Cadillac: Cadillac transmission, tv cable system, vacuum leak


Question
Hi Carlos,
I have a 1985 Cadillac Sedan deVille with about 150,000 miles on it. During the last few weeks it has begin to jerk when accelerating or going up a hill. I thought we had gotten some bad gas so had the fuel filter replaced last weekend when having the oil changed but that did not help. We then left on a long trip so I had a chance to try a few things and discovered that the symptoms could be overcome by manually shifting down when going up a hill so concluded that I have a transmission problem. The transmission fluid was low before the trip but added before leaving. What do you think? Is it serious or can it be corrected without a new transmission?
Thanks,
Richard

Answer
From what's you've said... since you can get rid of this problem manually by doing the shifting... that very much lowers the possibility of mechanical transmission trouble.  It sounds like the transmission is having some sort of electronic "freakout" or something.  Maybe the computer that controls the transmission might be bad, and by shifting manually you override the computer getting involved in making shifting decisions.  Now honestly I don't know if in 85 the Cadis had electronically controlled transmission, because many cars in the 80 were controlled by a cable attached to the throttle body that went to the transmission, and depending on HOW MUCH you pressed the throttle would delay the shifting longer, which is the way it should be.  It might be posssible if the transmission does make shifting decisions like this through what sometimes is called the TV cable, that maybe the linkage is messed up or the TV cable system inside the transmission is damaged.  Although, computer decisions are still present in this kind of a setup up since engine load is a factor the cable can't tell the transmission so VACUUM usually is a way of the computer telling the transmission when to downshift.  You might have a vacuum leak somewhere.  The transmission does not specifically have a sensor for that purpose, but the computer used the sensors used to deal with air/fuel mixtures like the MAP manifold absolute pressure), MAT (manifold absolute temperature) sensors, and many others.
Overall, I don't think you'd need a new tranny, but something is definitelly wrong with the systems dealing with the shifting decisions for the transmission.