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Discussion Starter #1
...and how can I get it to obtain the CORRECT 3rd gear ratio?

OK, seriously...

This repeatedly and randomly shows up when I check for trouble codes (it never illuminates the CEL).

I've noticed no driveability issues regarding 3rd gear and the tranny works OK overall (considering what it is). The fluid remains fresh.

What the heck is going on here?

I might add (and I don't know if this is relevant) that this tranny is a transplant; it came from a '91 with a 3.0... it's now behind a '91 with a 3.8. I understand that the 3.0 tranny has a lower final drive ratio than does the 3.8 tranny. Do you think the computer is somehow sensing this and regarding it as a problem? If so, why just 3rd gear?

Regards... Mike
 

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A different final drive ratio after the correct 3rd gear ratio is going to give it a bad "ratio" for 3rd gear. The different final drive is throwing it off.
 

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Discussion Starter #3
OK, thanks Millermagic.

If I install larger diameter tires, would that change the overall ratio enough to fool the PCM into thinking everything is OK? Maybe not... my head hurts just trying to figure that one out.

I'm not too concerned about what the PCM thinks then, as the DTC seems to be the only symptom.

I've been considering taller tires though, in an effort to get the upshift points back down to a more normal level... the engine currently revs more than I like it to before upshift occurs. I'll have to experiment.

Regards... Mike
 

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No, the tires won't help. The sensors are in the transmission and the wheels are outside of the transmission.
 

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As Quertz said, tranny monitors TSS (turbine speed sensor) and OSS / VSS (output shaft sensor / vehicle speed sensor) and looks for the correct RPM ratio between these sensors depending on the gear selected by the PCM. If the final drive ratio is different (inside the tranny) the PCM will pick this up and set a incorrect ratio code.
 

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205/75-15s or 215/70-15s would get you close (if they fit).
 

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Discussion Starter #9
I kind of thought bigger tires wouldn't fool the PCM but I wasn't smart enough to know that for sure.

Converter lockup works flawlessly, however.

The PCM only seems to be advising me of the incorrect ratio... it doesn't seem inclined to take any corrective action, so I can easily live with that.

I will, however, try taller tires in an effort to get the rpms back down to a more comfortable range, as I find that more annoying than anything.

Thanks for the advice guys... I really appreciate it!

Regards... Mike
 

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The code is meant to indicated something such as a slipping TQ or slipping bands... general transmission malfeasance. It takes the VSS input and compares it to the TSS input to get a ratio which it compares to a stored value. If it's off by more than a bit (to allow for tire size changed, etc.) it will throw the code. My 94 threw it once when it got really hot out one day and the TQ started slipping.

If you're running a tranny with a different final gear ratio, then you will get that code all the time, every time. The PCM cannot possibly know that you're running a different transmission and it can't correct for it. Remember, computers operate wholly within the constraints of their programming.

the 3.0 would've come with a much shorter gear ratio to account for the fact that it doesn't put out near the torque of the 3.8. I don't know what the ratio is, but you'd have to figure out the, um, ratio of the ratios and apply that number to the total diameter of the tires to account for it. i.e. if the 3.0L tranny is 35% shorter than the 3.8L tranny, your tires will need to be 35% taller to account for it.

Basically, the only way you're going to get rid of it is to a) put the right transmission in it or B) run super swampers. Which has been done before.
 

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Discussion Starter #12
hawkeye18,

Thanks for the explanation. According to my calculations, the 3.0 tranny final drive is only 5.5 percent shorter than that of the 3.8 tranny for that year. I suspect that a tire of around 5 percent larger in diameter will fit without clearance issues. We'll see.

Millermagic,

My 3.8 doesn't seem particularly quick, but then I don't push it hard because it feels like it's over revving before the tranny hits the next gear when I do. I'm sure it doesn't hurt anything... but it just doesn't feel right... like the engine isn't happy there. The 3-8 definately feels torquier (not a word, I know) under load at lower rpms than the 3.0 however. I think the lower final drive is actually having a somewhat detrimental effect on the performance characteristics of this engine.

Regards... Mike
 

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Here are the origianl final drive ratios.
[/b]
It's intriguing that the 3.0L SLO units and the 3.8L police units would have the same final ratio.

Now, you must keep in mind that that code really could be your transmission crapping out! So don't discount that completely.

Theoretically, if the tranny is the wrong ratio then all of the gears should be throwing a bad ratio code, but I don't know the trannies well enough to say for sure.

But, to answer your question, yes, running a tire combo that is 5.64% over stock. Running 215-70-16s gets you 6.5% slower, which should be pretty close... but it's gonna look like you're running pickup tires lol. 225-65-16 gets you looking a little more normal, puts you 5.2% slower, and gives you a wider contact patch to boot, though you may run into fitment issues.

This is of course assuming you have 16" rims. If you don't, go here and do the math.

Good luck.
 

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Discussion Starter #16
I don't really think this tranny is heading south, as it has been this way since I installed it 3 years/30,000 miles ago. It is subjected to severe stop-and-to service as well, and still, it hangs together. The symptoms are very stable, not having worsened whatsoever.

I've serviced the fluid/filter a couple of times and both times the fluid looked like new and the pan & magnet were exceptionally clean.

A fault pointing specifically to 3rd gear makes me wonder too, but perhaps it's just one of those unexplainable mysteries.

A different PCM must have been used in police applications, but I'll bet that they are about as scarce as hen's teeth.

I'm wondering if the TPS would also have a slightly different voltage curve to coincide with this engine/tranny combo.

An online lookup indicates that the TPS used on the '91 Continental (same tranny gear ratio as my Sable) is identical. The PCM is a different part #; I don't know if it's because of the different tranny, different body, or both.

Regards... Mike
 

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I'd bet that using (which means FINDING) a police 3.8 PCM would solve all your problems. I don't even know what the catch code for one of those is.
 

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A police PCM would cause far more problems than it would solve. I don't even think a 3.0 trans will play nice with the police PCM because they're programmed differently. Combine that with all the rest of the different electronics and the different injectors, etc. and it's definitely not going to solve anything.

That, and there's four different police PCMs for 1991.
 

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A police PCM would cause far more problems than it would solve. I don't even think a 3.0 trans will play nice with the police PCM because they're programmed differently. Combine that with all the rest of the different electronics and the different injectors, etc. and it's definitely not going to solve anything.

That, and there's four different police PCMs for 1991.
[/b]
I did not know that. Interesting. The limit of my knowledge is pretty close to where SHOs stop and regular Taurii start...
 
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