99 Sonata GLS V6/Auto. 178k on car now and over the last several months this problem is getting worse. I've done lots of searching and researching the web but found nothing solid. Basically, at any time and in any gear (even in OD at 70-75 mph) this thing will default to it's limp home mode (third gear) and turn on the CEL. This can be dramatic when it happens at high speed. It has set many different codes but the first thing I will replace (because it's cheap and easy) is the output speed sensor. I'm puzzled by the fact that this fault occurs at a steady state, not during shifts, etc.. Other codes present have to do with internal solenoids. Also seems important that "most" of the time, it works well and when it is working well there are no signs of mechanical failure-- slipping, etc.



Any ideas? This trans was used in many different models and even other manufacturers. Seems like something that would be a known problem?





Thanks!

 

Very strange - not sure where to start.

 

I started my transmission troubleshooting by replacing it with a standard but that's not for everyone. I'd say pull the pan and see if there is anything interesting and obvious going on. Like pieces of shattered needle bearing and lots of black goop, like what was in my trans. pan.



I guess it wouldn't hurt to also have a peek at the wiring harness and remove/reinstall the plug on the transmission computer. If you could snag a junkyard computer for VERY cheap I'd say try that before replacing all the electrical stuff in the transaxle.

 

Sounds good, Stocker. I'm going to the pick-and-pull tomorrow and last time I was there, there were three auto trans Sonatas so I intend to have my way with them. Computer, sensors, etc.. During my next round of troubleshooting I'll try to have a look in the pan and at the valve body for obvious signs of failure. One clue that I have discovered (maybe?) is that my trans. ECU has been replaced before and has junkyard yellow writing on it. Found that out while I was checking the connections like you suggested.

 

If you can get a guarantee of function (even a week or two guarantee) and can find the same model year of car with the right transmission, and it's not the computer, AND the price is reasonable, it might be worth trying a whole new transaxle for troubleshooting. But computers are much lighter and easier to install! Beware that the model year or at least the part numbers match. The electronics changed as the years went by.



I'll take this opportunity to repeat once again that I am so glad I have owned this car since it was brand-spanking new and did 99.94% of the maintenance to it myself. That is about the only benefit of buying new IMO and for a low-cost car it's arguably "worth it."

 

Definitely agree with your benefit of buying new. Though guys like you that keep their cars way past the point of paying them off are RARE. Today I picked up the input and output sensors for 4 bucks each from the junkyard. Beats 80 bucks or more from the parts store or dealer! I got the output sensors from two different cars and one input sensor. I decided against pulling the ECU from there because I need to double check that I am getting the right one. I'll start with the output sensor tomorrow morning and see what I get. The only problem is that I won't really know if I've fixed it or not until I drive it for a few days, as it is an intermittent pronblem. Oh well, gotta start somewhere.

 

I think I fixed it! I replaced the input and output speed sensors on the transaxle. On first test drive the problem was worse than before, and code read 'output speed sensor'. I replaced the output sensor again and now after about 25 trouble free miles I have no symptoms of trouble. Total spent was $12.00. Four dollars each for three junkyard sensors, one of which was bad. Fortunately I purchased two output sensors and the second one was the fix. Those sensors are currently $119.00 at AutoZone. So my four dollar junkyard items were a heck of a bargain, even though one of them was bad. So far so good..