P0721 Circuit Noise / Interference

[C420sailor]

Hey guys, I have a thread going in the Transmissions subforum, but it seems like this is more of an electrical issue and less a transmission issue, so I figured it would get more exposure in here.

98 Ex SOHC. Recently started some funky shifting business. Uncommanded downshifts, jumping erratically between 4th and 5th, etc. Finally threw a code the other night for P0721 Output Shaft Speed Sensor. I replaced the sensor with Motorcraft, and the problem persisted. I hooked up Forscan and noticed intermittent noise all over the OSS. Looked like interference. Sawtooth, with occasional spikes to wild numbers (15,000+ rpm). It would do it while driving and while stopped, ruling out a mechanical cause. It seems like it worsens as the vehicle warms up, but I'm not sure. I did notice that the noise is not present with key on, engine off---the waveform is stable at zero rpm. I'm only seeing this interference on this circuit.

I've been cleaning grounds like a madman. Thoroughly cleaned all underhood grounds, including the battery to starter ground. The only one I can't reach is the engine to body strap attached to the back of the cylinder head. I do have low resistance on it, though.

Cleaning grounds seems to have cleaned up the interference dramatically. I'm starting to think that this is a problem that is somewhat mitigated by good grounds, but that bad grounds are not the root cause. This is apparently a variable reluctance sensor that produces an AC waveform, so it's very sensitive to interference and/or alternator AC ripple.

The alternator has 75k on it. I used my multimeter to check the diodes. They appear to be intact. I did not detect any AC voltage. That said, this is a cheap multimeter---I do not have an oscilloscope.

The plugs and wires have 60k on them. Motorcraft. I do remember hitting the cylinder #3 wire/boot with the HVAC box a few months ago during removal, but the truck runs well other than a very slight stumble at idle. I have a spare used ignition coil laying around somewhere.

I will say, the radio is pretty noisy on AM with the engine running...

Visual inspection of the harness (what I can see of it) looks good. I don't want to fire up the parts cannon, but I'm at a loss here. Alternator and/or plugs and wires? Any chance this 360k mile PCM could be failing?

I've also had an intermittently wonky tachometer for the last six months or so. Sometimes it jumps around a bit, sometimes it just sits at 0rpm. Not sure if they may be related.

 

[amj441]

with removing you hvac box and considering that the spark plug wires for the passenger side of the vehicle cross over the connector that goes to the transmission harness, it sounds like you are on the right track. id wait for the sun to go down and get a mirror or something and see if your spark plug wire might be jumping to that harness. Also make sure that your radio capacitor is good and in place as it will act as a filter for the radio and absorb any strange voltage spikes. The capacitor is on the bottom center of page 1 of this wiring diagram. on the coil it should be a tiny white brick with a metal tab bolted onto the coils

 

[C420sailor]

Thanks! I’ll check the radio cap.

Parts cannon initiated, just bc these parts are old anyway…and I like having spares on hand if it isn’t the alternator. She’s def getting plugs and wires though.

 

[410Fortune]

Slight stumble at idle? Not allowed!

Following this thread and actually paying attention this time! 98 5r55e does not have the second speed sensor right it only has one oss and no tss

While you are replacing the ignition wires Check bottom of coil pack for any rust or cracking. I’ve heard people have issues due to corrosion at the coil pack to bracket ground connection which is also where the noise capacitor is Located

 

[C420sailor]

This one does have a turbine speed sensor…not sure if it’s internal or external. I know that changed over the years, I’d have to look.

I pulled the alternator field fuse and the issue persisted…

So…ignition?

This seems to be a relatively rare code, and there isn’t much info on how people solve it. It appears that many vehicles with this issue end up taking it to the grave….so if we can figure it out, it would be a huge win for me and this forum. Good diagnostic info.

 

[amj441]

I havent given up on my sploder, I aint giving up on yours lol. Ill pull some more wiring diagrams and see what I may be able to find.

 

[amj441]

Yes it does seem to have the turbine speed sensor

 

[410Fortune]

It could always be coming from the pcm side of things if we get an obscure code we can’t solve often times these pcms … well they are getting older
With a 98 model year and pats it is more difficult to find a “donor” pcm for testing, but not exactly impossible… there is no shortage of 98-01 explorers or rangers w the sohc and 5r. The trick is finding one in the yard or on facebook and the key/tumbler the pats module and pcm are actually all there… usually can be had cheap only takes minutes to swap over
Fix up your ignition wires and keep on the path you are on… I try to rule out a bad pcm as a cause, because it can have you chasing a ghost!

 

[C420sailor]

Thanks guys. It’s getting harder and harder to keep these things running. This one is my original Explorer, and has the most mileage (360k). She’s my ride or die. Had her since 2007. She’s my primary daily driver.

My other I got as a second vehicle back in 2019. She has fewer miles, but lived a much harder life. Previous owner did horrible repairs.

I really don’t want to give up on her. Thanks for all the help.

 

[C420sailor]

This is a wild thought, not grounded in much evidence. Pure spitballing.

A while back, I had a problem with my CHMSL (third brake light). I noticed it seemed to be flickering at a rate that was similar to what I thought was my torque converter locking and unlocking at highway speed. I now know that was the OSS freaking out, resulting in uncommanded downshifts/unlocking. I ended up disconnecting the LG wire that feeds the brake lights from the RAP and the brake light problem went away. Note that this flickering was occurring with the truck off (possibly with it on too, I just never noticed). Something was making the RAP leak voltage into the brake light circuit via the LG wire. I figured it was a bad RAP and forgot about it....until now.

As a side effect of my weaponized autism, I spent a few hours staring at schematics. I wanted to know WHY the RAP was making the light flicker. It didn't make sense. I noticed something interesting. It seems that the RAP is fed brake pedal position info from the BPP switch, which makes sense (auto lock doors when brake pedal released).

This brake pedal position input also feeds the GEM.

I started looking at the GEM schematic more closely. There are a lot of interesting circuits that go through the GEM, to include sensors like the throttle position sensor, which use circuit 359 as a signal return to the PCM, which is shared with the OSS.

The GEM also shares a power feed with the radio, which has been picking up a lot of system noise on the AM bands.

What if the GEM is going south, and wreaking havoc throughout the truck?

 

[C420sailor]

More stream of consciousness posting…

So the OSS is a variable reluctance circuit. This means that the sensor produces an AC waveform. This waveform varies in frequency and amplitude, and the PCM completes signal conditioning to turn it into something usable, like a square waveform.

I noticed that, when hot, I could induce OSS RPM spikes in Forscan by manipulating the high draw DC loads in the vehicle, specifically changing HVAC blower speed, manipulating the headlights, etc.

I also noticed that at no point am I seeing a lack of RPM. These are always spikes well above normal. This suggests that it isn’t a shorted or open circuit. This suggests noise. Improving of grounding making the problem better tells me that the grounding is improving the signal-to-noise ratio of the sensor/circuit.

Where am I going with this? Either something is pushing AC into the circuit OR I have DC making its way into the circuit, causing a positive offset of the AC waveform. What can cause that, especially with heat? A failing capacitor.

I need to pull the PCM and check the electrolytic capacitors on the board, and check traces for continuity.

Disclaimer: I’m not an electrical engineer, just a nerd…I may be well off base, but I need to get these ideas out and written down

 

[Turdle]

That little grey single wire plug on the coil pack is an ignition noise suppressor. It could be bad, or, not grounded well.

 

[C420sailor]

I now have a list of things to check when I get home from this work trip, thanks!

 

[410Fortune]

It’s like a mystery novel and I can’t wait to get to the end

The flickering led light was your sign and has now reaffirmed you are pointed in the right direction!

 

[C420sailor]

oh we’re gonna figure it out

 

[C420sailor]

Snuck home for a day to do some work.

Did plugs and wires—they were kinda due anyway. New Motorcraft for both. While in there, I removed the ignition coil. Looked good except the bracket and underside of the coil were very rusty. Cleaned them to bright metal, reinstalled. Radio noise capacitor looked good and appears to hold a charge.

Truck drove great for 30ish minutes, then the tiny spikes began to return in the waveform. They slowly got bigger and more frequent. This seems to be a heat related issue. It consistently takes 20-30 minutes from a stone cold engine for these issues to start popping up.

Doesn’t seem to correlate with engine/trans temp at all though. More of a delayed onset. Makes me think something electrical is heating up at a slower rate, then going haywire.

I pulled the PCM out and did a visual inspection. Don’t see any broken solder joints or broken/burnt traces. Caps don’t appear to be leaking or bulging.

I have a spare Bosch alternator. I don’t want to throw it in yet. Trying to avoid the parts cannon. I’ll wait for my oscilloscope to arrive and see if I can figure out what is going on in this truck—the Forscan data just isn’t enough, I need actual waveform samples.

 

[410Fortune]

I got $5 on the pcm

 

[Turdle]

Ok, my bet is alternator-regulator,

What are we wagering though? I might scrounge up a milkshake.

 

[C420sailor]

Lol, this would be fun if it wasn’t my daily driver

Alt has 72k on it. Some parts store special. It holds a solid 14.4V with no load, 13.6ish with a full load. But yes, could be excessive AC ripple. Diodes could be leaking as the alternator heat soaks.

Gotta find a PATS PCM with all the trimmings