MAF problem - need diagnostic help...

[Boatman]

My 96 3.1 is running well, but it threw two codes today:

P0102 - MAF Circuit Low Input

P1655 - Evap Solenoid Control Circuit

As usual, the Gates manual is useless.

My scanner can log real time data. The MAF signal is .26 and doesn't change while driving. All the other obvious signals seem reasonable - MAP, TPS, RPM, Temps. etc. and change with speed and acceleration. However MAF signal doesn't change.

Is this what the computer does (assume a constant value) if the MAF doesn't send a signal or sends a signal out of tolerance?

Is there any way to tell if my MAF is bad or my wiring is bad?

Anyone know if the Evap Solenoid code means the evap solenoid is actually bad, or is it an accumulated error code? If it's bad, where is it?

Thanks to all...

Boatman.

[SyntheticShield]

Well it should change values with an increase in air intake that would occur when you accelerate. So if that isnt happening, then I would suspect it to be bad. 0.26 is what I would expect to see perhaps at idle. But even if you revved it up some in park, it should still change values. Even sitting in park and just letting it idle it would change values as the air intake volume is not a constant.

[Boatman]

Put in a new MAF from Auto Zone, but the computer still throws the same codes and my scanner still reads a constant MAF output value of .26 while driving. The car does not start correctly dies once or twice and hunts at idle for about a couple of minutes. Drives OK after that.

It still throws the Evap solenoid circuit too.

Is there another problem that could be causing the car to incorrectly read the MAF or substitute a constant value for it?

This is getting confusing.

Thanks...Boatman.

[SyntheticShield]

Do you have the correct voltages at the MAF? Im troubled by the fact the MAF is not changing values, thats not normal. I assume the .26 is the air flow measurement and not some kind of voltage measurement, correct?

[TDCRacing]

Usually when sensors go bad (or unpluged) they read 99 on the scanner.

I know this for a fact that the iac and the map does.

[Boatman]

On a thread from another forum, I learned that there should be 12 V plus going to the MAF:

Pink = Batt +
White/Black = Ground
Yellow = voltage out

Well, the white/black is a good ground, but I'm getting no voltage to the pink wire. I'm thinking this is the root of my problem.

Anyone know why that 12 V source would all of a sudden go bad?
Anyone know if anything else is on that 12 V circuit?
Anyone know if the computer supplies voltage or the key/ignition circuit does?

Any help greatly appreciated.

Thanks...Boatman.

[02BlueGT]

Check your fuses before you start tearing apart the wiring harness

[Boatman]

This is starting to become clearer. Went through the wiring diagram and found that the MAF and Evap purge solenoid both share the same "pink wire" 12 V source. And my car is throwing both MAF and Evap codes. It's got to be the 12 V source.

Wiring diagram says that the source is the "Underhood Electrical Center 1". But is does not show that the source from the PCM is fused, just that is passes through this box.

Anyone know if it's fused?

Anyone know where the Underhood Electrical Center 1 is?

Thanks...Boatman.

[02BlueGT]

Above the battery is a large black plastic box, the electrical center should be that(i just cal it a fuse box ) when you take the cover off (big plastic thumb screw) there is a label inside, look for one with either Evap, maf, or even emission controls and check it

[SyntheticShield]

His is a '96 3.1L, will it have the same fuse box as the 3800 set up? Im not saying its not there, but I dont recall see a fuse for a MAF or EVAP solenoid on a car before.

[archemedes]

could it be a bad pcm?

[02BlueGT]

His is a '96 3.1L, will it have the same fuse box as the 3800 set up? Im not saying its not there, but I dont recall see a fuse for a MAF or EVAP solenoid on a car before.

Your right I dont read very well.....could that section if harness be cut/unplugged, cause the 3.1 has an independent Maf right??

And my fuse box was by the bat on my 94 3.1, but it was different from our 3.8 bays for sure

[SyntheticShield]

Since I have my harness all tore apart I went out and checked it. My wiring color is as follows:

BLK/WHT = Ground
Yellow = MAF signal
Pink = Ignition voltage

Both the blk/wht and Pink wires got back to the under hood fuse center. The yellow wire goes back to the PCM. I checked my fuse box, and while there are no fuses that are specifically marked MAF or EVAP, there are few in there that deal with the ignition, including one mini fuse.

Since our wire colors are the same, I will assume for the moment that the functions and wire terminations are the same. So to start with, you have to have the key on to check the voltage. Make sure that you are checking the voltage with known good ground (like the frame, or the block, something that is bare metal, not painted). If you still have no voltage at the MAFF then start tracing back from there. Make dang sure that ground is good. You should be able to do a continuity check from the ground pin to the frame or block and see little to no resistance.

Now, I checked All Data, and you are correct in that the EVAP and MAF come off the same fuse. I cant tell from the all data diagram which fuse it is other than to say it is a 15 Amp fuse. I have included the schematic for your below.

[SyntheticShield]

Oh, also, keep in mind this is for my L67 3800 engine, Im working from the assumption that since our MAF wiring color is the same that perhaps the connections are the same. Good Luck.

[Boatman]

Turns out that the two codes I got - one for low freq for the MAF and one for the Evap purge solenoid - were both caused by a blown fuse 12 (in the fuse box on the front passenger side under the front brace). Once I saw that both were on the same circuit, I know that was the problem...and it was.

Why the fuse blew is another matter. It was a self inflicted wound. I didn't like the fact that the radiator fan doesn't come on until 230 so I wired in a Flexalite adjustable thermal switch from the radiator into the fuse box to the fan relay. My problem arose when I misread the wiring diagram. I know that the two-wire thermal switch feeds the PCM, which in turn powers the relay. However, I mistakenly thought that the two wire switch was a positive fed resistor, like most two-wire thermal switches are. You know, positive voltage in and increasingly positive voltage out as temp increases. Well, it's not. It's a negative in and out.

With this wiring mistake, every time the fan went on I shorted fuse 12 and the car ran poorly - hard to start and hunting idle. Once I figured this out, I ran ground-in to the adjustable switch and then onto the fan relay (brown wire) and everything works great. I cleared all the codes and the car runs just fine. I used my scanner to set the fan to come on when the engine coolant hits 200. Now I can sit in traffic on a hot day forever (tried this out today in some Memorial Day traffic) and the engine never gets above 200.

Thanks to everyone who posted and all the helpful advice.

This is a great board.

Boatman.

[SyntheticShield]

Glad you got it all figured out. And chalk up the experience to a learning one and there is not really a bad experience if you learn.

I would even go as far as to set the fan turn on a bit lower, maybe in the 185* range. If you have any other issues, post up, we'll do all we can to help out.