code 730

[Nighteagle1010]

my dad has an 1996 Cadillac Seville SLS code 730 keeps coming up we have had 5 used transmissions put in it. same code comes up we have had 2 PCM's put in it and reprogrammed and still same thing none of the transmissions have worked they all shift hard. Any thing that you could sagest would be greatly appreciated

thank you

[Cadillac Jim]

P0730 Incorrect Gear Ratio

Someone put in a transmission from an STS. The transmissions are identical between the SLS and STS other than the final drive ratio. The differential comes out the right side of the transmission fairly easily and can be swapped for the right one.

[Nighteagle1010]

thank you very very much I am going to call my dad right now again thank you

[Ranger]

The SLS has a 3.11 final drive and the STS has a 3.71. A Deville trans should work too. It has the same trans as an SLS.

[Cadillac Jim]

Anyone have any advice as to the advisability or amount of work in changing out the final drive gears versus switching the complete transmission again?

[Nighteagle1010]

I would like to thank everyone for their replies you guys are GREAT you have no idea how much I appreciate your help thank you all very much I talked to the guy working on my dad’s car he clams it is in the wiring going to the computer, but I think you guys are dead on target.

[Cadillac Jim]

If the wiring to the computer was a problem, you would be getting lots of other codes. If all you are getting is the P0730 CURRENT, then you are running 3.71 gears when the PCM expects 3.11. It measures the gear ratio through the speedometer sensor, which measures rotation of the right half-axle, and the input speed sensor, which is inside the transmission. In 4th gear over 41 mph at a cruise, the TCC kicks in and the engine RPM can't vary more than 50 rpm without throwing a TCC code. Given those two rates, and the known 4th gear ratio, the final gear ratio is easily computed.

If this guy is going to tear into the wiring instead of fixing the problem, take the car out of there and get someone else to work on it.

[Nighteagle1010]

I am going to tell my father right now and again thank you. You have been such a blessing to me and my father. I wish there was somthing I could do you, for all the help you have givin us.

again thank you

[Cadillac Jim]

NE1010 asked in a PM about telling from looking at the transmission if you could tell which gear ratio it has.

I happen to have the 1997 Unit Repair Manual for transmissions, transaxles, and transfer cases, which includes the FSM for the 4T80E and all other GM transmissions for the 1997 model year. I found the section on the VIN derivative and the transmission serial number and didn't see anything there that would differentiate the final gear ratio.

I believe that tearing into the transaxle to look at eh final gear ratio is unreasonable here. That requires removing the right half-shaft, a retaining ring, and sliding out the differential assembly, then counting the gear teeth. Here's what I recommend:

• The 4T80E has a VIN derivative for the car it comes from stamped on it at several locations. The VIN derivative is the 3rd and the 10th through 17th digits. For a 1997 Cadillac Northstar the 3rd digit will always be 6. The 10th digit is the model year, V for 1997. The Seville and Eldorado are built in Hamtramk, for which the 11th digit will be U. The last six digits are the plant serial number for the car. I don't see where these are on the 4T80E but if you see a number like 6VUnnnnnn, where nnnnnn is a six-digit number, the six-digit number will be the last six digits of the vehicle VIN from which the transmission came.
• On the vehicle, the VIN is visible on the dashboard on the driver's side and is visible through the windshield. You can use the last six digits to match the car to the transmission. Look at the 8th digit, this will be a "Y" or a "9." The cars that have a "Y" have the 3.11 gears. You don't want a transmission from a car with a "9" in position 8 of the VIN.

If the recycling yard has separated the transmissions from the cars, ask them which went with which car. If they don't know, then you don't know which gear ratio the transmission has. The VIN derivative may or may not be on the 4T80E; the manual gives locations for the VIN derivative for the Corvette transmissions but not the Cadillacs, but I have heard that VIN derivatives are all over all GM cars and have been for many years.

A recycling yard should be able to tell you definitively the VIN of the car that a transmission comes from. This is absolutely necessary because any transmission that ends in "E" is operated by computer, not hydraulics, and must be matched to the correct car type you the car will produce an OBD code like the P0730 that you are seeing. If they can't you can gamble because the majority of cars sold in most parts of the country are VIN "Y" cars but its still a crap-shoot. Don't take wave-offs, reassurance that "they are all the same" and such; go elsewhere if necessary; you've been there, done that, and got the T shirt.