P0721 Circuit Noise / Interference

[amj441]

View attachment 466847

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.

 

[C420sailor]

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.

 

[C420sailor]

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.

 

[C420sailor]

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…

 

[410Fortune]

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?

 

[C420sailor]

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!

Edit: Two hours of driving on her as of tonight, 8/23/25. No OSS spikes on Forscan until my laptop went dead…kept driving. No uncommanded shifts, good driveability. Cautiously optimistic.

Above photo is the ignition interference on the OSS circuit, measured across the OSS wire and the signal return.

 

[C420sailor]

Nope. Still just as bad. Flashing OD OFF and P0721 again.

I’m going to try rerouting those ignition wires if I have slack. Likely going to send the PCM out since finding them is very hard now.

Edit: rerouted, and started truck back up. P0721 came back almost immediately. The longer you drive, the worse it gets.

PCM is the only thing left. After that, she goes to the junkyard unfortunately.

Edit 2: I made the mistake of thinking it was fixed and took it on this drive, a 200mi ride to work. Awful. Worst it’s ever been. Constantly changing gears, constantly cycling the torque converter. At low speeds it’ll audibly bang gears. The OSS trace is absolutely unhinged. Basically cycling up and down between 0 and 15,000rpm. The further I got into the drive, the worse it all got.

I’m pretty much at the end of my rope. Having a hard time justifying $350 to have some guy check out the PCM. May just junk the truck, as much as I love it.

 

[C420sailor]

Now picking up misfires in multiple cylinders per Forscan. Mostly passenger bank but some on other bank too.

 

[410Fortune]

I have a 97 sohc pcm you can try? It shifted odd with my 2005 5r55e but it ran great. It just would hold 3rd and 4th gear too long?

It maybe useful for you for testing
It will plug right in
Wiring for 97 and 98 sohc at the pcm should be identical
Your 98 should have 4 02 sensors and the power valve in the intake ..
So you could try this sucker
Just pay shipping
If it works and you like it send me $40 that’s what the pcm cost me
97 is return fuel and no pats, but that shouldn’t matter

 

[C420sailor]

You’re the best, PM inbound. That way, if it works, I can send my PCM out for repair.

Mine is an early 98 I think. Return system, 4x O2, vacuum valve in the intake.

 

[410Fortune]

Okay 97 pcm coming to you
Only difference beyween 97 and 98 is 97 was the only year of sohc return fuel and no pats

The pcm has no idea if the fuel system is return or returnless

My info says the 97 sohc had the 5r55e and not the 4r55e like the ohv hd so let’s see how this pcm works for you

I pulled it from a 97 sport with a sohc and auto 4wd build date of 10/96 so it was a very early sohc

 

[C420sailor]

What’s interesting is that I fired the truck up after letting it sit for six hours and the misfiring and stumbling was almost entirely gone.

It has to be the PCM. I can’t think of anything else that worsens dramatically with heat and affects both the engine and transmission—that I haven’t replaced, anyway.

@410Fortune has a PCM in the mail for me (thank you again!), hopefully I see a dramatic difference on that.