This article is one in a series that have been released in conjunction with Wayne's new book, 101 Performance Projects for Your BMW 3 Series. The book contains 272 pages of full color projects detailing everything from performance mods to timing the camshafts. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any 3 Series owner's collection. The book was released in August 2006, and is available for ordering now. See The Official Book Website for more details.
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The transmission fluid change and line replacement was the very first project that I performed on this particular car (my wife's 1992 325is). I purchased the car with a known transmission problem. Basically, when the car was stopped suddenly via the the brakes, and then the accelerator was immediately pressed, the transmission would slip, and then slam into gear, lurching the car forward. Not really a good sign, but I had a strong suspicion that the transmission was low on fluid.
Why did I suspect this A thorough inspection of the car had shown that the seller's wife had smacked the front of the car into a few parking blocks one too many times, and had damaged the underside of the car, right behind the radiator. One of the transmission lines had been damaged, and was leaking a slow, but steady drip of transmission oil. From the looks of the car underneath, I could tell that this had been occurring for quite some time.
BMW claims that their transmissions are "sealed" units. This is a load of BS, as they have very significant and very vulnerable transmission cooling lines that run to the front of the car. It's not uncommon for these lines to become damaged, and start leaking. I found that this particular transmission was several quarts low on fluid, and had been slowly leaking fluid for quite some time. If I hadn't caught the problem, the transmission would have run dry and probably would have been damaged beyond repair. As a result, I recommend that all BMW automatic transmission owners check their fluid often - the alternative could be very expensive.
From underneath the car, you can clearly see the two transmission lines. They have a unique bend to each one - specifically designed to fit around the engine and the front suspension components of the car. Figure 1 shows the pair of lines together. The lines are part hard-line and part rubber line. The rubber-to-metal interface is where most of the leaks come from. To replace the line, start by disconnecting it from the transmission (Figure 2). Use the proper wrench, as there will be quite some resistance to loosening the line. Pull the line away from the transmission when the coupler is loose (Figure 3 and Figure 4). Also make sure that you have a bucket or drip pan underneath the connection, as fluid will immediately start to drip out.
With the rear part of the lines disconnected, now move to the front of the car. (Figure 5) shows how the lines are attached to the front transmission cooler, which is located in front of the engine radiator. Remove the clamp bolt, and you should be able to pull the lines out of the connector. Again, have a pan or bucket underneath to catch the transmission fluid that will inevitably spill out (Figure 6).
The line can then be snaked out and around the suspension for removal. I found that it took me quite a while to figure out how to do this. One of the lines was far easier than the other one. Depending upon which engine you have installed, you may need to remove some suspension components (like the front a-arms) in order to gain enough clearance to maneuver the lines so that they can be removed.
Installation is basically the reverse of removal. Be sure to verify that you have enough fluid in your transmission, as you will have lost some in the process of changing out the lines. See the Pelican Technical Article, Replacing your Automatic Transmission Fluid for more details.
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