How to Troubleshoot a Turbo-Hydramatic 400 Transmission

Check the fluid level. The first thing to check if you suspect that the transmission is acting up is the automatic transmission fluid (ATF) level in the transmission itself. The dipstick on the TH400 can be found inside the engine bay, on the passenger side toward the firewall (opposite the radiator). Pull the handle attached to the dipstick, which will remove it from the dipstick tube. Clean any fluid off and put it back into place, making sure that it seats all the way. Then remove it again and look at the tip of the stick. There are level indicators there, which will tell you if the transmission is low on ATF.

Check the linkages. The linkages that are connected to the shift lever can get out of alignment at times, causing sloppy or difficult shifter response. Look for abnormalities such as bent or broken parts, loose attachment points, or blockages that would prevent smooth shifting.

Change the filter. Unbolt all 13 transmission pan bolts (on the bottom of the TH400) and remove the pan. The filter is found in this area of the transmission. Filter kits, which include the filter and a new pan gasket, can be found at any auto parts store for around $15. Replace the filter and fill the transmission with new ATF (the TH400 requires six quarts). Do not put the bolts back on too tightly, as the gasket can become damaged. Six to 8 foot-pounds should be plenty. If no torque-wrench is available, just put them on as tight as you can by hand, and then turn them 1/2 revolution.

Inspect the cooler lines. The number-one killer of automatic transmissions is heat. If the cooler lines that come from the radiator are damaged or leaking, it might be necessary to replace them. These items can also be found at the local parts store.