Crank Sensor Diagnosis

[ZVOLV]

If I wanted to test a CKP sensor on my 1990 240, how would I do it?

I have a multimeter. What values would I look for?

[propav8r]

Does your meter read frequency?

[sbabbs]

https://www.volvoclub.org.uk/faq/Eng...stition_Sensor

[Response] Measure the signal while the sensor is still installed by unplugging the sensor at the cable connections at the firewall to the drivers side of the engine. Measure voltage here as the engine is being cranked. Use a low voltage setting on a digital volt meter, preferably in an alternating current setting if available in a low voltage setting. If no signal is detected, the unit is suspect. The other simple measurement is to unplug the unit (without removing it from its location behind the engine on top of the transmission housing). Check all 3 terminals for resistance or an open circuit. The new sensor measured about 160 ohms between the center lug and a side lug (red and blue wires). The black wire is the grounded shielding wire. My old defective unit had no continuity and no measurable resistance.

Other thing is while cranking watch the tach and if it jumps then it's working, if not then bad. Also you could check the codes. IF it's bad the codes will say that.

[Khrrck]

Don't you have a fancy labscope that you could see the teeth on if the sensor was good?

[turbotankshane]

Originally Posted by Khrrck

Don't you have a fancy labscope that you could see the teeth on if the sensor was good?

We all get a little ahead of ourselves sometimes when we learn something cool, no need to be like that now.. I remember when I had just learned how to do alignments and tried to tell someone to set toe on the rear axle of a 240 cus I thought I was hot ****

[ZVOLV]

Originally Posted by sbabbs

[Response]...digital volt meter...alternating current setting

How many AC volts should one see?

[sbabbs]

Not much, but something. Remember the sensor is just a magnet..

[Khrrck]

Originally Posted by turbotankshane

We all get a little ahead of ourselves sometimes when we learn something cool, no need to be like that now.. I remember when I had just learned how to do alignments and tried to tell someone to set toe on the rear axle of a 240 cus I thought I was hot ****

Nah I was actually being serious, if he has the equipment he could use it and it seems like it would give more info.

I'm in no position to make fun of people ;)

[ZVOLV]

I got about 0.7V AC cranking and about 1.3V AC idling. Voltmeter averaged-out the signal for me.

[cleanflametrap]

Originally Posted by ZVOLV

I got about 0.7V AC cranking and about 1.3V AC idling. Voltmeter averaged-out the signal for me.

Sounds OK to me. You've got a good baseline. Just try to stick with the same meter and technique for connecting it when you do it next time.

The reasons this method of verifying the crank angle signal is not in the books might be:

1) An AC voltmeter was not standard fare in an automotive repair shop ca. 1989.
2) Those digital multimeters we have now vary considerably in how accurately they measure signals that are not strictly sinusoidal.
3) Test leads and connection technique can introduce unwanted signal and noise near the same level, for example from the injector flyback or secondary ignition wiring, leading to ambiguous conclusions.

Your lab scope would show you something like these traces, but every lousy CPS I've had will out itself by failing to show the reasonable coil resistance when checked with the ohmmeter - when I've been able to get it to fail in a solid manner.

The failures are internal connections between the fine enameled wire wound on a bobbin over the magnet and the flexible stranded cable assembly. The stresses of potting and its ability to hold up across time and temperature probably account for the infant mortality and out of box failures, and the cable sheathing break that conducts water into the core to swell it with rust accounts for most of the long term failures.





I took these scope pictures when I was younger and more ignorant. In the photo, I blamed the signal variation entirely on tone ring runout, but it is mostly due to the instantaneous angular velocity of the crank as indicated by the wave period of the slots or windows. That's basically why your idling voltage was twice that for cranking -- rpm.



Above is a good resistance reading using single-pin sockets pushed over the ICU pins in order to ensure all the cabling connections are included. That way I can give the cables and connectors a good shake and watch for a change in the resistance reading.



Extreme measures here to catch and prove an intermittent failure. The cable was tapped in order to leave the meter in the car while it was being driven, so the act of connecting the meter would not disturb a flaky connection.



Resistance reading is OK now. Remember the resistance will vary with the temperature of the sensor's winding. I've seen it as low as the 160 reported by sbabbs, I think, and over 200 as well.



But 922 is way out of line. If you see it well above or below 180, give the cabling a shake. This particular one responded by going completely open when shook at the sensor end, and it was only a year old.

[ZVOLV]

I have just repeated my test. Center wire at CKP connector. Red/yellow wire.


-0.4vAC Cranking (ignition disabled)

-1.3vAC Running at idle

-4.1vAC at 3000 rpms

[ZVOLV]

Originally Posted by propav8r

Does your meter read frequency?

Yes. What should the reading be?

[ZVOLV]

https://www.picoauto.com/library/aut...e-guided-tests

[ZVOLV]

Originally Posted by propav8r

Does your meter read frequency?

Yes.

[cleanflametrap]

Nicely done video. That's rare.

[ZVOLV]

That was with my "custom" 10 foot long test leads I put together with 20/22 guage wire. I backprobed center red/yel wire underhood at connector.

I went for a drive after making that idle video and got about 3000hZ at 3000 rpms. I see a pattern!

Then coming to a stop it would surge and stall. I disconnected test leads and problem went away. The ECU was getting grouchy so I stopped tampering with it and haven't had a problem since.