Consider the transmission pump to be the heart of the system. Just like a human heart, the pump sends oil (fluid) through a system of valves, gears, clutch packs and passages that must work under pressure and stay lubricated. Pumps can use two methods of circulating fluid: the gear-driven pump and a rotor or vane type. Either pump needs enough pressure to operate a check valve that allows fluid to enter the case. Gears or vanes can clog or become worn out, which will alert the vehicle owner to some obvious symptoms before complete transmission failure.
Listen for any humming or howling noise coming from the vicinity of the floorboard while driving. The noise will increase with intensity as the pressure rises inside the transmission, but the pressure required to activate the parts will be absent, forcing the pump to overwork itself. Put the transmission in gear from an idle, which will rule out the entire gear train. See if the noise persists. If it does, the pump can be blamed exclusively. Consider the second candidate for pump noise: the transmission filter. A clogged filter will not allow an unrestricted loop of oil through the system, causing air in the gears or vanes.
Shift the vehicle in park or neutral and set the emergency brake. Raise the vehicle high enough with a floor jack to place two jack stands under the front frame near each wheel. Start the engine and let it idle. Disconnect the vacuum line at the modulator. The modulator attaches to the side of the transmission and looks like a small diaphragm with a vacuum hose connected to it. Remove the hose slightly from the tip of the modulator nozzle, just enough to vary the amount of vacuum going into the transmission. Listen for a change in the humming or howling noise. Any change will likely point to a bad pump.
Test-drive the vehicle. Find a steep hill to climb at a gradually increasing speed. Determine if the transmission slips while it changes gears automatically. Note: the transmission fluid should be good quality and at the proper level. Any slipping, during automatic shifting, could indicate worn pump gears or vanes.
Determine that the pump has suffered a failure if the transmission begins to pour fluid out of the area between the engine block and the transmission housing, and the engine has overheated while pulling a heavy load. This failure is caused by a front pump seal that has overheated and expanded, purging all of the fluid from the transmission case. This calls for replacement of the pump bushings, gasket and O-ring seal.
Look for any slippage in the transmission while accelerating from a standing stop. Provided the clutches and bands function properly, and the fluid level and quality are sufficient, this symptom could indicate worn pump gears or vanes. This means that the pump has not created sufficient pressure to engage the clutches and bands for forward movement.