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!
Hello All,
I have a good one here. This is the type of car that normally rolls into the shop and you ask them politely to leave. But I'm a sucker and feel bad for this kid. He's going to wear out the barrel of his parts cannon on this one if I don't help him out.
I have a young friend with a 1992 Nissan 240sx that has a tweaked 2.0l SR20det turbo engine from a 89-92 Japanese market Silvia in it. The car runs extremely rich and I'm reasonably sure the maf sensor circuit is our problem. However, I'm using my snap-on brick scanner set to the American 240sx/2.4l engine to read codes and data so I don't know how much I can trust what I'm seeing. All the other sensors and data seem legit but the maf reading (being read in volts) never matches what my picoscope says the voltage is on the signal wire. I'm using the 3 wire analog maf setting in the scope to read voltage. Unplugging the maf the signal wire has 3.5v coming from the ecm. When you turn the ignition on it seems to jump to about 5v and then drop down immediately to 3.5v and sometimes it fluctuates between 3-4v. The maf power is 12v and the ground is less than 100mv backprobed with the sensor plugged in.
The maf sensor is from a Japanese Silvia. It's a 4 wire sensor but it runs on three. 12v/grnd/signal. I can't seem to get any details on how the sensor works. Being that I have voltage on the signal wire from the computer I'm assuming the maf should be grounding that signal as the airflow changes. Is that correct? Or should the maf sensor be generating the voltage and sending it to the computer? Since I'm using the wrong model to scan it and it reads voltage I'm not sure if this should be read in hz maybe? Either way I think this the ecm is bad because the signal wire should show either 5v or 0v, no? Also, the voltage reads the same at the maf or the ecm connector. Disconnecting the maf and pcm and checking for continuity in the harness doesn't reveal any faults.
Anyone have any experience with one of these swaps or can lead me in the right direction with how the maf should be working on this thing or the proper way to test it?
Thanks!
If you have 5v on other sensors ie TPS than the computer is good. The 5v regulator supplies a constant 5v to all the sensors. If the 5v is pulling down on the maf and no where else, it's a bad MAF. With MAF unplugged you should have 5v on sig wire. If you have 0v than you have an open.
When they did the swap, did they wire the MAF correctly?
Analog test of MAF while on sig wire snap throttle. Voltage should go up to at least 4v. Less than 4 bad MAF. Analog MAF signal is a wave form not on off square wave.
"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
Thanks for the reply. The maf seems to be wired correctly according to the schematic he has for it. The car had been running fine for some time and then started acting up. It has some sort of aftermarket kit harness as apparently this is a common swap to do. He also had another used maf that we put in and there was no change, even after clearing the kam. But who knows it's condition? You can't just find a new one at AutoZone because they never brought those Silvia's here.
I did check tp voltage and it was 5v steady. I didn't check any other sensors for vref. The maf signal wire fluctuates, like I said, from 3.5v-4.0v measured with the maf unplugged at the connector and also back probed at the ecm connector. Also, the scan data never matches what the actual voltage is on the sig wire. It reads just fine for the tp sensor though.
If you can confirm for me that there is definitely supposed to be 5v coming from the signal wire out of the ecm with the maf unplugged then I think I can find my problem from there. I'm just not sure if it's analog or digital? Is there a way to tell the difference? Does it matter either way if there's supposed to be 5v on that wire? If not, I need to find the reason for that first I'm thinking.
Thanks
I would say you need to check the wire from maf back to pcm for chaffing or loose fit pins from the sound of it but if it's fluctuating at pcm you might have a broken solder joint in pcm. I would do a pin drag test and load test the wire to maf to eliminate that part. As well as a visual inspection of the wire out of the harness wrap. Also with the pcm for a 2.4 and he has a 2.0 it would run rich irregardless of a actual maf problem which might require the pcm to be tuned down for fuel curves.
Thanks, those are good points, I'll try them. I tried rapping on the ecm a few times but that didn't seem to change anything. Neither did blowing some air over the circuit board. Everything looked good but a pin drag is definitely a good idea for sure. I'm pretty sure something is up with the pcm though. I should have mentioned, it has the 2.0l Silvia ECM in it and it has some sort of tuner crap inside it. My concern was that the brick scanner wasn't reading the Japanese unit properly, but all the sensors aside from the maf have reasonable readings that react normally and verify my rich condition. At idle, the scanner usually reads about 2.5v on the maf and the injection time is about 13ms!
I don't know, when I was this guy's age, the 240sx was the car all the hot chicks drove. Now they're hot rods??
Is the missmatch between scope and scanner voltage readings proportional?
How bad is that missmatch?
2.5 volts is to much at idle (general speaking should be about half of that voltage). That should be the reason because injectors are opening that long.
I would try to cheat Maf signal by pulling down voltage with a resistor in parallel (just for verifying maf reading is the problem)
Topic Author
