Stupid auto trans fluid question

[Mastiff]

I pulled my t-case on and off and lost an unknown, probably small, amount of fluid from the transmission. The dipstick reads above full with the engine off, but I'd like to ensure it is safe to start. In this case, I don't expect there would be a problem, but if you drop the pan or replace the trans or something, what's the right way to get it filled prior to firing it up? And what exactly is the reason for checking the fluid with the engine running anyway?

I replaced the trans filter on my CUCV a bunch of years ago, but heck if I can remember what I did. I may have just relied on engine-off dipstick readings.

 

[80 Chevy K20 4x4]

You check the fluid level with the engine running so that the trans is circulating the fluid through the trans and is at the recommended level to ensure no issues.

With the rig running fill it to only half way in the cross hatch marks on the dipstick. This will allow for fluid expansion as the ATF heats up and won't overfill the trans and possibly make it blow out the vent tube.

 

[Blue85]

Unless you dropped a couple quarts, it's probably safe to start. I've never heard a transmission pump run dry, so I don't know if you would hear it right away like a power steering pump (but they are a similar design). To flush a tranny, I will dump a quart at a time and never had any problems. I mean if the pump was starved, it would stop squirting fluid...

You check it with the engine running because then the pump is running. This moves a lot of fluid into the torque converter that otherwise would be sitting in the pan. It's the running level you care about.

 

[Mastiff]

You check it with the engine running because then the pump is running. This moves a lot of fluid into the torque converter that otherwise would be sitting in the pan. It's the running level you care about.

I see, so the converter drains back a lot of its fluid when the engine is off? That would explain why my reading is high with the engine off. On the other hand, I understand that when you drop the pan, only a fraction of the fluid comes out. I assumed it was in the converter.

 

[paratrooper307]

I see, so the converter drains back a lot of its fluid when the engine is off? That would explain why my reading is high with the engine off. On the other hand, I understand that when you drop the pan, only a fraction of the fluid comes out. I assumed it was in the converter.

It'll be fine. If there's that much fluid in the pan it's safe to start. Let it get nice and warm before adding any. The reading is high because fluid is not circulating through the system. It's common on many transmissions for the pan to contain only 4-5 quarts when dropped, but overall capacity of the system may be 10 quarts or more thanks to the coverter, lines, transmission cooler, etc.

 

[6.2Blazer]

If the fluid is reading high on the dipstick there is absolutely no concern with starting the engine. Just for perspective, when you drop the pan only about half of the fluid gets drained out so a lot of fluid still remaining in rest of the transmission circuit. I wouldn't be concerned at all unless you lost at least a couple quarts or the fluid wasn't even reading on the dipstick.

FYI: my local trans guy who has been in business forever with a very good reputation doesn't even say you need to prefill the torque converter after re build...says all that does it cause a mess when you try to install the converter full of fluid.

 

[Mastiff]

Just to wrap this up. Once running and warmed a bit, the T-case swap job required less than a pint to top off the trans. I got one of the funnels where you can pre-fill it and the fluid stays in until you twist the base to open it up. Very handy for avoiding mess at the back of the engine.