VSS gone (maybe) -> running weird.. suggestions?

[nkb93]

I think the VSS is shot, either the sensor itself (although it has only 1k on it) or the gear inside the MTX tranny is stripped. Either way, I'm getting a 452 code (insufficient input from VSS).

The question is, with the VSS not working at all, would it explain the engine wanting to cut out under moderate to full acceleration around 5k RPM? This behavior started right after the speedo quit. It's like a really agressive rev limiter, and it feels as though you're hitting a wall when it cuts out. It's not missing on a few cylinders, it's shutting the motor down completely until the RPM's drop back to 4.5k or so. Really strange.

-Nate

 

[SolidState]

I think the answer is rigth in front of you, and you have done your homework. Replace the VSS and/or gear and go from there...

 

[nkb93]

I got the vss off today, and the VSS gear in the tranny is definitely stripped (the shaft spins freely). Now I need to figure out how to get the VSS gear out without damaging anything. Hopefully the plastic gear on the diff is okay, as I really dont want to have to remove the tranny again.

Has anyone else noticed this distinct rev-limiting behavior when their VSS went bad or gear stripped out? With all the documented VSS failures, I'd think someone would have mentioned this issue in conjunction with the normal cruise control problems.


Nate

 

[SolidState]

VSS failures are rarely as extreme as yours....typically, it is just a malfunctioning sensor, and it is much more severe on an ATX, as it relies heavily on the VSS for its shift schedule.

 

[the mongo]

I'm trying to locate my vss to replace it on my 93 atx and can't find it. if any body can tell me where its located i would greatly appreciate it.

 

[nkb93]

It's pretty hard to get to on the ATX. It's located on the passenger side of the transmission, above where the axle plugs in. Here are a few links that might help:

http://www.shoforum.com/showthread.php?t=46946&highlight=ATX+VSS
http://www.shoforum.com/showthread.php?t=34897&highlight=ATX+VSS

run a search on "VSS replacement".

 

[Axianator]

The question is, with the VSS not working at all, would it explain the engine wanting to cut out under moderate to full acceleration around 5k RPM?

Yes, and here is why:

AUTOMATIC SHIFT SCHEDULE LOGIC

FUNCTIONAL DESCRIPTION:

This module handles the Desired Gear computation when PRNDL = DRIVE or OVERDRIVE based on shift curves of vehicle speed vs relative throttle position and engine based shift curves. The vehicle speed and engine based shift points are provided by the VSSP (Vehicle Speed Shift Point) logic. The gear availability flags are provided by the Gear Available logic. All gears are available unless failed. If they are not available they cannot be commanded by the 3 or 4 Shift Logic.

In the event that a vehicle speed sensor failure has been detected, engine rpm will be used to determine when it is appropriate to shift. The engine based upshift and downshift points will be provided by the VSSP logic and the vehicle speed based shift schedule will be disabled by the VSSP logic providing out of range VSS shift points. When VSS is failed, VS_D1, VS_D2, VS_D3 will be set to 0 and VS_U2, VS_U3, VS_U4 will be higher than an attainable vehicle speed. Calculated vehicle based shift points will be provided when VSS is functioning and the engine based downshift points will be set to 0.

In the event that gear failures have been determined by the gear ratio validity test or shift validation tests, desired gear availability will be determined by the Gear Available Logic setting the gear available flags. The flags will be set to control the shifts as follows:

- If the current gear is failed an upshift will be commanded if a higher functioning gear exits; otherwise the desired gear will remain unchanged until a downshift to a functioning gear is desired by the shift schedule. If all gears are unavailable, a default gear (DEFAULT_GEAR) will be commanded.

- Upshifts will be allowed only if a higher, functioning gear is available. In the event that the next highest gear is failed; but a higher gear is available, the failed gear will be available to be commanded. Once the shift in progress timer has expired, another upshift will be commanded (until the current commanded gear is a functioning gear).

- Downshifts to failed gears will be disabled by setting the failed gear available flag to zero for a downshift to that gear. The desired gear will remain unchanged until a downshift to a functioning gear is desired. (The lone exception is a 4-3 pull-in. If overdrive is cancelled, a 4-3 will be commanded even if 3rd gear FMEM is in progress).

Since your D4U1 PCM is expecting to see transmission inputs that are no longer present, you are already halfway down the road to the above-mentioned failure mode strategy. Combine this with a malfunctioning or absent VSS signal and your PCM is likely to limit your engine speed in a manner similar to what you've described.

It's like a really agressive rev limiter, and it feels as though you're hitting a wall when it cuts out. It's not missing on a few cylinders, it's shutting the motor down completely until the RPM's drop back to 4.5k or so. Really strange.

You've just described the PCM's "hard limiter" strategy, which, unlike the two "soft limiters" that limit engine speed through partial injector dropping, completely cuts fuel delivery to the engine until engine speed has dropped below a certain calibrated point.

 

[nkb93]

Thanks, Adam!

That explains why there haven't been many folks experiencing the same rev limiting behavior when the VSS fails. That's what I get for running the MTX with the D4U1, I guess. Lol, I ran the codes the other day, I bet it took 10 minutes to get through all the ATX and EGR related codes before it finally spit out the VSS fault.

Interestingly, I wasn't able to perform the KOER test. It would never blink the CE light while the engine was on, even though it was jumpered correctly (KOEO worked fine). I'm guessing it has to do with the way I have the MPS wired so that the PCM thinks it's in Drive in order to defeat the 4k rev limiter.. Perhaps the EEC wont run the KOER test since it thinks the car is in Drive and might take off.. Sound plausible?

If that's the case I'll have to rig up a switch to command Park instead of Drive. That'll also give me the ability to reinstate the 4k rev limiter for when I drop it off at the shop.

-Nate

 

["SHO"time]

Yes, and here is why:

Since your D4U1 PCM is expecting to see transmission inputs that are no longer present, you are already halfway down the road to the above-mentioned failure mode strategy. Combine this with a malfunctioning or absent VSS signal and your PCM is likely to limit your engine speed in a manner similar to what you've described.

You've just described the PCM's "hard limiter" strategy, which, unlike the two "soft limiters" that limit engine speed through partial injector dropping, completely cuts fuel delivery to the engine until engine speed has dropped below a certain calibrated point.

could the vss also be my problem?but the difference with my car is it acts up only after a couple hours of driving it seems like my trans is overheating .any advice on that?

 

[projectSHO89]

[QUOTE="SHO"time]could the vss also be my problem?but the difference with my car is it acts up only after a couple hours of driving it seems like my trans is overheating .any advice on that?[/QUOTE]


Since we have no idea what symptoms you are experiencing, it is unlikely we can offer any kind of useful opinion....

Steve

 

[haydenm315]

I think I may have a VSS issue but I am confused because my speedo works fine. My car displays erratic behavior if I put it in neutral while decelerating. Sometimes it will hold strong while others it won't. It doesn't seem to be temperature dependent.

My VAPS also doesn't tighten up now. It worked one time last week, but other than that it has been pretty loose for quite some time. I did most of the easy vaps tests and found that the steering was loosening and tightening during the test procedure, but not changing resistance during the voltmeter sweeps. I plugged another VAPS control module in this morning and it's still the same. I also noticed that my cruise control no longer works. Strange enough it worked a week ago when I was trying to diagnose the problem.

If the VSS sensor is done, can the speedo still work?

 

[projectSHO89]

...

If the VSS sensor is done, can the speedo still work?

Yes, provided the failure was due to the electrical portion of the sensor while still allowing the speedo cable mechanical linkage to be intact.

Steve

 

[ohfosho]

yes the VSS sends signals/outputs to the compu, which sends outputs to the VAPS system, the cruise control, and the ATX system. i dont know what effect a malfunctioning VSS will have on MTX cars besides the VAPS not correctly adjusting to vehicle speed, and the cruise as mentioned before.


i dont believe that there would be any engine troubles.