Automatic Transmission Fluid Leak: Find the Source

Steps

1. Confirm it's transmission fluid. Wipe the drip onto a white paper towel: red or reddish-brown fluid points to ATF. Engine oil from a valve cover or rear main seal can run back and look like a transmission leak, so don't assume.
2. Clean the area. With everything cool, clean the pan rail, case, bellhousing area, cooler lines, and the back of the engine with a nonflammable cleaner so old residue doesn't mislead you.
3. Run and recheck. Bring the transmission up to operating temperature with the vehicle safely supported or parked, then look for fresh fluid and trace it to its highest wet point. Fluid runs downhill and blows back, so the lowest drip is rarely the source.
4. Pan gasket and pan: look for wetness along the pan rail. Make sure all the old gasket was scraped off and check that the pan flange is flat. Over-tightened pan bolts dimple the pan rail around the bolt holes and cause leaks. Get the pan bolt torque from your service manual or the gasket/pan maker; never guess.
5. Drain plug (if fitted) and filter/pickup: check the plug and its washer. A misplaced filter seal can cause fluid problems after a pan job.
6. Cooler lines and fittings: check both ends, at the transmission case and at the radiator, plus any rubber hose sections. Don't keep tightening a fitting to stop a leak; replace the worn part. ATF in the radiator coolant points to a leaking cooler inside the radiator.
7. Dipstick (filler) tube seal at the case, which can be disturbed during a pan job.
8. Shift shaft (selector) seal and TV/kickdown cable seal on the side of the case, plus any electrical connector.
9. Front of the transmission (between engine and trans): fluid on the back of the torque converter suggests the front pump seal. Fluid only on the inside of the housing suggests the pump gasket or pump bolts. Front leaks usually mean removing the transmission, which is shop work for most people.
10. Rear of the transmission: check the extension housing (tailshaft) seal at the driveshaft yoke, the output seal, and the speedometer gear/sensor seal.
11. Still can't find it? Add a fluorescent leak dye made for ATF, drive, and inspect with a UV light.
12. After the repair, refill to the correct level using the manual's check procedure (overfilling causes foaming and overheating), then clean and recheck for leaks.
13. Older GM trucks (TH350/TH400/700R4): before buying gaskets or seals, identify which transmission you have, for example by the shape of the pan. This is general identification help, not a spec. Confirm with the service manual or a transmission shop.