Sensor ground at MAS question.

[gnasty]

How are you doing? I have a question about the sensor ground at the MAS. A little history about the car. I purchased a 2G MAS to install in the car with a plug and play harness adapter. After installing the sensor, the car would not idle and triggered a CEL code 12 (mass air flow sensor) I checked the wiring for the adapter harness, and all was good. I then installed my factory MAS and the car runs fine and no codes are triggered. I have a good feeling the 2g MAS i purchased is not a good unit.

Now, to my question. While trying to diagnose my code 12, I checked for my 5 volt reference signal, and ground at the MAS plug. With the key in the off position, I have a good ground at pin 6 (green/black wire) at MAS. When the key is turned to the on position, the ground is lost. Why is the ground lost when the key is turned on? I checked a friends car and his ground signal is still in tact even with the key in the on position. The only difference between our cars, is that mine is Automatic. I do realise if I had a problem I would get a CEL, but I want to understand how the circuit works. Is the ground circuit for the automatic different?



Sorry for the long post, and thanks for your help.

Kyle

 

[spoonman]

Check all of the wiring on the plugs maybe something came out. Did you reset your air fuel ratio? I notice on your profile you have a SAFC, a 2g MAS on a 1g car that is not tuned will run lean and idle erratically.

 

[gnasty]

Spoonman,

Thanks for the response. I have checked and double checked the wiring and everything is in tact and wired correctly. I did reset the fuel trims also. The funny thing is, after the 2g install, the car was runing rich. It pegged my wideband at 10.0. I was pulling -25 out to even get the air fuel ratio close. This is what led me to think the 2G mas was faulty.

I just got back in from putting the 1g mas back on the car. The car starts fine as drives great with no CEL. I went back and checked my sensor ground (pin 6) at the mas, and I still have no continuity to ground. Does this make sense? Surely if I was not getting a ground at the mas, I would have some driveability issue with my stock mas, correct? Where does pin 6 get its ground signal from? Is it a grounded inside the ECM?

Thanks again,
Kyle

 

[luv2rallye]

The sensors ground must go to ECU pins 17 & 24 on the 1g. The ECU grounds it internally. It MUST NOT be grounded anywhere else or ground loops and different voltage potential grounds can give incorrect readings to the ECU which can cause irratic behavior.

 

[gnasty]

Problem resolved. The running situation was a bad 2g MAS.

The ground issue is resolved as well. I got my hands on a factory service manual and buried my head in the wiring diagrams. The sensor ground is controlled by the ECU. On a 1g, the TPS, coolant sensor, and egr (for california cars) share the same ground, green with black wire. The way I understand it, which may be incorrect, is that the ecu will look at the sensor resistance readings. Each sensor has it's own preset resistance. If the sensor goes bad on the ground side, the ecu can determine by the resistance change in the circuit which sensor has failed.

Thanks for the help guys,
Kyle

 

[luv2rallye]

The way I understand it, which may be incorrect, is that the ecu will look at the sensor resistance readings. Each sensor has it's own preset resistance. If the sensor goes bad on the ground side, the ecu can determine by the resistance change in the circuit which sensor has failed.

You're right, YOU ARE INCORRECT. The ECU tells if a sensor is faulty by measuring it's voltage (or sometimes current) at specific times to see if it's within a specified range for that sensor. It's sometimes more complicated by also testing other things that affect it but that is the essence. The 2g factory service manual tells exactly what is tested and compared to what values and how in the "Inspection chart for diagnostic trouble codes" section (CEL codes). In the '99 Mitsu Eclipse factory service manual this starts on page 13A-25.

 

[gnasty]

You're right, YOU ARE INCORRECT. The ECU tells if a sensor is faulty by measuring it's voltage (or sometimes current) at specific times to see if it's within a specified range for that sensor. It's sometimes more complicated by also testing other things that affect it but that is the essence. The 2g factory service manual tells exactly what is tested and compared to what values and how in the "Inspection chart for diagnostic trouble codes" section (CEL codes). In the '99 Mitsu Eclipse factory service manual this starts on page 13A-25.

Thanks, I appreciate the help. I learn something new everyday. Mods can close the thread if they need to.

Kyle