In Front of the Flywheel - March - 2017

Crank and Cam… How are Your Signals?

Faulty crankshaft and camshaft position sensors can cause a variety of customer complaints. Some of these issues can prevent the engine from starting, cause the engine to stall, turn the MIL on, and even cause improper shifts. And these are only a few of the possibilities. This article is the first in a series about crankshaft and camshaft position sensors.

For many years, crankshaft and camshaft position sensors, as well as other position sensors, have mainly been one of two types: permanent magnet (PM) generators or digital sensors.

PM generators output an analog, AC signal that varies in frequency and amplitude with speed. In today is complex vehicles that incorporate systems such as variable valve timing, accuracy is critical. PM generators tend to be less accurate for measuring position than digital sensors.

Although PM generators are still widely used for measuring speed, digital sensors are used more often on today’s vehicles to measure position. Digital sensors output a DC, digital signal that switches on and off between zero and reference voltage. The reference voltage can be anything, but are usually either 5 or 12 volts.

These digital sensors usually take the form of either a Hall Effect sensor or a magneto-resistive sensor. The difference between the two is the way in which they create their digital, on/ off signal, but the signal is basically identical.

Today we’ll focus on these digital sensors, using a 2000 Dodge Dakota with a 4.7-liter engine. The customer’s complaint was the vehicle would intermittently stick in one gear, stall intermittently, and the MIL was on.

The first step was to retrieve diagnostic trouble codes: There were eight DTCs stored in two modules. Here are the codes we found:

PCM

  • P0320 — No crankshaft reference signal to the PCM
  • P1391 — Intermittent loss of camshaft (CMP) or crankshaft (CKP) signal
  • P0340 — No camshaft signal to the PCM
  • P0700 — EATX controller DTC present

TCM

  • P0725 — Engine speed sensor circuit
  • P0720 — Output speed sensor error
  • P1790 — Fault immediately after shift
  • P0731 — Gear ratio error in 1st gear

When the computer systems have a long list of codes stored, it’s a good idea to step back and look for relationships between the codes. Don’t clear the codes and see what resets yet: First, record all the codes and freeze frame data.

Knowing how the individual codes set can be valuable. Notice the PCM stored code P0320, which could indicate a missing crankshaft (CKP) signal. P1391 could indicate a missing crank signal, but could also be caused by a camshaft (CMP) sensor.

For reference, Chrysler products can often set a crankshaft sensor code when the camshaft sensor is the issue and vice versa. To muddy the waters even more, there’s a P0340 camshaft sensor code. So which sensor could be causing the problem? Moving forward, the P0700 indicated that we should check DTCs in the transmission control module.

The TCM had code P0725 stored for the engine speed signal; that’s taken from the crank sensor. It turns out that the P0720, P1790, and P0731 codes can all set due to discrepancies between the engine speed (CKP) signal and the output speed signal. Diagnosing eight DTCs might seem a little daunting, but with a little thought and research, a pattern began to emerge: All of these DTCs could trace back to a crankshaft or camshaft sensor error. Things were starting to look a little clearer.

A check of available service data indicated that all eight DTCs could trace back to a failing crankshaft sensor. In addition, all of the customer’s complaints could be caused by the same failure.

At this point, a lazy technician would probably have sold the customer both sensors, and there’s a good chance it’d have solved the problem. But that technician would miss out on two things: knowing what the actual failure was, and the pride of diagnosing and repairing the problem properly.

Then again, there’s always the chance that the sensors aren’t at fault and replacing them won’t fix the problem. Then they’d have to start over. There could always be issues with the ground or reference voltage that they both share. The smarter choice is to test and prove the failure.

To test a digital sensor, you’ll need a wiring diagram to determine where to connect your scope leads. On this vehicle, the camshaft sensor signal is located at connector C1, pin 18. Connect channel A (blue) at this location.

The crankshaft sensor signal is located at connector C1, pin 8. Connect channel B (red) here.

You’ll want to make both connections at the PCM to make sure the signal is getting back to it. In this case, the reference voltage to both sensors is 5 volts. So the digital sensor’s signal should probably toggle on and off, between 0 and 5 volts.

Once you have the scope connected and set up properly, start the engine and wait for the failure to show up. The waveform in figure xx shows the signal while the engine was running with no faults present.

For now, both sensors appear to have good clean switching on and off just as you’d expect from a digital sensor. The voltages, 0 volts and 5 volts, also appear to be correct for this vehicle.

This is one of the areas where scopes shine. If you use a voltmeter to measure the crankshaft sensor signal, it’d measure about 2.5 volts. That’s because voltmeters average the changes in the voltage signal. If a digital sensor outputs a 5-volt signal half of the time and 0 volts the other half, the average would be 2.5 volts. The scope reveals the detail you won’t get from a voltmeter.

As the engine continued to run, the waveform started to change, even though the vehicle continued to run correctly. The crankshaft sensor wasn’t pulling reference voltage all the way to ground, or 0 volts. Eventually the crankshaft signal disappeared and the vehicle stalled.

So there was the problem: the waveform provided proof that the crankshaft sensor was failing. This would have never been possible using a scan tool or voltmeter. Replacing the crankshaft sensor resolved all the DTCs and the customer’s complaints.

This particular case study covered a few points:

  • How to test digital sensors.
  • The importance of taking the time to step back and assess the problem before clearing codes.
  • The value of a sound diagnostic plan.
  • The reason to pinpoint the fault before throwing parts at the vehicle.

In the next issue, we’ll continue our diagnostic exploration and move beyond sensor voltages. We’ll address the camshaft and crankshaft position sensors’ relationship to one another, and discover how to expand the tests we’ve already covered.

Is there an engine or electrical diagnostic issue you’d like to see addressed? Let Scott know. Send him an email at scott@driveabilityguys.com and you just may have your question covered in a future issue of GEARS.