Only 2.5 volts reference to map sensor

[cameronc]

2003 gmc sierra 2500hd. 6.0 auto.

I have an issue I can't figure out. The truck doesn't want to run . It just starts and dies. Throws code p0107 (map sensor circuit low). I changed the sensor with two different ones with no change. I checked the wires between sensor and ecm with multimeter and all wires are good. I only have 2.5 volts reference coming out of ecm to gray wire on map. I changed the ecm and the voltage is the same. I don't know what to do now.

[tbarn]

Only 3 components for the map sensor. PCM, sensor and wires. Did you check the voltage at the ECM? Conn 1 term 48

[sefiroxx]

Measure the reference signal using a good ground. On 01's, it should be 5v

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[cameronc]

Yes I checked it at the wire where the pin goes in ecm. Only 2.5 with two different ecm got the same reading. All other components are getting full 5 volts. 

[sefiroxx]

At the wire where the pin goes into the ecm? It should be checked at the MAP end of the harness first. Only one of the 3 wires should be live (key on). If voltage found on other wires, then likely a short. If not 5v, then either a short or bad ecm. Check pin at ecm to confirm 5v.

When ever checking v, check against a good ground , ie battery negative. Do not check against supposed ground on wire harness. Once volts confirmed on wire, then check against supposed ground wire to confirm that ground connection is not a problem.

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[cameronc]

My tuner led me to the problem. The oil pressure sending unit shorted out and grounded that side of the ecm. Unplugged the oil pressure sending unit and immediately got my 5 volts back to map. Runs like a top. Thanks for the help guys. Another one solved.