Before taking drastic measures, try something simple: change the fluid and filter. You could just have a clogged transmission filter. By changing the filter and replacing the amount of fluid that comes out when you drop the pan, you could avoid headaches and continuing problems. Be aware of the Automatic Transmission Fluid (ATF) you use, however. Consult your owners manual for the specific type of fluid you should use. The wrong fluid can change the way a transmission shifts. It could even cause it to not shift at all!
If the simple things don’t make a difference, there are other tests to help you diagnose your transmission problems. And while you can perform some tests yourself, others require the equipment only a repair shop has on hand.
Air Pressure Tests – Usually done by a repair shop, air pressure is substituted for fluid pressure to test the bands and servos inside the transmission. By applying compressed air to certain ports, it will apply pressure to the particular band or servo, creating a dull thud when the band or clutch engages. Make sure there’s no hissing sound when the band or servo is engaged.
Line Pressure Test – Different parts of the transmission operate with different fluid pressures. If the fluid pressure is too low or too high, shifting can be too soft or too hard. A transmission pressure gauge, attached to the transmission, can check the pressures. Then simply compare the readings to the manufacturer’s chart to pinpoint the most likely cause of the problem.
Shift Point Road Tests – Every transmission shift should take place at a specific time, under specific throttle conditions. You can figure out what’s wrong with your transmission by noting when the shifts take place vs. when they are supposed to take place.
Stall Tests – Stall speed is when the engine RPM goes as high as the transmission will let it. If you can located the stall speed specification for your vehicle, you can perform this test yourself. The text checks converter one-way clutch operation and installation, the holding ability of the forward clutch, reverse clutch, the low and reverse bands, the planetary one-way clutch, and engine performance. You should perform the test with both your engine coolant and transmission fluid at operating (not cool) temperature.
- Firmly apply your service and parking brakes
- With transmission in ranges D, 2, 1, and R, press the accelerator to the floor and hold it just long enough to let the engine reach full RPM. NOTE: Do not hold throttle open for more than five seconds at a time, or you may damage your transmission.
- After each range test, move to N and run your engine at 1,000 RPM for approximately 15 seconds to cool the converter. Then perform the next range test.
Transmission Fluid Leak Tests – Try using an oil-soluble aniline or fluorescent premixed dye (one half teaspoon of dye powder to one half pint of transmission fluid) to locate the source of a leak. There aren’t too many places a leak can occur, so you can do a thorough check.
- Check speedometer cable connector at the transmission. Replace rubber 0-ring, if necessary.
- Check oil pan gasket. Tighten attaching bolts to proper torque and/or replace gasket, if necessary.
- Check fluid filler tube connection at transmission case. Check filler tube 0-ring seal for damage. If leakage is found, install new short oil inlet tube and 0-ring seal or clean the area and apply sealer around the tube.
- Check fluid lines and fitting between transmission and cooler in radiator tank for looseness, wear, or damage. If leakage is found, tighten fitting to proper torque, or replace the damaged parts.
- Check engine coolant in radiator. If transmission fluid is present in coolant, the cooler in the radiator is leaking.
- If leakage is found at the downshift control lever and manual lever shaft, replace seal.
- Inspect pipe plug on left rear side of transmission case. Tighten plug or replace, if necessary.
- Fluid leakage from the converter housing may be caused by engine oil leakage past the rear main bearing, by oil galley plug leaks, or by power steering fluid leakage from steering system. Determine the exact cause of the leak before beginning service procedures.
If there’s a leak from the converter housing, isolate the cause:
- Fluid leaking by the front pump seal lip will tend to move along the impeller hub and onto the back of the impeller housing.
- Fluid leakage by the outside diameter of the seal and front pump body will follow the same path as the leaks from the front pump seal.
- Fluid that leaks from a front pump-to-case bolt will be deposited on the inside of the converter housing only. Fluid will not be deposited on the back of the converter.
- Leakage by the front pump-to-case gasket may cause fluid to seep down between the front of the case and converter housing.
- Fluid leakage from the converter-to-flywheel stud weld will appear at the outside diameter of the converter on the back face of the flywheel, and in the converter housing only near the flywheel.
- The following areas of possible leakage should be checked to determine if engine oil is leaking.
- Leakage at the rocker arm cover may allow oil to flow over the converter housing and cylinder block, causing oil to be present in or at the bottom of the converter housing and cylinder block, causing oil to appear at the bottom of the converter housing.
- Oil galley plug leaks will allow oil to flow down the rear face of the block to the bottom of the converter housing.
- Leakage by the crankshaft seal will work back to the flywheel, then into the converter housing.
Fluid leaking from other areas, such as the power steering system forward of the transmission, may cause fluid to appear around the converter housing due to blow-back or road draft. Use the following procedures to determine the cause of leak before any repairs are made.
- Check transmission dipstick for color of fluid. Original factory-fill fluid is dyed red. Unless the fluid has been changed, the color should assist in pinpointing the leak. Leaks due to road draft should be eliminated before checking the transmission for fluid leaks.
- Remove converter housing cover and clean off any fluid from the top and bottom of the converter housing, front of transmission case, and rear face of engine and oil pan.
- Start and run the engine until the transmission reaches normal operating temperature. Observe the back of the block and top of the converter housing for evidence of fluid leaks. Raise vehicle on a hoist and run engine at fast idle, then at engine idle, occasionally shifting to the drive and reverse ranges to increase pressure within the transmission. Observe the inside of the converter housing and front of the transmission case. Run the engine until fluid leakage is evident and the probable source of leakage can be determined.
Rest assured, every symptom of a transmission problem has a cause that can be fixed. Expert mechanics can often identify what parts are needed just with a simple road test. If you’re in tune with your vehicle, you’ll also have an easy time identifying the specific symptom. Then simply consult your repair manual and shift yourself into high gear on fixing your transmission!
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