1998 F-250 LD 4x4 463,000 km.

My truck begins to flash the OD light after driving about 10 km. So I borrowed a friend's scan tool and got a P0503, and P1500. So having been pointed to the vehicle speed sensor, I connected TORQUE and watched closely vehicle speed. Driving at a steady 80 kph it will jump rapidly up and down, from about 72-92, while the gauge in the instrument cluster reads a calm and steady 80.

So, someone help me out a little. Which sensor and wiring do I need to inspect? Rear diff or on the back of transfer case? Oh, and the ABS light is NOT on.

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VSS sensor, it's gear, it's plug, harness.
Sounds like gear wear.
Clear 503 and 1500 will go with it. 1500 is a sympathy code from the 503 fault, for another reason.
Good luck.

 

Thanks Bluegrass. I am, however, still unsure which sensor is the VSS.

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The VSS is mounted to the transmission, left side, at the rear (near the output).

 

Thank you. Took it out this morning and it looks fine from the outside ... Do these habitually go bad or is it likely wiring?

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So today I wasted the $40 and picked up the VSS. Was gonna change it out but it rained all day, so I just put some km on it instead. Oh, and went to "help" a friend who got his truck stuck ... But couldn't actually get him unstuck...

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Since it didn't rain quite all day today I got the opportunity to install the new VSS. Took it for a drive and it has improved nothing. Next I'll change the sensor on the rear diff and we'll see what that does. If there's still no improvement I reckon I'll ignore the flashing light until a problem arises.

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The 503 codes indicates noise on the circuit.
Usually the VSS or gear gets worn such that rotation is not steady.
It could be noise generated from some other circuit such as ignition or the charge circuit.
Since it varies in a rhythmic pattern, it sounds like it could come from another circuit that is generating a signal who's phase changes just out of sync with the VSS signal.
The 1500 codes is set when the VSS issue interferes with other tests the PCM does.
This code would clear when the 503 is cleared.
Maybe you should inspect the harness for a problem or rub through.
The VSS sensor is a signal generating device vs RPM that is converted to pulses for digital use in other circuits that need speed indications for their functions.
It's as close as we can come for you here.
I'm sorry it's not simpler for you.
Good luck.

 

Well, before I purchased the sensor, I trailed the wires from VSS to the connector on firewall. I connected the ohm meter there, set it where I could see it from under the truck then got underneath and did a thorough "wiggle test" on the wiring. The ohm reading did not change at all during the entire "test". Given that, I am led to believe it's not the VSS wiring either.
Any other suggestions? How would I go about checking for noise from charging system and/or ignition?
Thanks for your input so far. It's appreciated!

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As much as I would like to ignore the little flashing light, my type of brain programming will simply not allow me to! It just bugs me!!! So I'm not giving up yet!

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This is out of the ordinary for most people but put in a situation like this I would get out my Oscilloscope and look at the signal wave form from the sensor line with the rear end on jack stands so I could run the drive train in gear.
Then chase it from there until I found the source of the noise on the output from the sensor, if that is what the scope shows.
Another way to test the sensor is to turn it with a drill and look at the output AC voltage with a voltmeter set to AC and look at the level, as a start.
Next look at the charging circuit for AC or spike noise riding on the DC wave form that should not be there etc.
If that were seen, the alternator would be suspect.
Most of the time looking for AC on the alternator output should show much less than a couple hundred millivolts AC at the most.
Good luck.

 

Thanks again. I spun the sensor with a drill a while back when I had it out to look at it, but I don't remember the what the AC volts were exactly. Someone know off hand what it should read?

Also, I just did some more testing. The problem appears to be relative to speed. For example, if I drive 40 kph, the readout on TORQUE will fluctuate from 37-43. If I drive 80 kph, it will fluctuate from 72-92. If I drive 100 kph, it's a massive 86-126, and I bet that's what sets the code instantly. So to understand if was relative to engine RPM or speed, I shifted into first gear and revved it up to 3,000 RPM. 27-32 kph. So, vehicle speed. I'll have to jack the rear up and see what it actually reads...

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Today I took the opportunity to disconnect the VSS, connect a pigtail and take it for a drive while monitoring AC voltage. I got a very steady voltage relative to speed. 80kph yielded an unflinching 5.3~ volts. No fluctuating. I then reconnected the VSS wiring, but removed the 2 wires from plug on firewall and connected the volt meter there. Same reading as direct on VSS. It has me thinking it's somewhere north of that plug. Tested battery for AC current. 27.5 volts with engine off. (Same as a few other vehicles I tested for reference). I then started it and got 31.5 volts AC.

I DON'T KNOW...

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ok, let get you corrected a bit.
The AC voltage is what you read not current.
The reading will be in the low millivolt range on a good system.
Review what and how you made the readings.
Most good DVM voltmeters are able to read AC without the DC showing.
That reading should be in the low millivolt range.
A millivolt is .001 volt. In a good system you should not have more than about 50 millivolts (.050).
The battery provides a lot of filtering such that this AC does not normally cause an issue with other systems.
If there is high amplitude spike type noise, you may not be able to detect it as more than a higher average reading thinking it is ok.
A scope is the real way to chase your type issue because you can see what's there.
The alternator does 'rectify' the AC it produces via the rectifier stack.
The result is an AC signal with a frequency that varies with engine RPM.
If the AC amplitude is high enough, the AC signal from the VSS can mix with the AC alternator output and can result is a varying signal do to the varying frequencies that are mixing as the truck is driven.
This may all sound complicated but you need to hear about how it can occur, at least.
Keep at it, your on the right track.
Good luck.

 

Somewhat going back to the beginning here... Why would it throw a code for VSS and all the while the speedometer has no problems. Nothing erratic or intermittent about it.

If I maintain speeds under 80 kph the light will not flash. Over 80(ish) and it begins to flash almost immediately.

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