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2007 Hyundai Tucson GLS with a 2.0 Intermittent No Start

Crankshaft Sensor Signal Intermittent - No Fault Codes

Today’s subject is a 2007 Hyundai Tucson GLS with a 2.0 4 cylinder engine. It was an auction purchase and brought in by the purchasing dealer for intermittent stalling and not restarting. When the problem was present, the engine would crank but not start. The symptom was very erratic and may not occur for days at a time. Looks like this one needed some patience.

After some trial and error, the shop discovered that the symptom of cranking but not starting would occur at first start-up. They never experienced any stalling, even on multiple road tests. The symptom never lasted very long, maybe 20 seconds or so, and then it would fire up and run fine until the next cold soak. After that cold soak, odds were 50/50 if it would start. These issues can be trying, and a logical stepped approach can yield positive results.

The shop began by monitoring check spark and injector pulse. When it ran, it ran well; it did not feel like engine timing or mechanical issues were at play, the shop relayed to us. They managed to determine that there was no spark but the engine started before they got to injector pulse. Generally we suggest monitoring both at the same time, as well as power to these components to determine what is missing and possible causes.

When removing the connector at the crank sensor for backprobing, they discovered it full of oil. Assuming they found the issue, the sensor was replaced and the vehicle was shipped. It returned a day later, symptom still present.

At this point the shop contacted Autologic for help with the diagnosis. We asked them to connect their ATS four-channel scope to the crank sensor, coil trigger and injector pulse.

 

  • Yellow trace is crank sensor output
  • Red trace is one bank of coil input
  • Green trace is other bank of coil input
  • Blue trace is injector pulse number 1

 

 

You can see by the trace that we are losing crank sensor output. The next step was to have them test if power and ground are present during crankshaft sensor loss. The next capture is of just the crankshaft sensor.

 

  • Yellow trace is crank sensor output
  • Red trace is crank sensor power
  • Green trace is crank sensor ground

 

Here we can see the sensor is losing the power. Now we are getting somewhere. Looking at the wiring diagram, the sensor is powered by a fuse in the engine fuse block. When power was lost, it was also lost at this fuse. Looking further upstream, we found that the main engine relay powers this fuse. Here is what the shop found when they pulled the relay. Plenty of corrosion on the output.

Problem found, excessive voltage drop at the relay terminals. This was quickly confirmed by adding a load to the fuse and measuring voltage drop, we have a 10 volt drop under load. The terminals were repaired and the relay replaced. Vehicle fixed.