Mexstan

Have been asked to take a look at a 1998 Ford 350 15 passenger Club Wagon. It has a V8 but unsure which one. It is in reasonably good shape but between 45 and 48 mph with light accelerator pressure, the thing shudders. Have driven it and the shudder is very hard and do not think it is a misfire. It may be a transmission problem. When this happens I have toggled the overdrive switch on and off, but nothing changes. The engine has recently been thru a tune up (new plugs etc), but that did not change the problem. It has also had the tranni oil changed. When the shuddering happens and I accelerate hard, it immediately takes off and the shudder vanishes.
The check engine light is on and the mechanic that checked the code tells me that the code translates to a speed sensor. He gave it to me in Spanish and the direct translation is "velocity selector". No idea what this or where it is located.
Can anybody give me some sort of clue how to fix this problem?

Hillbilly66

Does it have a two piece drive line? If so I would check the carrier bearing. Wouldn't be a bad idea to check the u-joints while you're under it. I would imagine the "velocity sensor" is the speed sensor. My guess on a 1998 that it would still be on the tail housing of the transmission.

Ronco95

Sometimes the rear ABS sensor also gets used for vehicle speed.

Mexstan

Thanks guys for the answers so far.

It is a one piece drive shaft.
There are no mechanical noises or squeaks when the shudder occurs. Actually, other than this problem, it is a very smooth drive.

Presume that there is a tone ring in the diff. Also presume that is virtually problem free. Am I correct? Could the problem be a bad sensor on the diff? What about dirty oil in the diff. No idea when it was last changed. If I do take the cover off, what oil do I put back and about how much will it take?

SIXSLUG

Rev it thru the rpm range in park and see what happens.

If it is driveline related, that will rule out the engine.

Could be a bad tire, out of balance driveshaft, trans going south....

Mexstan

Will not get to see the vehicle until next week, but will rev the engine in neutral and see if anything strange happens.

Really do not think that it is driveline or out of balance tire related because it is too slow and too hard. It is not a vibration, but more like a solid thump, like a tranni hunting very fast.

Hillbilly66

If it sounds like a thump, check all of the trans, and engine mounts.

It's possible the sensor on the rear dif is bad, could be loose wires or connection. Also the sensor should be magnetic so if it's attracted any junk it may not be able to read right. That gives me another thought....

It is possible that this thing has a limited slip rear, vibration is only under acceleration, correct? If you have limited slip, is it possible that under light acceleration that it is trying to partially engage?

Rear dif should be a synthetic 75-140. I'd guess 2-3 quarts at the most.

Mexstan

Thanks for the tips. Hope to be near this vehicle tomorrow, so will check it out.

Mexstan

Finally had a chance to look at this vehicle again. Removed the sensor from the rear end and cleaned it up. There were a few metallic fragments on the outside, but nothing on the 'face'. The oil that was on it did not look very dirty.

Drove the van for a longer period this time and it may be my imagination, but the 'thump' seems to have diminished, but now I can feel what appears to be a misfire at all speeds while accelerating. It is not very obvious. The obvious misfire or thump is around 38 - 48 mph. Above and below that speed it is almost non existent, but still there.

Not going to be in the vicinity of the van again next week, but will then check if there are any codes and if it is possible to delete the existing codes with the el cheapo code reader that I have.

Anybody have any other ideas? What can cause an intermittent miss under hard acceleration? I did try putting it in park and revving the engine slowly up to quite a high speed, but it was rock steady.

j_martin

Pressure raises the firing voltage of a spark plug. When you open the throttle under load, the engine takes in more air/fuel and compresses it to a higher pressure.

Plugs used to lead foul and need replacement. Modern engines don't deposit anything the plug, so the failure mode is erosion at extremely high mileage.

1 or 2 cylinder misfires under hard acceleration are almost always high voltage electrical issues, ie worn plugs, bad wires, etc.

Mexstan

j-m, I agree with you. Supposedly all the spark plugs have been recently changed and the HT wiring checked by a mechanic, but the problem is still there. As I get some free time (what's that?) I will continue to play with this beast and see if I can track the problem down.

j_martin

All HT parts are involved. Don't know that particular engine, but if individual coils, one of those could be bad. If distributor, the cap, etc.

More than once I've found a totally open HT wire that was "checked by the mechanic"