James Mackintosh 9 years ago 0

How To: Replace Your Driveshaft

Remind me later
Before I start out, may I advise:  if you have the necessary funds, this is one of those jobs better off left to a professional.  Not necessarily because they have all the technical training you don't, but because they have things like a 2-post lift.  And air tools. Nevertheless, here I am with another DIY mechanic's guide.  The driveshaft in my BMW has taken a turn for the worse in recent times, if I'm honest.  A diagnoses of a shot rear U-joint (universal joint) made a lot of sense.  There was a sharp "clunk" when engaging first or reverse, a dull thump when pushing in and re-engaging the clutch during gear changes, and a frankly alarming vibration at highway speeds when transitioning on and off the throttle - or using the cruise control.  Or exceeding 70mph.  Or on Wednesdays.  It was terrible. First things first, parts needed to be ordered.  Wholesaleimportparts stocks remanufactured driveshafts of all varieties at a price roughly half that of the OEM part at the dealer; good news when we're talking hundreds of dollars.  Why not just replace the bad joint itself?  Well, to prevent slack in the joint and thus unruly vibrations, BMW makes the U-joints permanent: meaning they're not easily replaceable.  As it turns out, sometimes it's cheaper to just get a whole brand new one than to pay the labor of having a single component replaced. The driveshaft transmits rotational energy between the transmission and the rear differential.  In between the transmission and the driveshaft itself is the flex disc- sometimes called a Guibo, for reasons I still don't understand completely.  The flex disc is a rubber hockey-puck lookin' thing with six sleeved holes - three connect to the shaft, three to the flange on the transmission, creating a sound but flexible connection between the two.  These wear out over time, creating it's own set of problems - so it's best to put a new one in while you're working on it.  I sourced my flex disc from Pelican Parts, which is a great source for BMW and Porsche parts of all varieties.  Now, on to the work. To get to the driveshaft, you must remove quite a few things, starting with the exhaust system from the downpipes back.  This turned out to be one of the biggest pains of the whole process.  I think that back in 1997, this exhaust system was multiple pieces connected by flanges to facilitate removal.  But at this point, it's become one single piece due to multiple cut-and-shut weld jobs - one of them while in my possession.  The BMW straight sixes run "true" dual exhaust, meaning that there's a header, pipe, and cat for each three cylinders that runs parallel all the way back, so you effectively have to do everything twice.  First, you have to remove the 2-point (or 4-point x-brace on cabrio/Z3 models) chassis brace that runs underneath the downpipes.  Starting at the header, remove six incredibly tight nuts from the flanges, being sure not to round them off or snap the studs either, and you get this: Moving rearward, disconnect the two center exhaust hangers, and the two rear exhaust hangers.  Drop the exhaust (gently!) to the ground, and from there it's easier to access the rear oxygen sensors, which are mounted on top of the pipes behind the catalysts.  All done, it'll look like this: wow, my exhaust is terrible.  I claim responsibility for everything from the crossover behind the cats to the tip; the new 2.25" pipe and Magnaflow I had done. Here, you can see a close-up of the shoddy work done from the cats forward.  That shiny section before the cats is actually smaller diameter than the rest...  why? At this point, the next step is to remove the heat shielding that sits in between the exhaust system and the driveshaft, which is an aluminum sheet of some sort, attached in six places.  You can also see the oxygen sensors dangling here - normally you'd mark which is which, but because one harness is longer than the other, there's no danger of confusing them. This is the heat shield, out of the car. Now we can finally see the driveshaft.  Of course, there's still more stuff to remove. The big aluminum bar with the bushings on it is the transmission brace; here you can see the UUC transmission mounts and enforcement cups I put on a while ago.  The transmission mount has to get out of the way for access to the flex disc bolts (the hocked-puck lookin' thing.)  Loosen the nuts on the top of the bushings, and take out all four bolts that hold the brace to the chassis.  Be sure to support the transmission with a jack or a jack stand.  Now is a good time to take out the three bolts which hold the driveshaft flange to the flex disc. This out of focus picture is the rear end of the driveshaft, where it goes into the differential.  These four bolts are welded in place, with lock nuts on the other side of the flange securing them.  Because of the limited clearance, I found the best solution to getting these rusty bastards loose was a combination of A) Liquid Wrench B) an open end wrench and C) A big freakin' hammer on the end of the open-end wrench.  This step, and the removal of the bolts joining the flex disc and driveshaft, require an assistant in the car to disengage the parking break and put the shifter in neutral so you can rotate the driveshaft, then locking it and placing it in gear so you can get the nuts loose.  There's also another chassis support behind the center support bearing to remove; four bolts there. Next we turn our attention to the center support bearing, which is just rearward of the center universal joint.  I was susprised to find mine was only mildly damaged from the rear joint flopping around violently, but the new shaft already had a new one installed, so no way was I going to reuse this one.  It's secured by two nuts - once removed, the center of the driveshaft sags, which allows you to pull the shaft away from the flex disc.  Then finesse the bolts on the rear of the shaft out of the holes on the diff flange, and this ugly thing's out. Old driveshaft and full exhaust system. Aaaand, here's the trouble.  I feel like I've gotten my money's worth out of this U-joint.  There was probably about 20° or a half-inch of play in it, radially.  Bad news - I'm glad I took it out before it actually snapped. A view through the tunnel of love, as I call it.  Above you can see the old flex disc still installed, and the small jack used to support the transmission with the brace removed. And here you can see the old flex disc up close.  The three remaining bolts fasten it to the transmission flange; the three empty holes are where the driveshaft attaches to it.  Replacement requires removal of the transmission brace so that you can get an open-ended wrench onto the nuts/heads on the back side of the flex disc; a breaker bar on the other side will generally get the bolts loose.  When doing this job, it's a good idea to replace all the nuts as they're self-locking with a metal cap on the end that the bolt taps into.  You can also see the bottom end of the shift linkage at the top of this picture; if you're thinking about putting a short shifter, now would be the perfect time to do so, although you can reach what you need to around the driveshaft if need be. As they say, assembly is the reverse of removal.  The rear end slides through the holes on the differential flange, the front end lines up and has bolts passed through it, and then you secure the center support bearing with the two nuts.  As long as you don't pull the driveshaft apart, it'll be balanced fine: they are specifically indexed, though. Now, back to what I first discussed, about doing this yourself.  It's a royal pain in the ass.  I don't have a lift, so I spend this entire job laying upside down on my back with probably 10" of clearance to work with, wrestling heavy components into place and fighting with rusted bolts.  It's just not any fun.  However, if you were to bring it to a dealership I reckon it would be a $1,000+ dollar job including parts and labor.  I was around $400 all in for this, so I say the $600 saved is well worth it.  Unlike a lot of BMW repair jobs, this one doesn't even require any specialist tools - just a lot of patience and upper body strength. Results?  Fantastic.  My 328i is driving like a BMW should again, finally!  Throttle response is sharp, there's no thump transitioning on and off throttle, and it just drives like the civilized car it should.  The car's so smooth now I find myself accidentally speeding on the highway.  It's probably the biggest repair I've made to the BMW up to this point, and it felt great to accomplish something like this and not mess it up.  Now onto the next project... Feel free to leave comments and questions below, or shoot me an email if you have a technical question!  I hope you enjoyed this DIY.