How to Troubleshoot a Slipping Powerglide

by Chris Stevenson
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The Powerglide transmission, produced by General Motors during the 1950s and into the early 1970s, had a 2-speed automatic gearbox and used a hydraulic-operated torque converter. It was known for its durability and simplicity, especially admired by race car enthusiasts. It was replaced by the Turbo Hydramatic, but the Powerglide was still used during the 1970s and was offered in the V8 and 6 cylinder engines. Troubleshooting a slipping Powerglide transmission involves a process of elimination, using some basic steps and tools.

Step 1

Start and run the engine until it reaches operating temperature, which is warm enough for normal driving conditions. Put the transmission selector in the "Drive" position and accelerate smoothly. Notice if any slippage or lag results from take-off, or just after placing the transmission in drive. Stop the vehicle and put the selector in "Reverse." Back up slowly, noticing if any lag results between acceleration and transmission movement. Slippage in either gear indicates a fluid level or transmission pump problem.

Step 2

Take your vehicle to a large parking lot, free of traffic and obstructions. Begin by driving in a straight line at low speed -- 10 to 15 mph. Make a tight accelerating right or left turn and notice if the transmission disengages temporarily, or if the engine races. If slippage occurs, it means fluid has sloshed to one side of the transmission case, temporarily draining the transmission pump. This results when the fluid level is low or contaminated, or pressure has been temporarily lost in the pump.

Step 3

Place the vehicle in "Park" and then shift into drive. Accelerate swiftly and listen for the shifting of the transmission. Drive fast enough to put strain on the transmission, with increasing acceleration, but do not exceed the speed limit. Look for slippage between gear shifts, where the engine races momentarily then catches again in gear. This likely results from a low or contaminated transmission fluid level.

Step 4

Place the vehicle in park and firmly set the emergency brake. Raise the hood and locate the transmission dipstick. Pull the dipstick from the tube and examine the condition of the fluid. The fluid should have a translucent red or amber color. If the fluid has a burnt smell and appears brown or black, or contains any white or brown foam, the fluid is contaminated and must be replaced. Any metal shavings appearing on the dipstick that reflect sunlight, or the rays of a strong shop light, indicates excessive bearing and gear material wear. Small dark particles will point to deterioration of the clutches and bands.

Step 5

Turn off the engine. Wipe the dipstick clean and reinsert it into the the transmission filler tube. Pull it out and check the "Cold" level line on the stick. Any reading below the line, or absent from the line, indicates insufficient fluid in the transmission. Fluid should be replaced to the line.

Step 6

Run the vehicle at an idle and in park over a clean, white piece of poster board for at least 15 minutes. Use a floor jack to lift the front and rear sections of the vehicle and place two jack stands under each side of the frame -- front and rear. Slide under the vehicle and look for any fluid droplets on the poster board. Examine for leaks the transmission pan bolts and gasket, the weep hole at the bottom of the bell housing, the tail shaft and the transmission lines leading to the cooler on the radiator. Any leaks must be repaired.

Step 7

Check the vacuum modulator on the side of the transmission. It will look like a small diaphragm with a hose connected to it. Examine the hose connection for a tight seal, and trace the vacuum line back to the intake manifold for kinks, splits or cracks. Any hissing noise coming from the vacuum modulator denotes a bad seal at the hose or a ruptured diaphragm. Re-splice any defective hose, and use a slot screwdriver to make sure all connection clamps are tight.

Step 8

Connect an adapter hose from a pressure gauge to the pressure port on the transmission. Refer to your owner's service manual for the location of the pressure port. Let the vehicle idle. Read the PSI (pounds per square inch) on the gauge. The PSI should read between 140 and 150 for a standard (stock) Powerglide transmission. Any reading above or below will indicate a problem with clogged lines, stuck check balls in the transmission or a defective pump.

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