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P0721 Circuit Noise / Interference

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So I haven’t used an oscilloscope since college physics, but I was able to get a semblance of a waveform out of the alternator. It would look acceptable, then this nasty would come out of nowhere. Seeing peak AC voltages in the ~500mV range (1.0VAC peak to peak), and it is just all over the place.

Letting her cool down, then throwing an alternator in.
This is so interesting to see, keep up the good work! I need a oscilliscope at some point.
 



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I THINK WE MAY HAVE IT

@Turdle may be the winner!

I installed a new Bosch alternator. I compared diode tests between the new and the old. The new alternator had about 33% less forward bias voltage drop through the diodes compared to the old. Whether that is of significance, I don’t know.

The AC signal on the scope was MUCH cleaner than the old alternator. Much less AC voltage.

I took her out for a long test drive. Almost two hours. I normally start to have driveability issues and uncommanded shifting by about 20-30 minutes into the drive, accompanied by significant spikes on the OSS circuit in Forscan.

I drove for a hair under two hours. Around town and highway. OSS trace looks great in Forscan. Transmission functioned flawlessly. No codes thrown.

My theory is that at least one diode in the alternator was beginning to fail/leak when heat soaked. This was pushing a good bit of dirty AC into the truck. The OSS, being a variable reluctance circuit, sends an AC speed signal to the PCM for conversion to digital to determine shaft RPM. Alternator gets hot, AC leaks, OSS circuit picks it up, PCM thinks it’s legit, transmission starts shifting funky, OSS signal gets so outrageous that the PCM realizes it’s noise and triggers the P0721.

Very insidious. Old alternator pushed out good DC voltage, no obvious issues.

I’m not going to call it solved yet, as these gremlins do often come and go. More driving required. I’ll report back.
 






UGH. It’s back.

Time to throw a PCM at it I guess? Anyone have a matched set of PCM/PATS/key for a 98 SOHC Auto?

I’m starting to see why most vehicles that come across this code just get junked.
 






Been chatting with an electrical engineer.

Latest theory is that the PCM uses a Schmitt trigger to convert the AC sinusoidal waveform from the variable reluctance sensor into a square wave. He believes that noise is finding its way into the circuit, and that the Schmitt is “squaring it off”.

We are leaning toward ignition being the source.

I’m going to scope the OSS circuit with the engine on and off, truck hot and cold, and see what is present. I’m also going to grab a spare ignition coil just for the hell of it. Truck does have a touch of a stumble/rough idle…
 






Interesting!!!

I do not have a 98 pcm
I recently searched for a 99-01 sohc auto pcm with keys and pats module it took a few visits to junk yard to find one that actually has the keys. Ended up finding a 99 pcm keys and pats module at a wrecking yard, the kind of yard where they pull the parts for you
Don’t forget to get the door and hatch locks as well

Getting harder to find
A holes at the junk yard like to steal the keys now… every vehicle that had a key with the ignition the little
Cable is cut and the key is mia
No junk yard etiquette anymore

I may win this one after all, my $$$ is on the pcm

Have you tested the oss? I forget
Looks like the scope has pinpointed the noise
Coil pack and capacitor?
 






Yeah, junkyard pickings are getting slim. I went today and only had a 1st gen, a few 2nd gens. None with SOHC, two V8s. Some asshole took the intake manifold and all that off the V8 and cut the harness up and cut everything instead of disconnecting. I wanted the coolant pipes, but they were all cut.

Yes, sensor replaced with Motorcraft and both ohm good.

Anyway, I probed the OSS connector with the scope and started the truck. Ignition noise, plain as day. If I moved the ignition wires going behind the manifold to the passenger bank, I could see some changes in the waveform. I put my palm on top of the coil, where the wires attach, and could see a dramatic change in the waveform. Disconnecting those ignition wires (rough!) resulted in the waveform vanishing, and me shocking the **** out of myself. I’m sure some of this noise is normal, but the question is how much…and is it getting worse as the truck gets hot.

I zip tied the wires as far away from the harness as I could—I know the OSS wires are in it. Not much room back there, but I got about an inch of clearance.

Did wires and plugs a couple weeks ago, so they’re new.

I replaced the ignition coil and radio capacitor with a junkyard coil out of an 08 Explorer. Ignition waveform looks roughly the same.

I’m wondering if the old coil was breaking down as it heat soaks, and creating a ton of EMI?

Anyway, I’m out test driving. So far so good, but we know how that goes….cross your fingers!
 






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