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Hi, I'm working on a 1999 Mercedes clk 320. The car had been to another garage for intermittent cutting out and refusing to restart until cold. The garage fitted a new crank sensor but then the car wouldn't start at all. The customer then brought the car to me.
I did some quick checks this evening, the car has no spark or injector pulse . It has voltage at the coils and injectors and also has it reference voltage at the cam sensor.
I put my scope on the crank sensor (it a 2wire magnetic type). And as soon as the ignition is on the voltage reads around 8v (is this a bias voltage?) then when I turn the engine over there is a weak signal with only around 0.3v amplitude and this is all happening around the 8volt mark.
I'm new to using a scope and testing these parts and I'm not sure about what I'm seeing, I'm guessing I should see more amplitude in the signal and suspecting a incorrect or faulty cps sensor. Can anyone please give me some advice here before I order a new sensor?
Thanks
Most 2 wire sensors don't have any voltage to them - but there is a fair amount of variety in this industry and some weird systems around. I've seen a Mercedes with magnets on the crankshaft to add to the pulses. I'll see if I can find a wiring diagram - sometimes components share a common earth and if it loses that earth weird things happen. Perhaps check engine earths.
Getting involved in discussions because I have a lot to learn still.
The Spec on this 2 wire Magneto-type CKP is 7.5-9.0 Volts AC(Can also be checked with a DVOM too.)
It very well may have a bias voltage on it, unplug the sensor with KOEO, check voltage level on either wire, if voltage is present, it is a bias voltage and that will also be the signal wire. The other is sensor ground.
About the scope, change your settings from DC to AC for the channel you are using, you'll see a picture very similar to the one that is posted on this thread.
Yes, you connect the scope to both the wires on the sensor, you'll get full amplitude waveform, connect to an alternate ground and the amplitude of the waveform is decreased.(if I remember correctly about the decreasing part).
In the posted waveform you'll see the large 'gap' 4 times, that is the reference to #1 Cylinder at TDC.
