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Auto trans fluid questions

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I have a '73 620 with an L16 and 3 speed auto.


Checked my trans fluid per the manual today (warm up engine, let idle in each gear, return to park, let sit a couple minutes in idle, check fluid)


I checked with the engine in idle, the book is a little vague. Is this correct?

The fluid was at the bottom of the dipstick so clearly I need to add some fluid. The book says dexron, is there just a generic dexron or is there different classes of fluid?

If the fluid is at the bottom of the stick roughly how much do I need to add?

The fluid is sort of a dirty red. Should it be totally red or is this ok?

How would I go about flushing the trans and how much fluid would it take to refill?

Do I fill through the dipstick hole?


Ok I think that's all :D

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> The fluid is sort of a dirty red. Should it be totally red or is this ok?

Is OK. Automatics, unless they leak all the fluid out, rarely go bad, but when they do it is often after someone changes the fluid.



> How would I go about flushing the trans and how much fluid would it take to refill?

I wouldn't.



> Do I fill through the dipstick hole?




> The fluid was at the bottom of the dipstick so clearly I need to add some fluid.

Not much. There top and bottom marks are only a half-pint difference.

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They mostly sell it by quarts. If you have old Dexron, Dexron II, Dexron III, Dexron IV or Dexron V that will work too, as will most ATF including most Ford types (if they say Dexron compatible).



The Datsun factory service manual says:

start the engine and run it until normal operating temperatures [oil temperature: 50 to 80 degree C (122 to 176 degree F). Approximately ten-minute operation will elevate the temperature to this range]


place the lever in park "P" position ... on a level surface ...


(1) Fill the oil to the line "H". The difference of capacities between both "H" and "L" is approximately 7/8 U.S. pint and, therefore, take care not to fill beyond the line "H."


Notice that it does not instruct to turn the engine off. Yes, you must check it with engine running, in Park. Let it idle, don't rev it while checking the fluid level.

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Beware Mercon ATF used in some Fords. Avoid! Must be Dexron.


I've heard about changing the fluid causing tranny trouble.... not completely sold on this train of thought. I've never seen this happen myself. Maybe this has come about from someone having transmission troubles (after driving the shit out of it for 8 years and 50K, never checking or replacing it, towing the tent trailer in the summer) and having it start to 'act up' so they decide to change the fluid to 'fix it' only to have it kack-up a week later anyway. Changing the fluid involves loosening and removing the pan to drain. A new internal filter is usually installed at this time but this is about as far as the home mechanic takes it. The bands can be adjusted for wear if you have a torque wrench. Most people are cheap and replacing the ATF is no exception. Usually a big jug of WallMart ATF and a Fran automatic filter kit and gasket. I don't see how changing the fluid would cause trouble but ATF can reliably work for upwards of 100K miles if looked after and using cheap fluid for such an important job makes no sense.


The dirty red is caused by heat (think about 6 month old frier oil at McDonald's) or clutch material or both. Heat breaks down the ATF and it's ability to lubricate and cool the internal gears and clutches is compromised. ATF should be almost see through clear and ruby red.

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Smell it -- if it smells distinctly burnt then it is bad, otherwise don't worry about the color. I've driven slightly dirty looking fluid (not pristinely clear) for up to 200,000 miles. In big american cars where they overheat and/or overpower, the transmissions can go bad, but then the fluid will smell burnt. The Datsun 3N71B is a tough transmission and has very little trouble like that.


You are correct. Don't use GM or Ford ATF that is not marked Dexron compatible.


The rule is, if it aint broke, don't fix it. This is because there is a small risk of introducing a problem any time you open up the engine or transmission or any other complex device.

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If you're flushing the system then you can go with Dexron IV. Otherwise, I'd use something that's Dextron III compatible. Dexron III was "obsoleted" and no longer licensed by GM (who owns the Dexron trademark) but you can still get "Dex III", Dex/Merc Or Dex/M fluid, which is as close to the original stuff as you'll ever find. Manufacturers can't use the "Dexron" label anymore because of licensing, which is why you'r ehaving issues finding it.


I would NOT top off anything that still has Dexron II or III with Dexron IV... Dexron IV is thinner than II/III. I'm a little concerned with Dexron IV and older transmissions as it is.


What you want to avoid at any cost is anything such as ATF+4, Mercon V, Mercon LV, or really old "Type F".

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Doubt you'd find anything marked Dex II, but maybe. Dex III for sure. Dexron III was backwards compatible with Dexron II, which it replaced in 1993. Dexron II replaced Dexron I in 1972, because the original Dexron used Whale Oil as a friction modifier and that was forbidden under the Endangered Species Act. They may have kept using Whale Oil in Japan longer, though.

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Serious. General Motors had problems with their positraction rear ends involving too tight tolerances. Dealers were instructed to drain and flush several times to try toclear out gear wear particles. When that didn't work, or when the owner bitched loudly enough the area manager would authorized "special lubricant" tube which at that time was about $300 and was sperm whale oil [from the head cavity, keep it clean] to be added to the rear end pumpkin. Cleared up the problems.

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