Intermittent VSS problem P0503

[Boss Hugg]

So my OD off light is flashing. Finally got a BlueDriver code reader that reveals a P0503 code which indicates an intermittant/erratic/excessive. This is a new sensor which i replaced as an attempt to fix without having to pay lots of money to get codes read. (Turns out the BlueDriver scanner seems to be pretty cool if you're having trouble getting your OBDII efi swap to work right.)

From what I"m reading the speedo drive gear may be worn, but I'm not sure I've ever heard of this. The sensor gear has slight wear, but it can't be enough to cause issues. I can't see significant wear to cause reduction in OD of the gear...

has anyone dealt with this issue before?

 

[904Bronco]

So my OD off light is flashing. Finally got a BlueDriver code reader that reveals a P0503 code which indicates an intermittant/erratic/excessive. This is a new sensor which i replaced as an attempt to fix without having to pay lots of money to get codes read. (Turns out the BlueDriver scanner seems to be pretty cool if you're having trouble getting your OBDII efi swap to work right.)

From what I"m reading the speedo drive gear may be worn, but I'm not sure I've ever heard of this. The sensor gear has slight wear, but it can't be enough to cause issues. I can't see significant wear to cause reduction in OD of the gear...

has anyone dealt with this issue before?

So a 4R70W? If so, would that be the OSS on the side of the tranny?

 

[Broncobowsher]

Guessing this is an Explorer EEC-V swapped engine and transmission? Little things like what you are working on really help us out. What helps as well.

Any drivability issues? HAve you tried driving with the VSS unplugged? A failed sensor (unplugged) is generally better than an erratic sensor. The computer just ignors the lack of signal better than it trying to figure what the weird signal is doing. This generally works for most any computer sensor except engine RPM.

 

[Boss Hugg]

yes, 4r70w. I suspected it would just be a temp sensor, but I forgot to change it when I had the pan off changing accumulator springs. (I was having trouble with it shifting slow and soft, but it turned out to the programming. Flashing light is what drove me to pull the pan. Found nothing wrong.) No drivability issues outside of the fact that I need to fine tune the upshift and downshift points.

I bought the new VSS (goes inline of the speedo cable, not the OSS sensor) and slapped it in. Went for a test drive and immediately got a CEL P0500. This morning got around to pulling it back out where I learned I had ignored it needing a gear on it. So put the old gear on, stuck it back in, and now its a P0503.

I gathered the gumption to move on to the AC system which I finished up some time back but couldn't get the compressor to engage. Played with that for a while and even gased it up and made some cold air. But when I went to put my fuse panel back in it's place and tighted the bolts up, it doesn't work. I also put some heat shrink butt connectors on at the pressure switch so it might be a bad connection there. So tomorrow I'll use my scope camera thing to check the output shaft speedo gear for wear and check into the AC issues... (The PCM was not grounding my AC relay as it should but I may have figured out why.)

Will follow up here sometime tomorrow afternoon.

 

[Boss Hugg]

So yesterday I unplugged the VSS and drove it up the driveway to my parents place and back and the light didn't flash. I'm not sure why I spent the time wiring a VSS if I didn't need it???

But I have another problem that started at the same time as my flashing light. In reverse the engine would just cut off like the power was cut or something. Yesterday before I unplugged the VSS, I had trouble getting to run in Drive. Which now makes me realize the PCM relay was really hot while I was working on AC wiring. I suppose I could have a relay losing connection momentarily? I need to get one of those clamp on type DC amp meters and go thru the sensors and stuff checking for high current.

 

[Steve83]

...P0503 code which indicates an intermittant/erratic/excessive.

That's not the COMPLETE code definition.

This is a new sensor which i replaced as an attempt to fix...

This is exactly why you should NEVER blindly replace any part in response to a code.

From what I"m reading the speedo drive gear may be worn...

That would only apply if you're using 100% of the hardware that the PCM is programmed to monitor & control. Are you? Every single wire from the PCM's original connector is connected either to the original hardware, or to something that produces the exact signal that the PCM expects on that circuit?

The sensor gear has slight wear...

WHICH sensor? WHICH gear? There are several sensors that produce signals proportional to vehicle speed, so you have to figure out which one that code refers to.

I suspected it would just be a temp sensor...

Why? I thought you said it was a speed sensor fault.

...change it when I had the pan off changing accumulator springs. (I was having trouble with it shifting slow and soft, but it turned out to the programming. Flashing light is what drove me to pull the pan. Found nothing wrong.)

You really need to slow down and DIAGNOSE these problems before disturbing/damaging things, or creating new problems.

I bought the new VSS...and now its a P0503.

Is this the 2nd new VSS in a week? Or are you jumping back in time, and describing the same one again???

No drivability issues outside of the fact that I need to fine tune the upshift and downshift points.

What about:

...started at the same time as my flashing light. In reverse the engine would just cut off like the power was cut or something.

Isn't that a driveability issue?

...the PCM relay was really hot... I suppose I could have a relay losing connection momentarily?

No, that would cause it to be cooler than normal. Heat indicates high current, but it's normal for the PCM relay to be warm because it's supposed to pass a lot of current (~20~30A).

 

[Boss Hugg]

That's not the COMPLETE code definition.

https://www.obd-codes.com/p0503 : When a stored code P0503 is exhibited, it means that the powertrain control module (PCM) has detected a voltage input signal, from vehicle speed sensor (VSS) A, that is intermittent, erratic, or excessive. The designation A usually refers to the primary VSS in a system which utilizes multiple vehicle speed sensors.

This is exactly why you should NEVER blindly replace any part in response to a code.

OCBR is less than 9 days away. I didn't have time to wait on tools and equipment to properly diagnose. And besides...I still have the old part so that I can replicate the original condition, which I intend to do this weekend.

That would only apply if you're using 100% of the hardware that the PCM is programmed to monitor & control. Are you? Every single wire from the PCM's original connector is connected either to the original hardware, or to something that produces the exact signal that the PCM expects on that circuit?

Worked great for nearly three full years. Now there's parts missing so I can't diagnose a code?? There's only one VSS. The only other speed sensor is an OSS which is wired EXACTLY as it was in the explorer. I changed it before I did anything else and there was no change, therefore the code is coming from the other speed sensor.

WHICH sensor? WHICH gear? There are several sensors that produce signals proportional to vehicle speed, so you have to figure out which one that code refers to.

There's only two sensors to read my bronco's speed as explained above. Since it went three years without a code, it's got to be a problem with one of these two--again, see above response.

The OSS works off of holes in a clutch drum, not something that can go bad without obvious shifting/driving problems. The VSS works off a plastic gear on the output shaft.

Why? I thought you said it was a speed sensor fault.

before I had scanner that would read Transmission codes... absolutely no change in shifting, no strange sounds. There is nothing wrong with my transmission except that I've never dialed-in the programming.

You really need to slow down and DIAGNOSE these problems before disturbing/damaging things, or creating new problems.

I've been troubleshooting for long enough I understand the issues. The transmission is fine so there's no harm in changing sensors. IF I had made a programming change and seen potential harmful response, I would've undone the change...

Is this the 2nd new VSS in a week? Or are you jumping back in time, and describing the same one again???

only one new VSS.

What about:Isn't that a driveability issue?

Not in terms of shifting and driving down the road...

No, that would cause it to be cooler than normal. Heat indicates high current, but it's normal for the PCM relay to be warm because it's supposed to pass a lot of current (~20~30A).

This relay is HOT to the touch. Like the contacts are burning due to low signal voltage. Will investigate this Thursday evening.

 

[Boss Hugg]

Soooooo... pulled the VSS back out. Took it off the speedo cable... cable is froze in the tube, won't spin. Not even connected to the speedo. So i'm left to believe that a VSS connected to a stuck speedo cable will give problems. Still hammering out details on the Environmental Climate Control system and when that's done a ride will be in order to confirm. And only 6 days till I leave for OCBR.

 

[Boss Hugg]

Well the code is back. I think it's a bad (or small) VSS gear. But I can't recall if I ever put anything in besides the gear from the original bronco speedo cable on the newer VSS. So I guess I need to pull that and investigate. I'll do some digging on this site and see what gear I need to be running. I feel like it's going to be a problem with the current cable gear being too small and that its actually slipping on the drive gear, thus creating an erratic signal. ( the speedo bounces around at all speeds...) From what I recall, there's no way to make the speedo match actual speed because of gearing, tires and OD. So what would it hurt to put the biggest gear possible on it and use the Quarterhorse to program a different Revs per mile to try to calibrate the PCMs understanding of actual speed.

 

[73azbronco]

so you mentioned this occurred while you were working on the shift programming. Maybe try flashing back to completely stock, give that a try?

 

[Boss Hugg]

I did that. I think it's going to boil down to speedo gear. I just learned that the 4r70w has either a 7 or 8 tooth drive gear, and I KNOW I don't have a cable gear with enough teeth to be right. I MAY have put a 19 tooth gear in, but I'm betting it's too small and slipping, and therefore wearing. I ordered a 21 and a 23 to put in it and see what happens.