'91 F250 7.3, e4od, harsh shifts, vss code

[Gone Huntin']

This is on my '91 F250. It has it's 2nd NEW speed sensor in the rear diff. The exciter ring is good, and so is the harness at the plug. I'm getting harsh shifts and a p0500. The other day the brake and abs lights came on together and the trans started shifting normally. This went on for about two hours, then the lights went back off while driving and the harsh shifts came back.The OD light has not come on or blinked at all. The trans was rebuilt at the same time the tach sensor was replaced. Any help would be appreciated. It's also got a bit of an RPM fluctuation while cruising, the tach sensor was replaced around 40k ago.

 

[LCAM-01XA]

You are replacing the wrong VSS - your '91 truck has two of them, the one in the axle is used by the ABS only. The one ya need to replace is on the transfer case, follow your speedometer cable from the floor board at the front of the cab down and back to the transfer case and you'll find the VSS. That VSS serves a dual purpose - it drives the speedometer gauge mechanically (though the cable I just mentioned), and also has an electrical component in it that sends vehicle speed signal to the E4OD brain. That sensor is like $20 and in stock at about any parts store, and I just stuck one in my own truck yesterday chasing a similar issue (delayed upshifts, but no blinking light, hell I didn't even get a VSS code).

That said, P0500 is not an EEC-IV code, but an OBD-II code - your truck's PCM should have stored a 2-digit code, like for instance 29 (insufficient input from VSS) - what scan tool did you use to retrieve the codes?

Also, the ABS and "brake" lights coming on together indicates an issue with the ABS system, bit in your truck that "should" not have any effect on the way the transmission behaves...

 

[Gone Huntin']

I took it to the transmission shop. They told me p0500 and to replace the sensor in the rear diff. He used a Snap On scanner, I think he has a MODUS.
I'll try a tranny speed sensor, it's only $18.99 at AZ. This truck does eat speedo gears. I've replaced the one inside the transfer case twice in three years. It eats the center splines for some reason, never damages the one on the sensor.

 

[LCAM-01XA]

Hm, interesting - maybe his scan tool has a feature that translates EEC-IV codes into OBD-II codes, and so whatever EEC-IV code the PCM had stored got converted to probably the most vague OBD-II VSS code there is, the P0500 ("VSS malfunction"). His recommendation, while possibly correct for a '92 and newer truck (assuming it is indeed the VSS acting up), is inaccurate for your '91 truck - as I said, your '91 has two speed sensors, the one responsible for transmission operation is on the trans tailshaft housing if 2wd truck, or on the transfer case if a 4x4 truck. The on-axle VSS in your truck controls only the ABS, nothing else. In '92 and newer trucks tho, the forward VSS was eliminated, and so the one on the axle is used for both ABS, and E4OD and speedometer.

What you say about the splines wearing out is weird tho, ain't the drive gear in the t-case steel or aluminum? Cause the VSS gear is plastic, one of the reasons for which is that a VSS gear is a whole lot easier to replace than the drive gear inside the transmission or transfer case... And yes, the AZ sensor is what I got too, Duaralast SU1019.

 

[Gone Huntin']

The drive gear is yellow plastic, and the driven gear on the vss is pink.

 

[LCAM-01XA]

Oh, okay then, that makes sense about your drive gear wearing out if tis plastic. I'm used to 2wd transmissions where the drive gear teeth are usually cut into the output shaft. That said, I have seen one of those wore out too, now how in the world that happened is anyone's guess

 

[Gone Huntin']

Well, I replaced the speed sensor in the t-case, same problem. If I had any hair I'd be pulling it out now. This truck has to tow 9000lbs,700 miles, in two weeks. I need to get it fixed.

 

[Gone Huntin']

Anyone else have any experience with this issue?