White Knight 95

Alright well I'm sorry for posting another MAF sensor problem but I have exhausted all my knowledge and I wonder if there might be something that I've missed. FOr reference I put a new GM MAF sensor on about six years ago, but have driven the car maybe five thousand miles since then.

I've had a code 48 for a while. At first it was just intermittent and I could get the sensor back up by wiggling the MAF connector. Then it just cut out altogether and I got a persistent code 48. So I bought a new MAF connector and soldered that on, but it didn't fix the MAF malfunction. So, I checked for 12V on the pink wire...I have that. I checked for 5V between the yellow wire and ground with key on ...I have that. My black wire (ground) is also good. I also checked the yellow wire at the PCM (B19) and I have connectivity between the MAF pigtail and the PCM. My GM service manual says given these conditions it's either a bad sensor or bad circuit. Well I'm pretty sure the circuit is fine, but I tried running all new wires for ground, B+ and to the PCM and that didn't make a difference. I went out and bought a new MAF sensor and that didn't make any difference.

When I fire up Datamaster I get constant 471 gps, which I understand is indicative of an open MAF circuit, but the circuit itself checks out. Is the PCM not sending or receiving the signal properly? Any ideas?

TIA

WS6T3RROR

Try swapping pcm's out and uploading your tune to see if it reports correctly then. It is possible that something has happened to the trace or some other component inside the pcm and it just is unable to interpret the signal properly now.

White Knight 95

Well I guess I'll source a new PCM and see if I can at least get a good MAF reading from that. Anyone else have any input before I pull the trigger? Does it seem reasonable that the PCM could be misreading or not reading my MAF signal?

WS6T3RROR

You have changed the weatherpack, verified the harness, and tried new sensors on it, that is about the only thing left to be bad is the pcm internals. Stranger things have happened, its gets miles on it bouncing down the road in that oh so kind under hood environment just like everything else.

White Knight 95

Good point. I will source a new PCM and report back in case anyone else has a similar problem.

Injuneer

Have you ever checked the output of the MAF sensor with a frequency meter? It should read around 2,500 - 2,800 Hz at idle. That would tell you if the sensor is putting out a valid signal. Check it at the MAF harness connector, then move toward the PCM.

White Knight 95

Fred - I haven't tested the frequency of the output from the MAF, since my multimeter doesn't have a frequency mode. I suppose that's the only way I can know for sure that the sensor itself is working properly, so I guess I should just drop the dough for a good multimeter that's reusable before buying a new PCM.

Thanks for the tip.

Injuneer

They put the frequency function in some pretty cheap multi-meters these days.

White Knight 95

I tested the frequency on the yellow wire and it reads around 7800 Hz. It jumps around a little, but doesn't respond in any reasonable fashion to changes in throttle position. All the while datamaster reads 471 g/s.

The stock frequency-g/s table indicates this should be about 150 g/s, so that doesn't seem quite right for LT4 heads and a hot cam at idle. I do have a mail order tune for this combo from an unknown tuner (Year One) but this tune actually ran fine for a couple years before any problem.

Now I am really stumped, because a mass airflow reading of 471 would correspond to a frequency of >11kHz, which I am below. So I don't know why my PCM thinks the MAF is pegged. I also don't know why the MAF itself is not outputting a change in frequency upon changes in throttle position.

Any ideas?

Injuneer

My Scanmaster indicates 471gps...... and I don't have an MAF connected to the PCM (engine is running speed-density off a piggy-backed MoTeC ECU). Sounds like the signal (erroneous as it may be) is not getting to the PCM. Can you measure the frequency at pin B19, not at the sensor?

White Knight 95

I get 7.8kHz at the yellow wire going into the PCM at pin B19. However, if I disconnect B19 from the PCM then I just get noise at the connector. Isn't the MAF supposed to be sending a signal to the PCM, and not the other way around? Or is it some kind of echo signal from the PCM, to the MAF, and back again?

White Knight 95

Does anyone know if B19 needs to be connected to the PCM in order to transfer a frequency signal from the MAF to the PCM? If not, that would mean that the "signal" I'm seeing is just noise from the PCM, and not a signal from the MAF, and would indicate that I need to make sure i'm getting a signal from the MAF to the connector at B19.

Anyone have any thoughts?

Injuneer

MAF generates the frequency, and sends it to the PCM. Is it possible you have a high voltage wire running very close to, and in parallel with the wire from the MAF to the PCM?

WS6T3RROR

Kidding.

I would perhaps go back over the wiring and make sure everything is going where it should and that nothing is wrong with the pins in the weather packs. After that I would check out what Fred suggested, maybe check the grounds or look at the alternator wiring harness etc.

White Knight 95

Believe me, I've thought of grenading the thing, and pushing it off a cliff, and driving it into the ocean...

...but I'm not going to do any of those things. I actually did try running a new ground from the MAF connector to the fender. I guess it may not have been the best ground but it didn't make a bit of difference when I tried it. Right now I'm just trying to figure out if the problem is the MAF, the wiring, or the PCM. I just can't decide if the "frequency" that I'm seeing with the multimeter means anything or not.

So if I understand this system correctly, then the "frequency" that I'm reading isn't coming from the MAF at all, since it ceases when I disconnect the yellow wire from the PCM. Hence it would be induced by some potentially high voltage wire running along a similar path. The problem is that the only non-stock wiring in the engine compartment is from my amp and from my MSD box. The MSD box has been disconnected and so has the (+) feed to it, so that's not doing anything. The (+) for the amp runs straight back from the battery along the passenger fender and below the door to the back. So the only way the amp feed could be causing a signal modulation would be in a few inches of overlap right near the PCM. But of course stranger things have happened. And it's easy enough to test, so I will check that out.

Regardless, any induction from a high voltage wire would only be causing an errant reading in the yellow wire with my multimeter. This would mean that my MAF is not sending the correct signal to the PCM, correct?

Thanks for all your help guys. If I was left to my own devices I'm sure this car would be at the bottom of a ravine by now.