C

cookm

New Member
Hi everyone,

Whilst investigating an uneven idle in my L seies Freelander my attention was drawn to the CPS. How to test it though? Then I came across a free program on the internet which turns your laptop into an oscilloscope..!

I downloaded it, then made up a lead out of an old headphone cable and voila! The resulting CPS waveform can be seen below:-

scope.jpg


If there is enough interest in this thread then I will publish more detail...:D

Martin
 
OK, more detail has been requested, so here goes:

The free "Scope" software actually uses your computer's sound card as a (albeit limited) kind of oscilloscope, the input being the Mic socket of your computer. This means, of course, that the bandwidth of the scope is only from about 20Hz to around 20khz, but for engine diagnostics this is not a problem.

Next step is to make up a cable. I used an old headphone cable, cut off the left and right earpieces and soldered a crocodile clip to each "signal" wire of the Left and Right cables. With the earth screen/braiding part of the L and R parts of this cable, I joined them together and soldered another croc clip (for the earth) making 3 clips in total.

Then I identified the CPS signal wire from the car's wiring loom (I found it easiest to pull back the corrugated wiring conduit right next to the ECU). In my Freelander it was a Light Green wire with a Grey stripe. Then I pushed a dressmakers pin through the insulation so that it made contact with the copper wire inside. I could then connect one croc clip to this pin and the earth croc clip to the metal screen wire which was near the CPS wire (or you could simply connect to battery negative).

Then I plugged this cable into the Mic input of my laptop running "Scope" and after playing about with the Amplitude and Time settings of the program, I got the results shown in my photo above. You can also click on the "Measure" button and it will tell you the frequency of the waveform. So in my example the CPS output is showing the car idling at 62.731Hz (i.e. 62.731 revs per second), which equates to 3763.86 RPM..! Ah, but the Freelander has 4 pickups in the flywheel so you need to divide this figure by 4 giving an idle speed of 940.965 RPM...much more credible! :D

Hope this all makes sense, but don't hesitate to say if you need more clarification...

Martin
 
That is a useful program. The wavefore shows distortion due to the AC nature of the MIC input on the PC. However, the pulses are clear enough to see the sensor is working, more or less. I say more or less because you can not determine the pulse width from this trace.

One thing that is interesting is the frequency. At the top of the screen it shows a frequency of approximately 62Hz. Roughly from the pulse positions on the trace I guesstimate the period to be 18mS and that gives a frequency of 55Hz. Therefore the 62Hz value is good. According to RAVE there are four equally spaced magnets on the flywheel that the CPS picks up. On that basis the crank is spinning at 62/4*60 = 930RPM. Sounds about right for a cool engine.

Therefore this is a good find and is useful. I do have a real scope but its an expensive alternative to this.

Good find that man!! :D
 
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