cam sensor signal voltage low.

Supertech1535

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Hello,

I just bought a 1998 GMC van with 5.7 vortec with 178500 miles. Went to take to town and died in driveway. Ran codes and most was due to it stalling. Would not start back up so I thought maybe it was out of gas since gauge was reading incorrect, added gas no change. i pulled the dog house so i check fuel pressure, getting 55 psi but still not start. I could put some gas down intake it would start…. Cleared all codes but P0340 was coming back.
Charged battery charge came back next day, turned on switch and fuel lump relay kept cycling and fuel gauge was way past full, wasnt like that when i bought it and brought home. So i thought maybe a ground issue and checked all grounds that i could find under the hood and cleaned.
next day tried to start and it fired right up not CEL but still read codes and still p0340 so i on new cam sensor, code still there…
so on the cam connector i checked for 12v and ground. Then next i checked for signal, i am only getting 2.8v. Pulled signal wire and checked was getting 12v when unplugged from ecm.
pulled plug at ecm and probed pin 3 to confirm and its the same. Ran separate wire from cam sensor to ecm and same 2.8 volts.
Is this correct? Bad ECM? Not sure where to go from here.
 

Supertech1535

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I dont have a manual. I assumed other ppl on here has had the same issue and could point me in the right direction.
 

Schurkey

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Some 1998 service manual links posted there.
 

stutaeng

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Is the camshaft position sensor actually required for starting the engine? I once had this code on my 4.3 Vortec. Driving around it intermittently acted like it was about to die (and tach went to zero,) but kept running. Starting was not an issue IIRC. A new cam position sensor did solve it for me.

I thought the the camshaft position sensor was for spark advance only, but not sure if that's true or now.
 

Supertech1535

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Ok so after reading what the manual says, said i should be seeing 5-7v for the signal. on the last step after replacing the sensor if the voltage is not corrected says to start back over at step 2? So to me keep replacing the sensor until it fixes itself??? Not really a fix and does not tell me if there could be something shorted in the ECM pulling the voltage down to 2.8v instead of 5-7v.

Is there something out there that could tell me how to troubleshoot the ECM? I did also check pin 3 on the ECM to ground and i am getting 320K ohms or this normal?
 

Supertech1535

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Read this process again and on "step 16 if problem is not found replace VCM" is the VCM the same as the ECM or computer?
 
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