Which wire to probe on crank sensor for lab scope?

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I'm working on a 92 Buick w/3.8 and trying to figure out why there is no RPM. The engine has no spark, fuel injector pulse or indicated rpm on the scanner. I want to verify the crank sensor operation. On the schematic, there are two wires, each coming from a buffer transformer, a ground and a 10volt reference (in the ICM). At the plug, I have two wires at about 6 volts, one at 12ish volts an one with continuity to ground. Which wire do I need to probe for the scope? Also, would this be the 18X or the 3X sensor (and what is the difference/how did they come about calling them 3x/18x)?
 
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I think to test the 18X CKP sensor you connect your red scope test lead to the Yellow wire or G terminal at the ICM connector and your black test lead to negative battery, and to test the 3X CKP sensor you connect red test lead to the light blue with white stripe wire or H terminal and black test lead to negative battery. I'm not totally sure, I would need to look at the service information. 3X and 18X are the numer of sync signals the sensors provide for each engine revolution. Let me see if I can find this website that has some info.
 

The_Eric

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I think I've found it. You were pretty close - dk blue/wht and lt blue/wht...
 
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Cool I was going to say the colors might differ but I was pretty sure terminal G and H were correct and once you have that you can determine the wire color. Lt blue and dk blue makes sense. Are you getting return pulse from the 3X and 18X?
 

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No, I am not. It came in like I described above. The kid had already put on a bone yard ICM and a crank sensor,but he damaged the wiring harness and the crank sensor. I put on a new harness end - still no RPM. That's when I inspected the sensor and found it damaged. Replaced it and still no RPM, spark or injector pulse. The cam and crank harness that goes to the ICM is good and the ICM (both the bone yard piece and the original) tested out fine. I've also cleaned the ICM and mounting bracket to get rid of the corrosion and also checked the ground wires that connect at the mount bracket. I know this is not an ECM issue, because it has no control over the spark and rpm when cranking. Those duties lie solely on the crank sensor and ICM. I suppose it's possible I damaged it on install, so that will be my next stop. One last thing, maybe worth noting, is that the speedo and tach are constantly wrapped around past max... I wonder if this has any bearing on the situation?
 
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Do you have 12V at terminal P pink wire of the ICM connector with key ON, and ground at I think it is terminal L blk/red stripe. And do you have 10-12 V at terminal N that powers the cam and crank sensors? If so and you still have no signal from the 3x or 18X at G or H, and the crank sensor is ruled out I'd assume there was a problem in the wire harness to the crank sensor. I agree with what you said the ECM should have no bearing at cranking speed and it's all about the ICM and crank sensor. If the ICM and crank sensor are ruled out, you'd have to assume there is a problem in the wiring, right?
 

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With koeo, I've got 12v on the pnk/blk wire. I haven't checked if it's got power while cranking - I'll do that soon. I have inspected the whole harness from the crank sensor - in fact, I removed it and took it out the tubing. So, if I've got power while koec, then I'll revisit the crank sensor. Actually, I don't know why I didn't check it sooner?
 

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Well, it turns out that this one is partly on me. When I repaired the harness, I wired it in backwards from the way it should have been. I think that I probably had one connector turned over when I was cutting the wires (I stagger them). Oh well, the car is running now...
 
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It happens. So what was the final verdict on why there was no CKP sensor output signal? Was it because the owner hacked up the harness in the process of replacing the crank sensor that you mentioned?
 

The_Eric

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I believe so. In addition to messing up the harness end, he also messed up the sensor. So after repairing the end (incorrectly) and replacing the crank sensor, it should have ran, but didn't due to me. Oh well... Like you said, it happens.
 
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