crank the engine.
- The meter should show a reading of 0.5 – 0.8 VAC (Volts AC). If the reading is below 0.5 VAC, the CPS is likely faulty and needs replacement.
Conclusion:
While a free OBDII trouble shooting code reader is a valuable tool for diagnosing many car problems, it’s not a universal solution. When faced with a “cranks but won’t start” situation, especially if your OBDII reader is not providing helpful codes or connecting at all, consider the Crankshaft Position Sensor as a prime suspect. By following the diagnostic steps and testing procedures outlined above, you can effectively determine if a faulty CPS is the cause of your no-start issue and take the necessary steps to get your vehicle back on the road.
