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Low Transmission Fluid

My 1975 Coupe de Ville was shifting raggedly recently, and so I added transmission fluid. That solved the problem but led to two questions. First, why was the fluid low? I put clean newspaper under the car and after 24 hours, it was spotless. So, no leak there. I checked the radiator coolant, and it was bright lime green like it should be. So, no exchange of transmission fluid and coolant. So, where did it go?

The second question is how much to add. I referred to the dipstick with the engine hot and idling on a flat garage floor, and it indicated I should add 4 quarts! That’s the entire capacity of the transmission system! I strongly suspect that was an error because, while ragged, the transmission still shifted. How can I get a reliable reading?

Your transmission holds FAR more than 4 quarts of fluid. If I remember correctly it’s something like 11 or 12 quarts.

If it was indeed that low, you have either an external leak or it’s being sucked into the engine. Skip the paper under the car, get under the car and look for yourself. If you see no fluid spread over the underside of the car check the vacuum line going from the transmission to the engine’s intake manifold. If it has any fluid in it at all the transmission’s vacuum modulator is leaking and needs to be replaced.
If you see no fluid under the car and the modulator line has no fluid in it, check them both again. There isn’t anywhere else for the fluid to go.

When was the last time the fluid was checked previously? This is a 1975 car, so was it EVER checked? A transmission could leak fluid very slowly and it would not be noticed. Also, in cars that old without electronic shifts, the fluid could be sucked out through the vacuum shift diaphragm, if it was defective. That would show some white smoke in the exhaust.

You’re checking the fluid properly if the engine is warm and idling in neutral or Park with the parking brake on. Make sure to push the dipstick all the way in.

Have the transmission checked by a competent independent shop; if it works OK otherwise it may be a low cost fix.

Many thanks for the tip about the vacuum line between the transmission and the engine. I will check that.

According to the owner’s manual, the capacity of the transmission system is only 4 quarts. That’s where I got that number. What makes you think it holds 11 or 12 quarts? That would be useful information, indeed, but in contradiction to the owner’s booklet.

If you change the fluid by dropping the pan you’ll get only about 4 quarts out of it. The transmission’s capacity is far more.

I think it holds 11 or 12 quarts because I’ve repaired transmissions in my past. It’s been a good many years, so I may be wrong about total capacity, but you may rest assured that it’s plenty more than 4 quarts. You may also rest assured that you’ve shortened your transmission’s life by running it low on fluid.

Drop the pan and clean the screen and pan, inspect for excessive debris, then fill with fluid.

Whoa. Somethings wrong. I believe that would be the TM350 transmission in it and it will hold way more than 4 quarts. A drain and refill is more like 7 quarts if memory serves me. But the important part is you would never see it down four quarts on the dipstick. That dipstick measures pints so if it were at the add mark on the dipstick, it would need one pint. If it was down one quart, it would not show on the dipstick at all. YOU CANNOT OVERFILL THESE WITHOUT DAMAGING THE TRANS SO DO NOT OVERFILL IT. It is checked hot, in park, on a level surface. Add one pint at a time and recheck. The engine should take about 4 quarts without the filter so maybe you just read the wrong line.

Thanks for confirming the consensus that the system capacity is far more than 4 quarts. The dipstick was read with the engine hot, idling, and on a level garage floor. According to that reading, about 3 quarts needed to be added. At least for the moment, the transmission is shifting smoothly and quietly again. The big mystery is how did it get so low? No obvious leak under the car. No sign of red fluid in the radiator coolant.

how did it get so low? No obvious leak under the car. No sign of red fluid in the radiator coolant.

Somebody has to get under the car and actually look - along with checking the vacuum modulator. As noted above, not all leaks will show themselves as puddles under the car.