Transmission shifts hard and sometimes slips

Hi James,

There's no simple, one-size-fits-all answer to your question; unfortunately there could be a lot of reasons for your transmission's bad behavior.

An automatic transmission (we are talking about an automatic, right?) is made up of bands that wrap around a drum and tighten, as well as clutches, a series of clutch plates and disks that get squeezed together much like disk brakes. Both of those mechanism employ friction material that eventually wears out, although the stuff they use in automatic transmissions is engineered for a very long lifetime. So that's one thing (or two, depending how you count them) that could be a cause.

Next, we have the actuation mechanisms for those bands and clutches. Those all work on hydraulic pressure, much like brake cylinders and calipers. Seals can go out and lose pressure, resulting in loss of function.

Finally, there are the control systems. Older vehicles used valve bodies consisting of mechanical components to control the shift points, the activation rate and pressure applied to the solenoids and valves. Newer cars and trucks have gone to electronic systems to accomplish the same thing, but those can fail as well.

The main conclusion here is that it's difficult if not almost impossible for us garage mechanics to diagnose a malfunctioning automatic transmission from our armchairs. They're simply too complex with many different interrelated and interacting parts to be able to figure out their problems without specialized diagnostic tools, equipment and expertise.

About all you can practically do yourself is to check that your fluid level is correct and that it's not contaminated. Although a fluid flush is often recommended as a fix for problems like yours, I've personally never seen one actually fix a problem, but all I can say is, it can't hurt and it's far cheaper than pretty much any other kind of service you can do on the transmission.

If it was me I'd consider taking it to an experienced shop that specializes in transmissions and paying them for a diagnosis. At least then you'll have a starting point to figure out what your next step should be. Transmission rebuilds tend to be fairly expensive, but cost far less than replacing the transmission with a new or rebuilt one. Your cheapest option, should extensive work be needed, would be to pick up a used one at a wrecking yard and replace it yourself, but it's a fairly big job that I wouldn't advise unless you're a reasonably experienced backyard mechanic.