: 96 Seville/97 Deville - different gear ratio?



H3nr1
03-19-07, 04:03 PM
Hello,

About a 97 Deville. After heavy acceleration I get message "service transmission" and code "incorrect gear ratio". The Deville just got engine swap from 96 Seville, transmission included, identical as I was told. Is it possible that there's difference in gear ratios and the computer gets confused or is the problem somewhere else, can it be fixed any other way than switching back the transmission? What are my options?

Thanks in advance :) ,

Henri

Ranger
03-19-07, 04:50 PM
Did the engine and trans come out of an SLS (compatible) or an STS (not compatible)?

H3nr1
03-19-07, 05:19 PM
They came from STS.... :( am I in trouble now?

Henri

clarkz71
03-19-07, 05:22 PM
You'll have to live with the code & service trans message.

H3nr1
03-19-07, 05:52 PM
The thing is, along "service transmission" comes on, the gearshift switches to save mode, oil pressure gets high and shifting becomes really "rough". Stopping the car and restarting solves it.... well, 'til next harder acceleration. I definitely need to do something about it. In theory - would it help to put back the original transmission?

H

clarkz71
03-19-07, 06:11 PM
The final drive ratio's are different, 3:11 for your old transaxle, 3:71 for the new one. The STS has less torque, hence the lower gear. It will be better then what you have, no problems other then feeling a little soft off the line. I don't think you have any other option.

Ranger
03-19-07, 06:57 PM
Tree moons need to be in line. The engine, trans & PCM. Your STS engine & trans are mated to each other, but not to your PCM. If you swap the trans, it will be mated to the PCM, but not to the engine. I know your next question will be, "can I swap in an STS PCM". The answer is, I don't know. Ewill3rd or someone else might know.

clarkz71
03-19-07, 07:10 PM
PCM requires a vin # and is flashed with the cars vin. I don't think you can just install another PCM. As you said, ewill3rd will know. The main thing is the PCM & transaxle. You can get away with the taller gear.

ewill3rd
03-20-07, 05:50 AM
Yikes, this is a lot of pressure!

I'll see if I can answer your question.
Step 1, if possible, is to get me the last 8 digits of the VIN for each vehicle. From there I can check the transmission RPOs and check the final drive ratios, however.

The computer monitors the input shaft speed and the output speed of the transmission, it compares the two and calculates the gear ratios, compares them against stored data in the PCM and if the gear ratios do not match what is stored it will set codes and go into a limp mode where it can command max line pressure and use a default shift pattern to "limp" you to a place to get it checked.
I think Deville and Seville did have different final drive ratios and there may not be a way around finding another trans.
Those don't have pluggable PROMs, it is software programmed and the VIN is part of the programming. If you tried to swap the PCM you'd have to reprogram it anyway to get it to work and using a different VIN will not get you far because you'll have other problems.

The only thing I might be able to tell you by VIN checking is what the ratios are (if I can remember how to look that all up) and why it keeps setting a code. I'd say that you might be in for another trans swap.

H3nr1
03-20-07, 06:44 AM
Thank you all for your time and know-how as I try to solve this problem :) I really appreciate it!

I agree, swaping the PCM is not something I'd like to consider, I rather go back towards original. I will try to get hold of the Seville's VIN and post both here asap. I have possibility to put back the original working 97Deville transmission - then the possible match difference will be between engine and trans, I wonder how that might affect to the PCM operation? Unfortunately don't know the details of the difference in particular Deville/STSSeville engines or gearboxes, are they rather mechanical or electronic?
(Though, if changing back just to old transmission would clear the problem, I'd be a happy man :)

H

ewill3rd
03-20-07, 07:23 AM
Why was the trans replaced to begin with if I may ask?
The internals should be pretty much the same, but the final drive might be different. That will cause the computer to see illegal gear ratios and set codes.

clarkz71
03-20-07, 07:28 AM
He replaced the complete engine/transaxle in his DeVille (vin Y) with one from an STS.

ewill3rd
03-20-07, 08:55 AM
I know WHAT he did, I was wondering WHY he did it.
;)

clarkz71
03-20-07, 08:57 AM
More then likely headgasket failure. I'm sure he will let us know.

ewill3rd
03-20-07, 09:20 AM
I see, this is starting to make more sense now.

H3nr1
03-20-07, 02:25 PM
The 8 last digits of both VIN codes:

Deville: VU204060
Seville: TU820071

That's right, blown head gaskets was the reason so engine/trans were replaced, taken from a 96Seville STS... unfortunately, as it seems.

H

Ranger
03-20-07, 03:41 PM
then the possible match difference will be between engine and trans, I wonder how that might affect to the PCM operation? Unfortunately don't know the details of the difference in particular Deville/STSSeville engines or gearboxes, are they rather mechanical or electronic?
The difference between the two engines is the intake cam, giving the Deville more low end torque and the Seville more high end HP to match the different final drives.

ewill3rd
03-20-07, 05:18 PM
Well one problem I see is the engine in the V (97) model year car needs a fuel rail, or needed one. Open recall.
It has an RPO MX0 transmission.

Now the funny part, the T model year (96) ALSO has the open recall for the fuel rail, so either way you need to get that car in and get the fuel rail replaced.
It looks like it also has an MX0 RPO for the transmission but they might still have different final drive units.

Let me do some research on what the actual final drive ratios might be and see if I can identify a difference.
I show 4 final drive ratios in my reference guide:
2.84, 3.11, 3.48, 3.71
I don't exactly know how to identify which one has which.
It may have to do with the ID tags on the side of the trans, but I don't work with them very much so I'll have to do some checking to see.
My guess would be that the Seville has a different final drive than the Deville.
(as I mentioned)

I'll see what I can find out.

ewill3rd
03-20-07, 05:22 PM
Ooops, there it is.


The '96 has FV4 which is 3.71
The '97 has FV3 which is 3.11
(and shows as a limo)

I think we found the problem!

clarkz71
03-20-07, 05:43 PM
The final drive ratio's are different, 3:11 for your old transaxle, 3:71 for the new one. The STS has less torque, hence the lower gear. It will be better then what you have, no problems other then feeling a little soft off the line. I don't think you have any other option. I think I already said that.:)

H3nr1
03-20-07, 05:52 PM
Thanks for that, I guess I am starting to understand now, bit by bit. As I found out STS uses L37 type N* and Deville has LD8, there's significant difference in characteristics, for reasons you described. One has optimum torque range on higher RPM than other, am I right? So even if switching in the Deville transmission(suited for low end torque) the PCM still might get incorrect reading from the engine when comparing to the data installed? Followed by another code and "limp mode" possibly?

(Things I believe are relevant, currently the cruise control is not working and the fuel ecnomy is quite poor.)

H

clarkz71
03-20-07, 05:55 PM
So even if switching in the Deville transmission(suited for low end torque) the PCM still might get incorrect reading from the engine when comparing to the data installed? Followed by another code and "limp mode" possibly?



No, the only difference in the engines is the cams. The PCM won't see that. Put the DeVille transaxle in, you'll be fine.

ewill3rd
03-20-07, 07:04 PM
Clark, you sure did say that.
I am good with transmissions but I work on one every other year so I am a little rusty on them.
I quit memorizing a lot of things that I don't deal with often.

I agree the engine shouldn't matter. It may not run at it's peak performance level but the PCM will undoubtedly not see much of a difference.
The only reason the trans is freaking out on you is because of the final drive unit.

clarkz71
03-21-07, 05:05 AM
I agree, I hope it works out for him.:thumbsup:

H3nr1
03-21-07, 10:14 AM
Thanks guys, you've been such a great help!

Is it reasonable to replace just the final drive unit in that case, will it be basically the same as swaping back the whole transaxle?

H

clarkz71
03-21-07, 10:49 AM
You have to pull the transaxle to get to the final drive. At that point you might as well just install the DeVille transaxle and be done.

ewill3rd
03-21-07, 01:01 PM
I haven't done it, but looking at the manual it looks like all you need to do is remove the extension housing and take the snap rings off to get the final drive and sun gear out.
You might have to remove the internal shaft which means both axles will have to come out but it looks like a feasible in car repair.
You might experiment on the drivetrain on the ground before you try doing the one in the car.

clarkz71
03-21-07, 01:51 PM
Wow, didn't think it could be done in the car. That's great news for this guy.

ewill3rd
03-21-07, 03:25 PM
On the old ones (and some of the smaller ones) you have to remove the final drive after you gut the transmission, on the 4T80 you just remove the right side extension housing (and whatever it takes to get that off) and remove some snap rings and both pieces should come out.
At least that is how it looks from the pictures, I have never actually taken one out.
It will still be some work but at least the trans can stay in the car.
Just replace the planetary carrier and the sun gear for the final drive and I'd say you'll be good to go. You will probably need an extension housing seal.

jadcock
03-21-07, 03:30 PM
I have also heard that you can R&R the final drive assembly "simply" by removing the right side extension. Probably not all that easy, but still much easier/quicker than R&Ring the entire transmission.

I would be real interested to hear from the original poster after replacing the final drive. This is what we've talked about a lot, "in theory". "In theory", what if you swapped STS camshafts into an SLS car (which is essentially what will have been done here)? We believe that overall performance, especially down low, will suffer due to the taller final drive. But an actual field report will be invaluable, and we may learn that when you let the L37 run all the way out with those tall gears, it does pretty good. I look forward to a follow-up post after the work is done.

My personal favorite would have been the other way around -- swapping an SLS engine into an STS car, and let the higher torque of the LD8 engine work with the shorter final drive of the STS transaxle. That's how they sold the Pontiac Bonneville GXP -- the 275 hp engine with the 3.71 final drive. Ideal in my opinion. :)

H3nr1
03-21-07, 06:28 PM
I wondered also about the chance and I will see if it works out with replacing the final drive unit with trans in place... fingers crossed. As I understand, the real difference then left will be just the cams...
I will be wiser on Tuesday next week. I will let you know how it turns out and how the car performs after.

H

H3nr1
10-06-07, 04:05 PM
Hey guys! Coming back to this old story.
Just remembered to tell you it really worked out very nice! Removing the extension housing and snap rings was enough to to get and replace the final drive unit (didn't do it myself though).
No more messages, car drives beautifully! I cant tell much about the effect of the STS intake cams since I didnt drive the car with the original engine. What I can tell is that I definitely feel some extra kick appearing at higher rpm, I love it! The average gas milage is 22 mpg which is great for me.

Again, thanks for your help! :)


Henri