CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

1998 Chevy 3500 check engine light

The fact it detects the error without even starting the engine indicates that the ECM is not seeing a valid input from the MAF. Seconded by the lack of MAF data on your scan tool. IMHO, that means one of the following exists;

Bad MAF- ruled out
Bad wiring FROM the MAF to the ECM (the reading of 5 volts at this input is not definitive that the wiring is OK)
Bad ECM input circuitry

If you ohm out the connection from the MAF to the ECM connector pin and it reads OK, I bet you have a dead ECM input and it needs to be fixed/replaced.

BTW- getting snippy isn’t helping win any people over to helping you. If YOU read what you wrote, lots of the detail in your last post is not in any of the prior posts. Typically, you reap what you sow…

P0101 Mass Air Flow System Performance.

One of the first steps in the daignostic test is to check for 12 volts at the MAF from fuse #15.
You stated that there is 5 volts.

By looking at all of the information I can gather the input and output voltage on the MAF is supposed to be 5 volts unless accelerating. The MAF is reading 5 volts from the input and the output of the MAF. Also you get the same at the ECM for the MAF connections (as traced back from the MAF to the ECM). I have not read in any repair manual about testing for 12 volts on any of the systems, let alone checking with a specific fuse in the fuse box/.

I am unable to load an image, you will have to refer to your wiring diagram. The pink wire in cavity C is powered by fuse #15 in the under hood fuse box.

I know you said you “checked the wiring”, but have you determined that the ground wire has continuity with a known good ground (example: negative battery terminal)? This is often overlooked and will certainly cause a lack of communication with the ECM, causing a code to be set before the engine is started. Without doing any actual research, based on what I know about MAF sensors and how they work, I would imagine there has to be a 12V source somewhere going into the sensor to heat the elements (the 5V stuff is reference signals from the ECM), so I would certainly look into what Nevada_545 has said.