P0721 OSS Noise

[C420sailor]

5R55E. Fluid is cherry red, clean. No burning. Honestly drives and shifts great except for occasional uncommanded downshifts.

Popped a P0721 code. Replaced OSS with a Motorcraft part. Problem persists. Datalogged with Forscan. Signal is all over the place, even while stopped. Very noisy. Multiple spikes to 15,000+ RPM, clearly impossible.

Wondering if maybe this is a signal return issue? I’m going to pull the connectors and clean them. Any ideas or thoughts?

 

[wollimann]

https://charm.li/Ford/1998/Explorer%204WD%20V6-245%204.0L%20SOHC%20VIN%20E%20EFI/Repair%20and%20Diagnosis/Transmission%20and%20Drivetrain/Transmission%20Control%20Systems/Testing%20and%20Inspection/Pinpoint%20Tests/F%3A%20Turbine%20Shaft%20Speed%20%28TSS%29%20and%20Output%20Shaft%20Speed%20%28OSS%29%20Sensors/

Replaced OSS with a Motorcraft part.

New one?

F7RZ-7H103-BA​

 

[C420sailor]

Yep, new Motorcraft.

The plot thickens!

I removed the PCM connector for testing. Got 12,600 ohms on the OSS circuit. But the sensor ohms out at 392 ohms. This 12.6k is between the OSS wire and the signal reference (circuits 136 and 359). Very odd.

I reseated the connector.

Crawled under the truck to inspect harnesses and found a rotting body to chassis ground strap. Replaced it with a good used one, cleaned everything up. I did all under hood grounds a couple weeks ago.

OSS traces on Forscan are clean now. Standard artifact noise, very minor and inline with other sensors. Zero spikes on test drive, sits at zero rpm when stopped. Night and day difference.

Not gonna call it solved yet, needs more testing…but vast improvement!

 

[410Fortune]

Nice work
Which ground strap?

 

[C420sailor]

The ground straps that connect the body to the frame on both sides, between the doors-ish?

So…that didn’t fix it. But the truck does run better…so it wasn’t for nothing.

I then found the PCM ground stud was partially unthreaded from the firewall, and rusty. I removed the stud, chased the threads, cleaned up the stud, nuts and strap. So far? Dramatic improvement. Drove for a good while, only occasionally seeing spikes in the waveform. Whatever is going on, it seems that improving grounding makes it much, much better. This seems like a signal-to-noise ratio type issue. It is not mechanical, as I was seeing significant noise while stopped--and ONLY with the engine running.

This all said, I'm running out of grounds to check, and I'm starting to wonder if the grounds are simply mitigating the actual source of the interference. This is a sensitive AC sensor circuit (variable reluctance), so it is extra susceptible. The two usual culprits are charging and ignition.

I checked the alternator with my DVM. On the diode function, they checked good, and I'm not seeing any AC voltage. That said, it's a relatively cheap DVM. I really need an oscilloscope.

I'm thinking about throwing plugs and wires at it, and I do have a spare used ignition coil. I hate firing up the parts cannon, though.

Thoughts? Any of you guys have any idea where this interference could be coming from?

 

[wollimann]

This seems like a signal-to-noise ratio type issue. It is not mechanical

So my thought is the alternator, it could produce electronic whistling.
Have a similar issue but only at startup with cold conditions,
and a flickering volt gauge while driving when using the turn signal.
More load (lights and trailer) more flickering.

The two grounding straps (body to frame) are a important thing, they
must have a very good contact, i used similar new copper straps and screwed
them up (not the clip style).
Hate too to change parts on suspicion, but as i know all my alternators did not last very long.

These issues are annoying and i hope you will find the culprit.

 

[410Fortune]

The 302 main drivetrain ground is direct from negative battery post to the front of the engine down at the timing cover studs

You have that ground connected and clean?
That’s the one where anything attached to the drivetrain gets its main battery connection

 

[C420sailor]

This is the SOHC truck, sorry. All four grounds under hood (G100-104, G201) are clean and tight. Negative terminal to starter ground clean and tight. Engine to body ground clean and tight at body—can’t reach the cylinder head bolt to get at the other end. Ohms out good though.

Parts cannon initiated. Got plugs, wires, and an alternator coming. And an oscilloscope, but that will be a while….

 

[410Fortune]

When the heck do I always think everything is a 302 hahaha my bad wishful thinking
If it was a 4r7 it probably wouldn’t be having any trouble hahahaha

Dang 5r55e! All stock ish? The 0ss is for sure making the noise not the tss?
The ground thay always gives me fits on the v6 trucks is the one under the drivers kick panel. I think it’s ground 100 in the book? Not sure but it has the fuel pump and a couple other things all run up and ground there

 

[C420sailor]

Which one exactly? The one next to the park brake on the A-pillar? That one looks good. G100 is in the engine compartment, I believe.

What’s strange is that I can hear every damn electrical component over the radio on my 4.0, and nothing on my 5.0. Blower, fuel pump, turn signals, the pop as I turn the parking and headlights on. I’ve also been able to correlate some of the OSS wackiness with messing with things like the blower, lights, etc. Electrically, it’s a noisy ass truck.

But…with the field fuse pulled, it persists. So it likely isn’t the alternator? Bad battery maybe? I know batteries will often act as capacitors/filters for the electrical system.

And yea, def the OSS. TSS is steady as a rock, no codes for it.

I have a brand new Siglent oscilloscope coming soon. Always wanted one, and this is my excuse. I’m going to find this problem. And if I can’t, this thing is getting a manual trans conversion.

 

[410Fortune]

Battery came to my brain as I was reading your post too

Should not have all that noise popping through the speakers
Back in my Audio install days we would call this a ground loop and try to fix the ground locations for all the heavy stereo equipment by keeping the big power ground as short as possible and by adding better ground on the battery side of things (the big three)
If it is the battery it will be impossible to fix until you swap it out… a quick test swap battery from truck to truck

 

[C420sailor]

I considered that…battery does act as a filter for the alternator’s noise too. That is a very strange failure mode of the battery though…

I’ll give the swap a try while waiting for the o-scope to arrive.