How to determine if CPS is bad?

COTTONMAN

MEMBER
1999 CHEVROLET SUBURBAN
186,000 MILES

I have a 1999 suburban. 5.7. Truck got to acting up while driving at highway speed. It would shut down for just fractions of a second then take off again. The tach would actually drop and then come right back up. I had an engine light for an O2 sensor that I had not taken the time to replace. One time that it failed with the engine dropping off it actually reset the ecm codes causing me to think that ECM may have been lowing power. It had gotten gradually worse until now it will not start. I am getting other codes now. P0336 and p0339. Crankshaft sensors. I found a test online to test the cps. 12 volt going to sensor. Shows good ground with ignition on. Showing 10 vdc continuous on while turning engine by hand. Should show 5vdc(on) and 0 vdc(off) oscilatiing while turning motor according to web source. The ground suggests that ecm is working properly however 10vdc should be to high for the on signal, correct? Also when I unplug the harness connector to the cps the ground and the signal wire ohms shorted together. CPS, ECM or wiring problem?

4 Replies

RASMATAZ

MECHANIC
75,992 POSTS

Check the wiring back to the computer while connector is removed from the PCM and also -Go to this link: http://www.youtube.com/watch?v=6ws8EVQ6NjE

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Sunday, July 22nd, 2012 AT 11:44 PM

COTTONMAN

MEMBER
3 POSTS

I did not remove the sensor from the engine but I did try to get some of the readings suggested in the video. I could find no resistance between any of the sensor contacts. Everything is reading as an open circuit.
Checking the wires from the ecm is also wierd. I have the 12vdc supply checking good. The other two wires show a dead short to ground and to each other with the power off. With power on (ignition on) they both show a little (around 500k) resistance.