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.

 

[amj441]

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?

I had smack my brother once while leaving the junkyard when he came with me to get explorer parts, little gremlin came out with about 20 explorer keys.

 

[C420sailor]

Installed a known good PCM from @410Fortune. It's out of a 97, while mine is a 98.

Double checked the OSS circuit for continuity. Showed 0.6 ohms on both legs (OSS and signal return). Showed 392 ohms across the two with the sensor connected (sensor resistance is 390 ohms). So that checks out.

Truck started right up, idled and ran well. Took her out on the road. About 5 minutes in, it got bad. Bucking, stalling. OSS was all over the place, torque converter was locking and unlocking. Barely got home. She stalled out multiple times.

P0500 (VSS), P0705 (Transmission Range sensor), and P0721 (OSS performance).

Not sure if the 0500 and 0705 are due to the PCM being from a different model year.

 

[C420sailor]

Okay, more testing. Learned interesting things.

With the 1997 PCM installed, I backprobed the OSS wire and signal return at the PCM and checked resistance. Up to 447 ohms. The truck was hot, so I'm attributing this to heat at the sensor. As the truck began to cool, the resistance dropped, ohm by ohm.

I decided to hook up my oscilloscope to the OSS backprobe at the PCM and ground it to the negative battery terminal. I pulled the transfer case fuse, jacked the rear, chocked the wheels, and threw her in manual 2. AC waveform frequency varied with rear wheel speed. It was interspersed with spikes and what I can best describe as noise. Forscan shows no data for VSS with the 1997 PCM, but I get valid rear wheel speed when scanning the ABS sensors. What's weird here is that I was getting 0rpm for the OSS on Forscan, with occasional spikes...with the rear wheels spinning in the air. I know I had enough speed (20-25mph) for it to register. I took the truck down the street and it was spikes separated by 0rpm segments.

I quickly installed the cold 1998 PCM and repeated the test. The occasional spikes were there, but smooth traces between them. Additionally, I had a good VSS trace---which perfectly matched the OSS when it was smooth. This leads me to believe a few things.

1. This is not a PCM issue.
2. The 1998 PCM seems to reference VSS data when OSS data is invalid---in this case, when it is zeroed out. The spikes still punch through, but instead of showing 0rpm, it falls back onto the VSS data. I'm not sure if this is a difference in architecture/programming in the PCMs between 1997 or 1998, or if the 1997 PCM just doesn't play nice with the 1998 wiring for the VSS. This explains why the truck was bucking like crazy and undriveable---it had no reliable speed information other than the OSS, which was alternating between wild spikes and 0rpm. I think the 1998 PCM uses the higher value of the VSS or OSS, in order to prevent an uncommanded downshift at speed---but this is just me spitballing.

The fact that I'm seeing good oscilloscope waveforms with the rear wheels spinning, and the frequency is correlating to wheel speed, tells me that my tone ring is there and intact. I see nothing indicative of a missing tooth or similar.

I got under the truck with it running and plugged and unplugged the OSS a few times...and noticed some weirdness in the baseline noise level on the scope (wheels not moving). Fishy. I want to replace the OSS sensor pigtail and I'd like to see if I can figure out how to re-pin the OSS and signal return wires at the PCM.

 

[C420sailor]

FOUND IT!!!!

I had previously inspected the harness. It looked good, but I had trouble seeing behind the engine. I knew there was a connector there that the OSS wire and signal return both went through, C113.

I decided to take another peek. I noticed it was kinda odd that the connector was sitting against the block/bellhousing, but the wire bundle coming out of the connector looked okay. Decided to get my inspection mirror and take a peek at the back side of that bundle.
Holy chafe!

Insulation chafed through the dark blue/yellow OSS wire and the black chassis ground. They were both intermittently contacting the block/bellhousing. This is where all that noise was coming from!!!!

Holy relieved. I'm going to get some liquid electrical tape on it and get some tape in there for now, just for testing purposes. I need to track down a 1998 5R55E transmission wiring harness.

Will report back!

 

[DintDobbs]

@C420sailor Good going! Exposed wires can cause all sorts of issues when they start touching other metal.

I'm a bit late to the party, but IIRC it was in 1998 that the VSS was relocated from the rear of the transmission to the top of the rear differential. I don't know how the early models calculated the vehicle speed, but the '98+ use a Hall effect sensor and calculate speed based on the differential gear ratio and the tire size (both of which can be reprogrammed in the PCM). Not sure whether or not that change would be relevant, as I don't know what the PCM does with VSS info, just didn't see any one else mention that change in this thread.

 

[C420sailor]

Whatever it does, it works well. The 1997 PCM's lack of blending with the VSS (at least as installed in the 98 truck) exposed the fact that the signal was zeroing out in between spikes. The blending logic in the 1998 computer was hiding that. Once I saw the zeroed out segments, I knew I likely had a short or intermittent contact somewhere.

I will say, the thought of not having the VSS signal as a backup is kinda scary. Imagine doing 65mph down the highway and the OSS cuts out with nothing to fall back on...that could be very expensive sounding downshift.

Test driving is going great so far. Gonna give it a good bit before I mark it solved. I want to write up a summary as well...I'm sure this will happen to someone else down the line.

Thanks everyone! @410Fortune is the hero here---I would not have figured this out without the '97 PCM he sent. Stand by for an inbound message, good sir!

 

[410Fortune]

Excellent! Stupid chafe wires right where you can never see them
How come they never chafe right in a visible spot? So wild

97 pcm is expecting a tailhousing mounted vss
98 gets its vss signal from the abs computer

97 dtrs is analog
98+ is digital

So we knew the pcm would be confused

Imagine this pcm has been sitting In a 97 sport for 28 years then here I come along snatch it up. Toss it into a 98 ranger with a 05 sohc and auto for a few weeks then ya m it out, mail it to you and you shove it into a 98 sploder… the poor thing never had a chance. I will keep it around to help others as needed hahahaha

 

[C420sailor]

I also figured out that the 97 PCM activates the AC compressor clutch in this truck…100% with key on…even with the HVAC controls off. Had to pull the AC relay.

The little differences in these rigs from year to year…