[Kaje]

Hey guys, new BMW owner here. I just recently purchased a 1985 635csi (automatic transmission) and was driving when all of a sudden I noticed the car began to shift improperly and wasn’t accelerating near as fast as usual. About a mile down the road, it stalled and was unable to go anywhere. No warning lights, no strange noises. When the tow truck arrived about 2 hours later, noticed red-ish fluid underneath like it had all drained out while sitting there. At first, I thought this was transmission fluid. Shortly after finally having time to get under the hood, I noticed the hose (in attached picture) connecting to the radiator had completely disconnected. Could that ACTUALLY be transmission fluid coming from that or is that just coolant?


In short, I bought this car to BECOME mechanically inclined and work on everything, but have a ways to go before I can diagnose things myself. Any ideas as to if this could be the reason for the scenario I described? Again, describe things to me as if I am clueless, because I kind of am. I really want to get her back up and running without taking her into a shop.

[V1_ROT8]

I’m unsure about your particular car, but many vehicles use a portion of the radiator to cool the ATF. So, yes, that is transmission fluid leaking out, especially if it’s red.

[RockCrusher]

As V1_ROT8 noted it is quite common for an automatic transmission to be connected to a transmission oil cooler.

Very quickly after I started working on my cars I learned from some professional techs "tricks" to help identify the type of fluid that was leaking out or puddled on the ground under the car.

Coolant (antifreeze/water) is water soluble and between the fingers very slippery.

It also is supposed to have a very sweet taste. (Which is one reason to avoid leaving any coolant out in an uncovered container if there are dogs/cats about.)

ATF being oil is not water soluble and between the fingers doesn't have quite the slippery feel coolant has.

[cerealwars]

Sweet 6er!

That type of red rubbery hose is 100% for oil use. You'll want to either replace that hose, or snip just the end and put it back in place if it has enough slack. You will likely have to snip the existing clamp with diagonal cutters or something like that. You can use band clamps as typically found at any auto parts store.

Make sure you look up and obtain the correct trans fluid! This is very important. This also isn't a 6-series-specific forum, so you may need to chase down this information elsewhere. bigcoupe or bf look like a promising forums for you. I simply googled "bmw 635csi auto trans fluid change forums". More specifically, this thread seems relevant. Search and you will find your way.

I am not a BMW tech, so regard all of my advice in this paragraph to be general and not particularly accurate for your model. In my experience with older auto transmissions, there is either a fill plug, or you fill slowly through the dipstick tube. Hopefully you have a dipstick lol (sore subject for us more modern BMW owners). Generally, you take the dipstick reading with the transmission warm (having driven the vehicle for 10-15 minutes and gone through all gears a few times), and add until the level is in the crosshatched area on the stick. Maybe start with adding 1 or 2 quarts, and try to cycle through the gears every time you check. Don't drive far, and consider bringing the fluids and filling apparatus with you.

This would also be an opportune time to perform a trans fluid & filter change (you're already halfway there lol). This is a more moderate endeavor, but something makes me wonder what caused the hose to blow off in the first place. Another thing to help decide whether or not this is needed is by checking the condition of the fluid. Does it smell burnt and terrible? Compare it to the new fluid you will be buying. Obviously it won't appear to be brand new, but if it is very dark and metallic and it reeks, it's probably a wise time to change it.

Two other things I can say about trans fluid changes:
1) Careful when dropping the pan. Fluid will likely go everywhere. Do your best to not get it everywhere, but also accept that it probably will. Freaking bloodbath almost every time I've done an ATF change. Blue shop towels, and I personally like Tub o Towels citrus wipes for cleanup.
2) Buy a torque wrench to tighten the oil pan bolts with. Most likely you will need a 1/4" drive wrench for this. A beam style is cheap and will work fine. It is critical that you do not overtighten these (it's very easy to do), you may start crushing the pan rails and bending up the sealing surface.

Hopefully the hose is simply old and deteriorated. If this is the case, purchase new trans oil hose at a local parts store (they sell it by the foot, bring in your old hoses to measure from) Maybe there's an obstruction in the oil cooler? Disconnect the hoses on either side of the cooler, place one end in a drain bucket, and try blowing on the end of the other hose. This is also why I suggest changing the fluid and filter - your car is certainly old enough where it wouldn't hurt, if the condition is unknown.

Also, this way you can obtain the info for how much you can expect to refill. Good luck dude! I started out by wrenching on glorious '80s machines, and fifteen years later I'm still in love with analog cars. Long live the E24.

[B58 parts]

Is this your daily driver or a project car?

I know everyone on this forum really loves their automatics, but you are eventually going to manual swap this, right? I would just start that direction now. There's no point in servicing an automatic transmission from the '80s. It would be a good rite of passage for you, you would end up touching many things on the car, and by the time you were done everything else is in the bag.

[Kaje]

Originally Posted by B58 parts

Is this your daily driver or a project car?

I know everyone on this forum really loves their automatics, but you are eventually going to manual swap this, right? I would just start that direction now. There's no point in servicing an automatic transmission from the '80s. It would be a good rite of passage for you, you would end up touching many things on the car, and by the time you were done everything else is in the bag.

Hey, it actually has crossed my mind to manual swap. I’ve just been dealing with this issue the first week I drove it up here and am really just trying to get it out of my brother’s garage and into my own garage first! That’s when the real fun begins. But yes, I definitely would like to soon.

[Kaje]

Originally Posted by cerealwars

Sweet 6er!

That type of red rubbery hose is 100% for oil use. You'll want to either replace that hose, or snip just the end and put it back in place if it has enough slack. You will likely have to snip the existing clamp with diagonal cutters or something like that. You can use band clamps as typically found at any auto parts store.

Make sure you look up and obtain the correct trans fluid! This is very important. This also isn't a 6-series-specific forum, so you may need to chase down this information elsewhere. bigcoupe or bf look like a promising forums for you. I simply googled "bmw 635csi auto trans fluid change forums". More specifically, this thread seems relevant. Search and you will find your way.

I am not a BMW tech, so regard all of my advice in this paragraph to be general and not particularly accurate for your model. In my experience with older auto transmissions, there is either a fill plug, or you fill slowly through the dipstick tube. Hopefully you have a dipstick lol (sore subject for us more modern BMW owners). Generally, you take the dipstick reading with the transmission warm (having driven the vehicle for 10-15 minutes and gone through all gears a few times), and add until the level is in the crosshatched area on the stick. Maybe start with adding 1 or 2 quarts, and try to cycle through the gears every time you check. Don't drive far, and consider bringing the fluids and filling apparatus with you.

This would also be an opportune [...]

Thank you so much for this. I do have one question, do you think it’ll be a requirement to snip the hose / replace or will it be as easy as attaching it back on with a new hose clamp? I don’t want to cut corners by any means but if the hose didn’t “break” off, I’m wondering if it’s as easy as what I mentioned above. Again, as you stated, it’s probably best to just go ahead and replace that and possibly the filter as there’s no fluid left to leak out.

I do believe there’s a dipstick in there so that will help me be able to slowly fill as I go; your advice about filling slowly and driving a little at a time seems to align with most everything else I have read and would be the safest option without “shocking” the system with new fluid. I’ll be updating as I go and let you know how things turn out.

[Kaje]

Originally Posted by RockCrusher

As V1_ROT8 noted it is quite common for an automatic transmission to be connected to a transmission oil cooler.

Very quickly after I started working on my cars I learned from some professional techs "tricks" to help identify the type of fluid that was leaking out or puddled on the ground under the car.

Coolant (antifreeze/water) is water soluble and between the fingers very slippery.

It also is supposed to have a very sweet taste. (Which is one reason to avoid leaving any coolant out in an uncovered container if there are dogs/cats about.)

ATF being oil is not water soluble and between the fingers doesn't have quite the slippery feel coolant has.

Thanks Rock. I’ll definitely keep this in mind!