The Corvair Powerglide automatic transmission remains one of most stable and resiliant transmissions Chevrolet made. The following troubleshooting techniques apply to all models from 1960-69. Amazingly, the Corvair air cooled engine seldom overheats or above 400F. The two issues covered in this discussion cover common issues with the automatic transmissions and engine overheating. Usually these occur after a car has been idle for many, many years.

Powerglide Modulator

The modulator, should it fail, can cause problems with shifting and if the small rubber connector tube is clogged, contribute to idle problems. The device contains a diaphragm and valve. If your engine is hot and you cannot obtain a reading when checking the PG fluid level when the engine is running ( it reads empty on stick), the modulator may be the issue as it is allowing the fluid out of its area. Loss of tranny fluid without locating the leak may indicate a ruptured diaphragm. This allows the oil to be drawn into the intake manifold causing excessive white smoke. If there is no smoke, check the rear axle lubricant as tranny oil can enter if the pinion shaft seal is worn or bad.

This is a 2-3” diameter like vacuum mushroom looking thing that is installed in a less than 30 min (with the right tools) in the side of the automatic transmission. Access is difficult so jack up the car and remove the rear right tire. It is located in a recessed area and using the special tool from Clark’s Corvair makes it easy. Odds are nothing has ever been done to it so prior to loosening, spray some lubricant to loosen it. Remove the short rubber tube, verify it is not clogged etc. Verify that the tubing attached to the rubber tube remains solid with no holes etc.

After a few torques, remove it. There is a small valve in the hole, if it sticks or binds while in the bore hole (it does not turn freely), remove it and inspect for roughness. Use a fine slipstone to polish roughness out by rubbing the valve on the stone. Rotate the valve as it is moved across the stone surface. Insert it and make sure it freely turns. Part of the valve is actually sharp and square, leave these areas alone.

If this valve does not need smoothing, leave alone and install a new modulator with the gasket centered on the threads. Carefully thread the unit back on with the special tool or Water Pump pliers.

To test the modulator before removal:

Attach a vacuum gauge to the lower end of the modulator hose and start engine. At idle speed, the gauge should read within 1 inch. If the reading is not this, something is blocking the tube. Open the throttle and there should be a quick drop in vacuum, if there is none or a delayed drop, the restriction is internal within the vacuum line somewhere.

If both readings are OK, the oil pressure at idle with fluid level full and engine hot should read as follows with modulator hose connected:

In Neutral\Drive:         47-57 psi

If there is no oil pressure, there is a broke front pump drive shaft, or shaft disengaged from the front pump drive hub. If there is low oil pressure, it may be caused by a dislodged plug from the front of the hollow front pump drive shaft.

You are done.

Normal shifting during driving (assuming no issues) should occur at 11 mph and at 44 mph, ± 3 mph. Going 45 mph and more and a downshift to L (low) will not cause the PG to shift. When the speed drops below 45 mph while still in L, the downshift will occur.

NEVER shift from D to R while moving, doing so will damage the PG.

You can push start your PG at 18mph.

If you are going 41 mph or less and you floor thru Detent with the accelerator, the PG will shift into Low and up shift at 44 mph or so.

Detent = is pressing or pulling through a definite resistance.

Additional problems

If your engine starts while the car in any of the gears (the car should start only in N):

1. Loosen the mounting screws and move the switch housing forward or rearward until the starter operates in only Neutral. Tighten screws and retest. Verify the backups also work.

2. Make sure the selector lever is D and check. Rotate the throttle lever clockwise. The hole in the large arm must be below the oil pan rail and measure 3\8”. If the hole is above the pan rail, the cable ball end did not enter the slotted lever and must be reinstalled.

If your shifting is rough or too early or late, your Govenor may be defective or

improper Throttle Valve (TV) pressure may be the cause. To check the TV:

1. Attach a 0-100 psi gauge to the TV test port on tranny (this will hard to locate). It is at the 8 o’clock position.

2. Disconnect the accel rod from the tranny throttle lever.

3. Raise the rear wheels 5” off the ground.

4. Start engine and idle.

5. Disconnect throttle rod from crossshaft  and modulator hose from Balance Tube.

6. Slowly pull rod toward the rear of car. Pressure should be 40 psi to the Detent position and be 52-54psi through Detent position. If lower than 52 psi, low pressure is causing it to shift early, if more than 54 psi, excessive pressure causes it to shift later.

7. Pressure in Low should be 66-77 psi. Pressure in N or R is 0.

8. To adjust the TV pressure, remove tranny oil pan. Using an allen wrench, hold the Throttle Valve plunger as you turn the jam nut CCW 1 full turn to raise TV pressure 3 psi. One full turn CW will lower it 3 psi. To change only 1 psi, move to the flat area of jam.

You may have a defective Governor.

TRANNY OIL

Check tranny oil after engine has run 15 min and is running. If the oil is more than 1\2” above the FULL mark, drain excessive amount to avoid oil foaming and other problems

Engine Overheating

The main cause for this is debris clogging up the cylinder fins which are vital for proper cooling. If your engine top shroud or oil is hot to touch after running for 5-10 minutes you might want to “de-flash” your engine. Corvair engines do not normally run so hot that the

top engine shroud is too hot to touch, warm yes, but not hot. The oil from the stick should not be hot either. If it pings, smokes, the passages are blocked. Clean the Oil Cooler, fan, remove the top engine shroud if you need by removing the carbs and alternator. Spray compressed air in from the bottom and see what comes flying out of the fan area. Once you do, you'll probably see that you need to pull the lower shrouds to clean it properly. This is especially the case if the vehicle has been parked for a period of time. Cleaning the fins out is VERY important and easy to do.  Slow timing may also cause engine overheating as does running too lean in the carbs.

Shine a light under the car so that you can see what needs to be de-flashed. Use a cheap keyhole saw from the local home depot or tool store to insert between the fins to punch out debris or excess metal. If the exhaust manifolds are still on, they block the light from below and limit how far the blade can go thru the head, but you can still do the job with them on.