Changing the E39 M5 Rear Suspension from stock to coil-over:
The rear is relatively simple to change over to coil-overs, but it’s a lot more work than the front. You don’t have to disassemble the actual strut assemblies; you simply remove the factory ones and install the new ones. That said, its heavy work, particularly if you don’t have help, and you should expect to spend a couple of hours per side. It will take longer if you’ve never done it before. There are some things that look simple that aren’t, and if you mess them up it will take much longer to do. Proceed with care.
The logic of the job is pretty simple:
- The strut assembly is longer than the space it has to fit through to come out. The adjustable coil-over replacement can be compressed more (shorter) so it’s not constrained.
- The bottom of the strut is in a box formed by suspension members. Undoing the upper rear control arm opens the box.
- You have to remove the brake caliper because it gets in the way of the upper rear control arm as you try to swing it out of the way.
- You have to disconnect the hub speed sensor because the wire is too short to bend out of the way.
- You have to block the whole swinging mass down because it will jump upwards when you undo the damper bolt, making the “box” even tougher to get out of.
- You have to remove the fender liner because it constrains the movement of the strut. You don’t have to remove the fuel plumbing (right side) or emissions equipment (left side) because they are not in the way.
- You don’t need a spring compressor if you block the suspension members down when you remove the strut bottom bolt. You do if it gets a chance to jump up on you.
I’m going to start with driver’s side (left, looking forward), but it really doesn’t matter. Remove the interior to access the strut tops (see other DIY instructions) and begin with the rear end up in the air on both sides and the rear wheels off.
- Undo the sway bar link at the swaybar and fold the link down toward the middle of the car. You need a thin open end wrench 16mm (I got one at a bike store) and a 16mm socket. This gets the link and the bar out of the way.
- Remove the fender liner – it’s the second worst part of the job. There are three pushpins at the lower rear side to remove, two nuts on posts at the inside on either side of the strut tower, and three screws (two longs and a short) under the bottom of the car at the front. There is a pushpin underneath (not on the other side though) and a 1” diameter rubber plug on the inside of the outside fender well. Remove all the screws, nuts and pushpins, reach up from underneath the edge of the liner and pop it off the rubber plug, then just bend it and twist it until it comes out. As I said, taking it out is the second worst part of the whole job. The worst part of the job is putting it back in!
- You can now see a bunch of emission control equipment hanging there inside the wheel well. Leave it alone – there’s no need to touch it.
- Undo the slide bolts on the brake caliper, remove the anti-rattle spring, push the piston back a bit and remove the caliper. Hang it on a wire hanger back out of the way.
- Remove the now-exposed wheel speed sensor (blue) connector from its box on the inner fender well. Press in the thumb release with a small screwdriver and pop the connector apart. Trace the wire to the grommet beside the brake line fitting and slide a small screwdriver under the grommet and pop the grommet out of the notch. Use the screwdriver to unclip the plastic wire bracket on the rear upper control arm (they snap together at the top – a quick flick with the screwdriver and it unlatches). Leave the bracket attached to the wire. The wire is now free, so unthread it from its route and let it hang forward of the wheel assembly.
- Get a 21mm box end wrench and a 10mm socket on a ratchet and undo the nut on the upper control arm that you just removed the wire bracket from. The ball joint is not on a tapered rod, so it will come free when it’s undone – a ball-joint remover is not needed.
At this point, you have the opportunity to make your life easy or hard. I tried both and easy is definitely better. The problem is that unlike the front suspension, the rear suspension jumps upward about two inches (not down) when allowed to hang free. Getting the strut assembly out is a lot harder once it’s up there.
I solved the problem with a short length of 2x4 between the bottom of the body and the top of the suspension member, so that when the strut is released from the suspension knuckle, the suspension can’t just jump upward. Of course getting the 2x4 out is a challenge, but it’s easier than wresting with the strut!
- Put a block of wood as described or similar in place to keep the suspension from jumping upward when you disconnect the bottom of the strut from the knuckle.
- Use the 21mm box wrench to release the bolt that holds the bottom of the strut. I spun the bolt out using a 7/8 socket on a 3/8 ratchet – it turns fairly easily once it’s free, but it’s torqued to 127 n-m when you start, so you need to get a positive grip on it.
- Undo the three nuts at the top of the strut assembly behind the back seat.
- If you’ve blocked the rear suspension members down, then you can just pull the now-unbolted upper control arm down and back as far as it will go and slip the bottom of the strut through the gap created. Once out, you’re done.
You now have the old OEM unit out, and you’re about half done. To get ready to install the new unit, wind its lower spring perch down so it’s near the bottom of its travel range. This will allow you to shorten the damper by pushing on it so you can get it into place inside the wheel well opening.
- Compress the damper by hand and put the strut assembly into the wheelwell.
- Line up the three top screws with their holes, push them through, hold it in place and reach inside the passenger compartment and put the nuts on the top of the screws. It’s awkward, and much easier with two people, but I did it myself and once you’ve got one in place, you can get the rest. Torque to 18 ft-lbs.
- Pull the 2x4 out and let the suspension swing up (who am I kidding – you couldn’t stop it if you wanted to).
- Put the bolt into the sleeve in the bottom of the damper and fit it to the knuckle. Do it up and torque to 127 n-m.
- Put the upper control link back together and put its nut back on. Torque to 145 n-m.
- Return the wheel speed sensor wiring to its former location, and reinstall the brake caliper.
- Spin the lower spring perch to the top of its travel (adjust it later).
- Put the fender liner back in. It’s not as hard as it sounds; except for getting that stupid 1” diameter rubber plug back in. I used a plastic hammer to drive the plug into position. A week later, it hasn’t fallen out.
Switch your tools and get ready to do the passenger’s side (right, looking forward). At this point, the driver’s side is complete except for re-connecting the sway-bar link. You’ll go back and do that after you finish the other side.
The second side is the same as the first, and unlike the front, you don’t have to undo the headlight sensor link because the suspension never actually droops. Also, as on the first side, once you get the fender liner out, you DON’T have to remove any of the fuel plumbing. It’s not in the way. Similarly, you don’t have to do anything with the brake pad sensor wire – leave it in place in the pad and it’s out of the way when you hang the caliper back. You do have to release and unthread the wheel speed sensor wire, though.
When you’re finished on both sides, reattach the rear swaybar links.
It’s possible that you can do the job without undoing the rear swaybar – I don’t know. I was replacing the bar itself when I did mine, so it was completely removed when I did my work. Taking it out does create space to work around the lower damper bolts.
Put wheels on, put it on the ground, and check the ride height. You’re done.
I strongly recommend that you don’t reinstall the rear package shelf until after you’ve driven the car for a week or so. You’ll find all the squeaks and noises, and if you need to tune the damper settings it saves a lot of time.
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