That moment when your car cranks but won't start, or suddenly dies at a red light - been there. You'll immediately suspect the crank sensor because it's the brain telling your engine when to fire. But before replacing it blindly, let's learn how to test a crankshaft position sensor properly. I've tested dozens of these over the years, from Hondas to Fords, and learned some tricks the hard way.
Why Your Crankshaft Position Sensor Might Be Failing
These sensors don't fail often, but when they do, it's dramatic. They live in a harsh environment - oil leaks, heat, vibrations. I've seen more failures after engine work where someone accidentally nicked the wiring. Common culprits:
- Heat damage from exhaust proximity
- Oil contamination (especially on sensors near timing cover)
- Corroded connectors from water splash
- Physical damage during belt replacement
- Internal component failure after 100k+ miles
Watch this: On some Toyotas, the sensor gaps get clogged with metal shavings from worn timing chains. Cleaning the tip often fixes it temporarily.
Crankshaft Position Sensor Failure Symptoms
Sometimes it's obvious, sometimes it mimics other problems. Last Tuesday, a customer swore his Dodge needed a fuel pump. Turned out to be crank sensor. Here's what to look for:
| Symptom | What It Feels Like | Probability Link to CPS |
|---|---|---|
| Extended cranking | Engine turns over 5+ seconds before starting | High |
| Stalling | Engine dies randomly at stops | Very High |
| Misfiring | Jerking acceleration, rough idle | Medium |
| No-start condition | Engine cranks but never fires | Extreme |
| Check engine light | Usually P0335-P0339 codes | Certain |
When Symptoms Get Weird
On my buddy's Chevy Silverado, the sensor failed only during rain. Water seeped into the connector. Moral? Check the wiring harness!
Tools You'll Actually Need
Forget fancy labs. Here's what works in real garages:
| Tool | Purpose | Budget Option | Cost Estimate |
|---|---|---|---|
| Digital Multimeter | Resistance/voltage tests | Harbor Freight $7 model | $7-$50 |
| Basic OBD2 Scanner | Reading trouble codes | BlueDriver Bluetooth | $70-$100 |
| Test Light | Checking power/ground | Parts store special | $5 |
| Wire Pins | Back-probing connectors | Paper clips (emergencies) | $2 |
I won't lie - a $400 oscilloscope gives definitive results. But if you're learning how do I test a crankshaft position sensor without breaking the bank, skip it.
Step-by-Step Testing Procedures
Safety first: Disconnect battery negative before touching sensors. Now let's get dirty.
Locating Your Sensor
This varies wildly. On Nissans it's often behind the harmonic balancer. BMWs? Under intake manifold. General locations:
- Front timing cover (common on GM)
- Bellhousing area (Ford, Chrysler)
- Near crankshaft pulley (Honda)
- Side of engine block (Subaru)
Pro tip: Google "[your car year/make/model] crankshaft position sensor location" for diagrams. Chilton manuals work too.
Visual Inspection First
Don't skip this! 30% of "failures" are just bad connections. Check:
- Cracked sensor body
- Oil contamination on tip
- Pins bent/corroded in connector
- Chafed wires near heat sources
Spray electrical cleaner into connectors. Many times this solves intermittent issues immediately.
Resistance Test (Ohmmeter Method)
Best for magnetic sensors found in older cars. Unplug sensor. Set multimeter to Ω (ohms).
| Step | Action | Expected Reading |
|---|---|---|
| 1 | Touch probes to sensor terminals | 200-2000 ohms (consult spec) |
| 2 | Check for open circuit | OL/infinite = bad sensor |
| 3 | Check for short to ground | Touch one probe to body ground. Should read OL |
Caveat: Hall-effect sensors won't show resistance. You'll need voltage tests.
AC Voltage Test (Dynamic Test)
Works while engine cranks. Requires helper.
- Back-probe signal wire with multimeter set to AC volts
- Ground black probe to battery negative
- Have assistant crank engine
- Observe voltage: Should pulse 0.5V-5V AC
No pulses? Sensor dead. Low pulses? Check gap alignment.
Power/Ground Verification
Many overlook this. With ignition ON (engine off):
- Test light between power wire and ground: Should light
- Multimeter between ground wire and battery negative: Less than 0.5V
- Reference voltage (if applicable): Usually 5V or 12V
On my project Civic, the ground wire looked fine but had 3.8V resistance. Fixed the wire, fixed the car.
Advanced Testing Techniques
Oscilloscope Waveform Analysis
The gold standard. Shows actual signal pattern. What to look for:
Good Waveform
- Clean, uniform sine waves (magnetic)
- Square digital pulses (Hall)
- Consistent amplitude
Bad Waveform
- Missing pulses
- Amplitude drops
- Erratic patterns
Using OBD2 Live Data
Monitor RPM readings while cranking. If scanner shows 0 RPM during crank, CPS isn't sending signal.
Interpreting Test Results
So you tested - now what? Decision chart:
| Test Result | Likely Issue | Next Action |
|---|---|---|
| Good resistance + no AC signal | Gap too wide or reluctor ring damage | Check gap (0.5-1.5mm typical) |
| Intermittent signal | Damaged wiring or bad connector | Wiggle test harness |
| No power/ground | Wiring harness or ECM fault | Trace circuit diagram |
| All tests pass but problem persists | Related component failure | Check cam sensor synchronization |
Installation Tips If Replacing
If tests confirm failure, replacement tips:
- Clean mounting surface thoroughly
- Use correct gap tool (business card thickness often works)
- Apply dielectric grease to connector
- Torque bolts to spec (over-tightening cracks sensors)
Aftermarket sensors have high failure rates. Stick with OEM for critical sensors like this. I learned this after three replacements on a Jeep.
Crankshaft Sensor Testing FAQ
Can I test the crankshaft position sensor without removing it?
Absolutely. Back-probing the connector gives access for all electrical tests. Physical inspection requires partial removal sometimes.
Will a bad crank sensor throw a code?
Usually P0335-P0339, but not always. Intermittent failures might not store codes. That's why live testing is key.
How do I test a crankshaft position sensor with just a test light?
Limited but possible: Connect test light between signal wire and ground while cranking. Should flicker if working. No flicker = no signal.
Why does my sensor test good but car still won't start?
Check timing belt/chain! Broken belts or jumped timing mimic CPS failure. Also verify fuel pressure and spark.
Is testing a camshaft position sensor the same process?
Very similar. Same principles apply - resistance, voltage, waveform. But location and values differ.
Personal Testing Nightmares
My worst experience? A '04 Silverado with P0335. Replaced sensor twice. Turned out the reluctor wheel had a cracked tooth, only visible after removing transmission. Moral: Verify tone ring condition if all else fails.
Another time, aftermarket sensor connectors didn't match OEM wiring. Had to repin the whole connector. Now I always compare old/new parts before installing.
When to Call a Professional
If you've tried testing and still have issues:
- No-start with confirmed good sensor
- Hidden wiring harness damage
- Need oscilloscope diagnostics
- Timing chain/belt inspection required
Diagnostic shops usually charge $80-$120 for full CPS testing. Cheaper than guessing with parts.
Wrapping Up
Learning how to test a crankshaft position sensor saves money and prevents misdiagnosis. Start simple - check codes, inspect wiring, do basic voltage tests. Remember that testing a crankshaft position sensor isn't magic, just methodical. Got questions? Hit me below.