BMW Z4 Hydraulic Pump Repair Without Removing Top: The Bellis Maneuver

Preface

8 min readDec 15, 2020

The BMW Z4 convertible has an unfortunate engineering flaw, in that the hydraulic motor that lifts the roof is situated in a drainage trough that fills with water as soon as its drain plug gets clogged. I’ll guess that unless one avoids driving in any rain, it is almost destined to fill with water at some point and ruin the somewhat water-sealed unit. I know one other owner; it happened to him too. Youtube, when I started investigating, had a great video that shows removing the entire roof, a very hard procedure that the dealer asked $2500 of me. Not gonna happen. (BMW=bring money with.) After successfully removing my roof and fixing it, I realized there was an easier solution, opening the roof half-way to relieve its tautness, and then just lifting the back half of the roof to access the pump… hereafter known as The Bellis Maneuver. The video is helpful, but text instructions are essential, and provided here in Medium.

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Entire article copied from my old Google Doc page, to Medium because Medium is now the place for all things text.

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BMW Z4 Roof Hydraulic Unit Repair

Update, April 28, 2019: Three Youtube viewers have now performed the procedure.

Update, October 9, 2018: One Youtube viewer has confirmed that my procedure (to access the pump by lifting only the rear window section of the roof) worked for them.

Overview

I repaired my 2007 BMW Z4’s convertible top hydraulic unit by UNNECESSARILY removing the entire convertible top, with the help of a great Youtube video from Pray4UrLife12, below. Others have posted that you can get to the motor by reaching it through the trunk. I tried reaching it that way and decided it was too difficult. [Update, May 10, 2023: I subsequently saw one Youtube that I think mentioned removing an additional piece of plastic in the trunk, that I did not remove, when I tried reaching through the trunk. It’s possible that the removal of that additional piece is the key to that maneuver. That piece of plastic is almost certainly the piece through which I drilled a hole to route the electric and hydraulic lines to my newly relocated pump. One would still have to cut some retaining cable ties at arm’s length, but it deserves investigation.]

When reinstalling the whole top, I realized that you should be able to access the pump by removing only the rear, aluminum “C-channel” that holds the rear of the top, and lifting up the rear window section. In the following video I show the procedure, although not every step (since my roof was already detached by that point.

I am pretty sure this procedure enables accessing the motor without removing the whole convertible top but I won’t consider it definite until someone confirms it. Please post a comment on the Youtube page if you use this procedure successfully. Three people have confirmed that it works as of April, 2019. Thanks!

My Youtube Videos

Accessing pump by lifting only rear window section of top: https://www.youtube.com/watch?v=JRwSJq64wBM

Reviving water-logged electric motor for the hydraulic pump:

https://www.youtube.com/watch?v=lYv5A1XdD3I

Replacing with Mustang Pump in Trunk:

https://www.youtube.com/watch?v=8dWMaZv-WQ4

Confirming Video from Another Youtuber

The Beat Don: https://www.youtube.com/watch?v=KBVEiP9XPa8

Optional Full Roof Removal

“Pray4UrLife12”

https://www.youtube.com/watch?v=k31nU0FWmtE&t=137s

I made a gantry out of my garage door tracks to remove my roof and will post videos some time.

Moving the Original Pump to Trunk

https://www.tophydraulicsinc.com

https://www.tophydraulicsinc.com/bmwz4instructions.pdf

Dc_wright on https://www.zpost.com: https://www.zpost.com/forums/showthread.php?t=1215892

Replacing the Hydraulic Unit with 2009–2014 Mustang Unit, and Moving It to the Trunk

Antoine JeanMarie

https://www.youtube.com/watch?v=O7lfvKerfhg

I have completed this substitution and it saved me US$300 compared to the Z4 unit, and the new one is stronger and faster.

The larger Mustang pump (right) and smaller Z4 pump (left); Google Docs keeps rotating the image, making the perspective look odd and the Mustang pump look smaller. Notice the center hydraulic pump section, between the red lines, is virtually identical and absolutely the identical size. The only difference is that the Z4 has the little bronze bypass button, circled. This is controlled by the red handle in the Z4 trunk.

Procedure to Access Hydraulic Motor by Lifting Rear Window

Preparation: URGENT TECHNIQUES YOU MUST KNOW

Some people call this section “tips” but that is incorrect. If you don’t learn these techniques, you might not be able to do the job.

Getting the rear, aluminum “C-channel” out is mainly accomplished by angling it up, side-to-side. I found that lifting the passenger side first makes room for the hydraulic unit to fit out. In other words, it’s unlikely to lift straight up and out.
When the roof is halfway up, everything seems to have more room to be manipulated.
When the roof is all the way down it is totally impossible to remove the roof because it forms a very solid semi-circle shape from all of its metal bars collapsed together.
When the roof is all the way up (with the front latches loose), it is also somewhat of a solid unit. In this position my procedure below (to lift only the rear window) does seem to work, but I’m pretty sure it’s easier with the top halfway.
How to hold the roof in the halfway position with tape:
Use longitudinally-reinforced (nylon, fiberglass???) strapping tape from the windshield to the black roof bars.
Fold the tape lengthwise over on itself as it passes the front windshield gasket so it doesn’t stick.
Fold the tape over at each end so you have quick-removal ‘handles’ to adjust it.
With this easy-to-adjust setup, you can adjust the length so that the roof is falling slightly backwards, perhaps just one inch past the centerpoint.
The nuisance aluminum clips that hold the aluminum C-channel in place can be reached through the trunk but really require you to stretch. But I am pretty sure you can also reach them through the passenger compartment when you have the roof in the HALFWAY POSITION! Reach between the roll bars and roof fabric. You can’t see the brackets, but they’re held in with a star bolt.

If you use the miraculous Husky or Stanley “Rotator” ratchet you can probably remove from the cabin. This wrench lets you just twist (!) the handle’s grip, not spin the whole arm.

How to use strapping tape to hold the roof in the halfway-up position:

Steps

These steps are written after doing the job, so they’re the best I can reconstruct.

For most of the details of the initial steps see the Pray4UrLife12 video.

Set the roof to bypass the hydraulic unit by pulling the red loop handle in the trunk so it is approximately 3/4” to 1” out from its fully-in position. Turn it a quarter turn so it stays in the out position. This creates a pathway for hydraulic fluid to pass between the up and down circuits of the system so you can manually move the top. If the top moves freely WITHOUT the red loop in the out position, I believe this means your hydraulic circuit has a fault… meaning either there is air in the system or a leak.
In the trunk, remove the top’s rubber edge from the removable deck… if it’s still connected.
In the trunk, remove the plastic deck by popping out the plastic clips (see the Pray4UrLife12 video) and angling the deck out.
In the trunk, remove the 7 bolts that hold the 4-foot long, aluminum C-channel to which the roof fabric is attached. (See the Pray4UrLife12 video)
Pull free the rear weatherstripping that connects the top’s canvas from the rear edge of the car body.
Cover the edge surrounding the top with removable painter’s tape, wrapping inside the alcove a bit.
Add a layer of Gorilla Tape on top of the painter’s tape, for better toughness against scratches.
As described next, remove the two aluminum clips that the C-channel sits in, within the trunk, at the frontmost extent of the C-channel. These are located roughly behind the driver’s left shoulder, and the passenger’s right shoulder.
Find them by using a flashlight in the trunk.
I’m pretty sure you have to pop the C-channel out of the clips to access the T-bolts that hold them on. You might be able to use a screwdriver from the trunk.
You can also access the clips, with the top in the halfway position by reaching in from the cabin, between the rollbars and the roof fabric. See above for instructions on holding the roof halfway.
From either location, a “rotator ratchet” wrench might be necessary. WIth it, you can simply twist the handle back-and-forth to unscrew the T-bolts.
Put the roof in the halfway position. See above for instructions on holding the roof halfway with tape.
On the passenger side, pull the weatherstripping toward the center of the car to pull the C-channel up and out. Pull it a little forward. Only if the aluminum clips are still in place is it likely to hang up on anything. If you did not pop the C-channel out of the clips at all, the channel probably won’t release at all.
Work your way backwards, pulling the C-channel out and toward the passenger side of the car.
On the driver side pull the weatherstripping and the C-channel up and toward the center of the car. It will be pulling the hydraulic pump, 4 hydraulic lines, and 3 electrical lines, so pull gently. Only if the aluminum clips are still in place is it likely to hang up on anything. If you did not pop the C-channel out of the clips at all, the channel probably won’t move at all.
Lift the entire C-channel and window up and put a soft brace under it to maintain access to the motor area.
If this worked for you, please post a comment to the Youtube video to that effect. Thanks!

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