Questions and Answers

Ford Thunderbird Transmission Concern Q. Hi Vincent, I have a 1995 Ford Thunderbird LX, 4.6 liter V-8, a little over 50,000 miles with automatic transmission, P/S, A/C and cruise control. This is one of those intermittent problems that doesn't seem to have any particular cause, and doesn't occur at any particular time under any particular circumstances. It seems to occur most often at highway speeds between 55 and 75 mph, but it will also do it at speeds as low as 30 mph.

It does it in warm weather, it does it in cold weather. Sometimes it does it when the cruise control is on, sometimes it doesn't. Here's the problem: While driving at constant speed, usually on the highway, the engine will suddenly rev up. For example, at 70 mph the tachometer normally reads about 1,800 when everything is running okay, but when the engine starts acting up and does it's revving thing, the tach will jump up to about 2,800 to 3,000 rpm.

It usually holds at the increased engine rpm for three or four seconds and then in what seems like two or three steps will gradually drop back to normal. The entire sequence takes about eight to ten seconds. It might do it once, and then not do it again for ten minutes, if at all. There is no increase in the forward speed of the car when this happens, but there is a slight lag in forward speed just as the engine starts revving.

Usually there is at least a one or two minute gap between these revving incidents, but on two or three separate occasions, it has done it three or four times in rapid succession. When this happens, the engine usually revs higher, 4,000 rpm, and the lagging that occurs as the revving begins can make the car kind of jerky or lurchy.

When the car was new, this particular model did get a batch of bad transmissions that Ford replaced under warranty. This one didn't start having transmission problems until about a year after the warranty expired, and Ford's fix at that time was to install a transmission cooler, and change the transmission fluid to some special blend high-temperature fluid. That was done probably back in 1998 or 1999.

About two and a half years ago it was in the shop for what I thought might be transmission problems, but turned out to be bad spark plug wires that were causing the car to run poorly. They also did general tune-up work, and I think they changed the transmission fluid and filter at that time. It has probably been driven about 15,000 miles since then.

I've asked a couple of friends who are pretty good shade-tree mechanics about the current symptoms. One said it sounds like a transmission problem. The other says it sounds like a computer module malfunction.

I was hoping that someone like you with a lot more knowledge and experience than my friends and I could narrow it down to either a specific fuel delivery problem, an electronic problem, or a specific transmission problem so the mechanic doesn't need to go poking around in two or three different areas trying to figure it out when I take it in for the repair.

Because the engine never revs when the car is just sitting idling, or at low speeds around town, and since the forward speed doesn't increase when the revving does occur, I'm thinking it is a transmission thing. Any help you can provide in getting a more accurate diagnosis will be greatly appreciated.

Sincerely,
Mason
Fairhope, Alabama

A. I have a feeling you're right Mason. When you have a sharp increase in rpm with no corresponding increase in vehicle speed, or a decrease, it almost always means a transmission problem. It might just be the overdrive. Try leaving it out of overdrive and see what happens.

Another possibility, albeit a small one, is the Throttle Position Sensor (TPS). They get a "dead" spot in them and when you hit it, the engine and transmission will do some weird things.

Check the computer for DTCs, there might be some set that will aid to confirm or deny this theory.