Transmission Is Not So Automatic

Question: I purchased a 1984 Ford Thunderbird with automatic shift. Each time it shifts gears, there is a very pronounced jerk. Is this normal for this model? E.B.

Question: Since this past August, my 1986 Ford Thunderbird has developed a clunk in the rear when going in and out of overdrive. It is getting progressively worse. Ford representatives tell me that this is “normal.” I ask, is it normal to hear what sounds like a heavy hammer hitting a casting every time the transmission shifts gears?

Answer: I guess it is normal if you have “driven a Ford lately,” as the advertising motto goes. The transmission in your Thunderbird is probably the A4LD and the clunk can be caused by a variety of unrelated problems--including the emission control system, which is controlled by the same computer as the transmission.

All automatic transmissions need to be told when to shift. The shift points are based on engine speed, engine load and car speed, among some other factors. As cars have become increasingly computerized, a central computer determines the correct shift point. About 75% of Ford cars now have electronically controlled transmissions.

The computer receives information about the engine load from a throttle position sensor. If that sensor is out of adjustment, it can cause a hard shift since the transmission is shifting at the incorrect engine speed. Even something as seemingly minor as a dirty air filter can cause a transmission shift problem, because filters can affect measurements of engine load.

These are only a few possibilities that do not include fundamental problems with the transmission itself. If your Ford dealership mechanics are unable to properly diagnose the problem, perhaps a transmission specialty shop is a better bet.

The simple solution to the problem may be changing your gear selection. The transmission should have a D position and a D-circle position. The simple D position will keep the transmission from going into overdrive, which may help the problem somewhat. At speeds above 45 or 50 m.p.h., you could then move the shift selector into the D-circle position.

Q: My 1986 Ford Bronco has a 32-gallon gas tank. At about 25,000 miles, the fuel filter became clogged and had to be changed. This has happened twice since. At no time have I ever used more than 20 gallons for a fill-up. How do I go about getting my tank cleaned to correct this problem? T.P.

A: If the tank is contaminated, it will have to be removed from the vehicle, emptied and flushed clean. It’s a technically difficult job, usually performed by radiator repair shops or dealers.

Once the tank is removed from the vehicle, it is washed out with a solvent. The inside of the tank is coated at the factory with a nickel alloy and if there is any deterioration in the coating, the tank will have to be replaced.

If the tank is damaged, it could have been caused by the use of gasoline with excessive levels of methanol in it. Cars properly equipped to burn high levels of methanol often require stainless steel tanks.

Note: In a recent Wheels column about oil filters, the name of Fram oil filters was misspelled.