Join Date: Jun 2017
Location: Denver CO
Model: GL1500 Aspencade 1980 GL1100 STD Vetter
Notes on oscilloscope display measuring MAF sensor:
There's a three terminal connector to the MAF (Mass Air Flow) sensor on the '99 Pontiac, +5v, Signal and Ground. Connect scope to "Signal" and "Ground".
The MAF measures the amount of air flowing thru the intake plenum after the throttle body.
With the engine idling (air flow is minimum) the signal output from the MAF is supposed to be a nice 5v Square Wave at a relatively low frequency.
As you slowly rev up the engine the square waves smoothly increase in frequency (get closer and closer together on the 'scope display) directly corresponding to the amount of air flowing thru the plenum.
The ECU uses this information along with many other sensors to determine how much fuel to inject.
If you punch the throttle, the scope display will show the square waves tighten right up in a high frequency burst corresponding to the big inrush of air when you blip the throttle.
The bad MAF I was testing showed a very unstable square wave (random square wave pulses at random frequencies) which bore no relationship to the air flow thru the plenum.
This confused the **** out of the ECU and the car ran so bad it wouldn't idle, and spit/sputtered going down the road. We disconnected the MAF and the car ran much better - that put the ECU in a fail-safe mode and it relied on the inputs from the remaining sensors. So we were able to drive back to auto parts and exchange the bad sensor. I changed out the sensor right there in the parking lot so I'd know it was good before leaving....
1972 CL-350 (1980-1988) sold
1978 Suzuki GS-550 (1985-2005) sold
1977 GL1000 (2002-2006) sold
1980 GL1100 STD Vetter (2005-)
1993 GL1500 Aspencade (2017-)
1983 Trav-Lite Camper (2010-)