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Discussion Starter · #1 · (Edited)
I have a unique problem. After about 30 miles of driving, my 2001 Saab 9-5 Aero (74000 miles) has a check engine light come on and an exlamation point alert next to it. After driving it a little more it goes into "Limp home mode." I took it to a transmission place and they held it for 2 days then said they would need to take the engine out to see if they could find anything. But they charge $900 just to look. I'm a recent college grad so I dont have $900 to blow just to see if they can find something, then pay more labor and parts on top of that if they actually find something. I know its not an easy process but it's more than I can afford. I can definitely feel a problem when the cars shifts gears (automatic). The code that comes up is P0730- Incorrect Gear Ratio. Please let me know if you might know whats wrong.
 

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this diagnostic trouble code is generated when the ratio from input (engine speed) to output (vehicle speed) is not "correct" for the gear that the car is currently in.

This code will not be generated until the transmission is at operating temperature (above 20-degrees C) so you may have an issue before you get to that temp, but the DTC won't get kicked. Once that happens, the transmission is in limp-home-mode (different from engine limp-home-mode) and will only go into 4th gear in drive and 2nd gear in Low.

There are many causes for this. In order of "lowest cost" to "highest cost" they are:

1: the fluid is low, bad, old, whatever and that is causing the transmission slip. A band or a clutch is not holding properly. The remedy is to do a 3-x drain, fill, drive 20 miles with the proper Dextron III (or equivalent) fluid.

2: you have a bad wire from the transmission speed sensors or a failing TCM. The two speed sensors are each 2-wire systems, one is the input voltage that is sent to the sensor and the other is the returned pulse signal indicating speed. You can measure the resistance of the wires, check for opens/shorts, etc with a cheap voltage meter and a wiring diagram (the diagram is in the wIS) Don't discount the TCM having gone bad, that was what was wrong with my AW55 that was acting up a few months ago. TCM's are cheap for your car, less than the speed sensors we'll talk about next.

3: you have a bad input or output speed sensor. There are two sensors on the transmission that measure the input shaft and output shafts and the transmission computer is always looking at those and since it knows what gear the transmission is in, it knows a defined ratio between the two. On the newer AW55 transmissions these sensors are the same, I don't know about the AW50 that you have. but you could buy one, swap it (they are easy) and if you still have the trouble then swap the other one. The sensors for the AW55 are about $100 each, I suspect that the AW50 sensors are similar.

4: There is a bad solenoid or passage in the valve body. These are electrically controlled transmissions and there are solenoids that regulate the pressure to the internal components. If one of those if failing and the pressure to hold a band is too low, then the transmission will slip. The valve body can be taken out with the transmission in the car, it's a couple hour job. But a valve body will cost nearly as much as a transmission, you just aren't paying for the labor to replace the whole transmission. There are places that rebuild the valve bodies and the solenoids. (PM me if you want to go there, I had mine rebuilt before I realized I had an electrical problem)

5: something internal to the transmission is worn out. A band, a clutch, the main pump, whatever and it's causing slipping to the point where the transmission computer detects a mismatch. This, of course, will require that the transmission comes out and is rebuilt or replaced. Your car only has 74K miles on it and this transmission is known to last a good bit longer than that. I would think that this is not the issue, but there are always defects.

Depending on your level of skill, you can do all of these jobs, the parts (and the labor) get more expensive as you go through the list.

good luck!
 

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There are multiple possible causes. What worked for one person isn't necessarily your solution.

Unclemiltie's post is spot on. Follow his approach, either by swapping parts or using Tech 2 to find the problem.
 
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