Anyone have experience with a '91 Ford diesel?

[gone huntin]

'91 F250 7.3, e4od, 4x4,harsh shifts, vss code. It has it's 2nd NEW speed sensor in the rear diff and a new one in the t-case. The exciter ring is good, and so is the harness at the plug. I'm getting harsh shifts and a VSS code. 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.

 

[Fordum]

I don't have a lot. I special ordered my with a C6 when those were just coming out because I knew that it would take a while to get the bugs out.

The MLP/TR sensor, transmission lever position sensor, can cause all kinds of problems if it gets moisture in it.
Some people call that a neutral safety switch, but its more than that on that tranny.

The hard shifts are due to the code being set.
That is it's version of limp home. The transmission does not know what amount of line pressure is needed, so it maxes it out to prevent burning up clutches.

I hate to say it, but more than likely you are going to wind up going somewhere that can read the VSS pulses directly to see if they are correct.

I have seen new bad sensors, and plus they are critical as to placement. The distance between the ring and sensor is important as is angle.

If it is setting a vss code, then something is wrong. Have the harness double checked, or swapped out if you can, and make sure that the sensor are mounted tightly and correctly.

 

[Confedneck]

my guess is that the excitor ring is bad, cracked and shifting. how did you conclude that it was fine? which one is fine?

 

[Fordum]

Good point. You need to verify that the exciter ring really is good. BTW, if the tranny is producing max pressure, that can cause an RPM fluctuation also.
Get the transmission right and then see if the fluctuation is still there.

 

[gone huntin]

The local tranny shop had me checking the exciter ring in the rear diff. After talking to a FORD transmission tech I found out that in '91 that one is only used for the ABS. The only one that affects the trans is the one mounted in the t-case output shaft. The code is for an open circuit in the vss circuit for the one in the t-case . I did a continuity test on the vss wiring between it and the pcm connector and had good continuity. I removed the tailshaft and looked at the teeth on the drive gear for the vss. I also removed the diff cover and inspected the exciter ring back there before I found out that had no bearing on trans operation for that year.

 

[gone huntin]

Guys, thanks for posting, I posted this over a week ago on three different diesel boards and have gotten a total of one reply between the three of them.

 

[Fordum]

Guys, thanks for posting, I posted this over a week ago on three different diesel boards and have gotten a total of one reply between the three of them.

Like they say, try the rest, then come to the best!!

 

[brian wafer]

i replaced a fuel pomp last winter on a 89 or 90 7.3. when the guy came back a few days later he complained about a hard shift. the trottle position sensor on the drivers side off the fuel pomp was off. played with it and everything came back too normal. its slotted and has alot off play i hadn,t install back in the right place!

 

[Confedneck]

open circuit code for the vss means either the sensor is bad, the circuit is faulty or the pcm has an issue. a cracked/slipping exciter ring will not cause an open circuit code. you need to test that new vss... do you have access to a good scanner, either factory or aftermarket, where you can monitor the vss while driving the rear wheels on jackstands?

 

[gone huntin]

i replaced a fuel pomp last winter on a 89 or 90 7.3. when the guy came back a few days later he complained about a hard shift. the trottle position sensor on the drivers side off the fuel pomp was off. played with it and everything came back too normal. its slotted and has alot off play i hadn,t install back in the right place!

TPS adjustment has been checked, it was right on.