Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
2003 Cadillac CTS with high voltage on the 5 v reference line
The 5v seen on MAF pin 4 from ECT pin 51 (on this car) and other sensors is a supply voltage from the regulator, different than the varied 5v on a signal wire. Signal voltage is from the sensor to the ECM as a result of the sensor detecting changes (0 to 5v). On some systems when testing a MAF, with the MAF plugged in KOEO 5v is seen on the ref line, 5v on the sig line and .100mv on the ground. When unplugged KOEO 5v is seen on the ref line 0v on the sig line and 0v on the ground. When 5v is seen on the sig line when unplugged it is due to an internal resistor between the 5v regulator and the signal wire. This is bias voltage. Not all systems have this. BUT, a voltage must be present on the signal line when plugged in KOEO.
The MAF does use 12v on pin 2, this is to power the hotwire. The sensor uses 5v on pin 4. Do Not jump 12v into the signal wire! :ohmy:
From what I read, no voltage is present on MAF pin 5 or at ECM pin 56 (yellow MAF signal wire). PID shows a value but the voltmeter when back probed at pin 5 MAF and pin 56 ECM shows 0v. Either the back probe is not making contact or there is no voltage on the MAF signal wire. The PID value can be just a substituted value.
At this point, clear any and all codes. Start the engine and read any new codes.
"an open mind let's knowledge flow in and wisdom flow out for a man who has neither never listens to those who have both".
Being wrong doesn't bother me, it's being right and not understanding why that does
Ok, I'll verify the voltage on the MAF signal wire near the ECM using a wire piercing probe- with the KOEO, and also try it with the engine on. If there is very low voltage measured by my meter on this line, then apparently the 5 volt reference coming from MAF pin 4, and though the heated sensor line -is somehow not being dropped across the internal resister at the ECM for this sensor line. The car runs fine, it must be mapping the air flow to the engine based on load and rpm or something else, and not using the signal from the MAF, unless I'm not grounding my meter properly to read the volts on this sensor line.
