Replacing Brake Vacuum Pressure Sensor

I have to remove the master brake cylinder to replace my vacuum pressure sensor and I was wondering if I can simply empty the reservoir and refill it with clean fluid once I'm done or do I need to just flush the whole thing?

I also might replace the vacuum pump while I'm at it, but it's only throwing a code for the sensor. If I did replace the pump, I can access it more easily just from the wheel well and all I need to do is unhook the vacuum hose. Would I be able to just pinch the hose shut and do this or am I back in the situation above?



***Since this post is now being cross posted and referenced, here is the part information***

Part number: 34302283605 (includes both sensor and vacuum line)

Real OEM diagram (#12): http://www.realoem.com/bmw/enUS/showparts?id=WD93-USA-09-2009-E92-BMW-M3&diagId=34_1627

Purchase link: https://www.ecstuning.com/b-genuine-bmw-parts/vacuum-line/34302283605/

If you remove the master cylinder completely then you would need to bleed the system as you would have introduced air into the lines. That isn't necessary however to replace the sensor. If you simply remove the reservoir to gain some room for your hands, then you can just fill the reservoir when you're done. My hands are small enough to reach in to remove the sensor with everything intact, though it does usually tear my arms up a bit.

For the pump, it's an easy swap. Just detach and pull the fender liner forward and you will have access to replace it. Pinch the connector clip to remove the hose and detach/slide upward the pump/bracket assembly to remove it. No need to pinch any hoses or anything. It's just a vacuum hose and the car should obviously be off and on jack stands.

Awesome thanks for the tips! You saved me a lot of trouble.

The brake boost sensor what your talking about? Just had to replace mine, was throwing me in limp mode. The gasket was pretty shot, assuming thats what was causing the problem. Was pretty easy swap, other than needing to do a full brake bleed/refill. Needed one anyway though.

Yep same code. So annoying to be brought down by something so small. I didn't think about the gasket, I figured it was the sensor shorting out because it got wet. I'll have to check mine, I haven't changed it yet. I took off the expansion tank and there was no way I was gonna get it without taking off the master brake cylinder. I need some magic hands like Deansbimmer!

M3MPH1S wrote

Yep same code. So annoying to be brought down by something so small. I didn't think about the gasket, I figured it was the sensor shorting out because it got wet. I'll have to check mine, I haven't changed it yet. I took off the expansion tank and there was no way I was gonna get it without taking off the master brake cylinder. I need some magic hands like Deansbimmer!

I was able to get to it with just the expansion tank off, was very tight fit and I have large hands. Took a little fishing around to get it back in place, but you dont want to go taking off master cylinder. Usually causes all kinds of seal leaks etc down the road.
I unscrewed it pulled it to surface then unclipped it, made much easier to pull outa that mess with sensor wire holding onto it. Then just clipped new one in, and wiggled it back into place.

Hmm I will definitely try again. What about the vacuum line it's tapped into? Is there enough slack to pull it up while it's still in or did you pull it out? Any tricks for getting the tip back in the hose?

I had to swap my sensor back in the fall. Yes it was a major pain in the ass err arm/hand but I did not have to remove the reservoir or brake system/lines.

I did unbolt the ABS system at its mounting point but only to push it out of the way about 1/2"and I removed the bracket around the vaccuum lines wear they go through the firewall towards the engine. If you remove that plastic piece that keeps the lines from shaking around it allows you to move the vacuum line a decent amount.

I wear a XL glove for reference, so most peoples hands should fit just fine.

Which one is the ABS unit? The thing on the right in my attached picture?

M3MPH1S wrote

Which one is the ABS unit? The thing on the right in my attached picture?

Yes the silver block with the hard lines going into it. Its teh ABS module and used for the stability control. Its two or three small nuts (10mm I think) on studs that hold it down. If you are careful you can move it up and over a bit to make a little more room.

Its not an easy task to swap the sensor but it can be done. I used a socket and a few random extensions and swivels to remove the two screws holding the sensor in place. Also used a bit of yelling and cussing. It eventually worked. Took maybe 15 minutes start to finish.

This is the best I can do with what I have lol. Doing all this will give you a bit more room. Then you just prepare to have your hand/arm mildly wounded by your own car and attempt to reach the sensor.

lol you da man!

M3MPH1S wrote

Hmm I will definitely try again. What about the vacuum line it's tapped into? Is there enough slack to pull it up while it's still in or did you pull it out? Any tricks for getting the tip back in the hose?

Line will not come up, it routes under the master cylinder and has many connections. Sensor pops right out with your fingers once you remove the two torque screws. Make sure to use a magnetic tool, do not want to drop the suckers. Can use a screwdriver to move it around until its lined up with the screw holes when putting back in place, a light is a must as its pretty dark down in there.

Ok I finally had some time to go at it again and got it finished. Thanks for all the tips! In the end I was able to get it with the expansion tank still on. I didn't even need to move the ABS unit, all I did was unhook that plastic around the hose on the left and pull it out just a bit. I will say it was definitely due to having a second pair of hands. One of us was able to use a long screwdriver to slightly push the sensor in one direction then the other to get it unscrewed and one of us used the magnet once the screws were loose. Additionally getting it back in was the real tricky part. Luckily my friend had a long hook tool. Once we plugged up the new sensor we fished it back down while holding on to the cable then we put the hook tool through one of the bolt holes on the sensor. Hook on one side and a long flathead screwdriver on the other side to guide it back in place. Even with the tools it took about 5 minutes of finessing to get it lined up. After we got it lined up I gently pulled up on the vacuum hose while he pushed down on the sensor with the flathead and it went it without much fuss. Lastly we were able to use the long magnet to barely start the screws then get in there with the torx socket (T15) with 2 wobble extensions. The whole thing took like 30 minutes.

Perfect! This is one of those jobs that is easy to overthink. Best to just dig in and git'r done.

M3MPH1S wrote

Ok I finally had some time to go at it again and got it finished. Thanks for all the tips! In the end I was able to get it with the expansion tank still on. I didn't even need to move the ABS unit, all I did was unhook that plastic around the hose on the left and pull it out just a bit. I will say it was definitely due to having a second pair of hands. One of us was able to use a long screwdriver to slightly push the sensor in one direction then the other to get it unscrewed and one of us used the magnet once the screws were loose. Additionally getting it back in was the real tricky part. Luckily my friend had a long hook tool. Once we plugged up the new sensor we fished it back down while holding on to the cable then we put the hook tool through one of the bolt holes on the sensor. Hook on one side and a long flathead screwdriver on the other side to guide it back in place. Even with the tools it took about 5 minutes of finessing to get it lined up. After we got it lined up I gently pulled up on the vacuum hose while he pushed down on the sensor with the flathead and it went it without much fuss. Lastly we were able to use the long magnet to barely start the screws then get in there with the torx socket (T15) with 2 wobble extensions. The whole thing took like 30 minutes.

I'm about to attempt this.. just ordered sensor with hose and will be replacing it using your method since I'm not too excited about draining the brake fluid! I'll let you know what happens lol

M3MPH1S wrote

Yep same code. So annoying to be brought down by something so small. I didn't think about the gasket, I figured it was the sensor shorting out because it got wet. I'll have to check mine, I haven't changed it yet. I took off the expansion tank and there was no way I was gonna get it without taking off the master brake cylinder. I need some magic hands like Deansbimmer!

How would you say the sensor may have gotten wet? Have you played with the hood vents or cabin filter area at all? Or were you 'assuming' it got wet and that wasn't the actual diag?

Oddly, after my car being out (parked, not driving) in a rainstorm overnight, I threw the same code the next morning when I was driving. Since then and after clearing the codes, the car has been driving fine. Although I do hear the pump priming more often (if that means anything).

JRob23 wrote  

M3MPH1S wrote

Yep same code. So annoying to be brought down by something so small. I didn't think about the gasket, I figured it was the sensor shorting out because it got wet. I'll have to check mine, I haven't changed it yet. I took off the expansion tank and there was no way I was gonna get it without taking off the master brake cylinder. I need some magic hands like Deansbimmer!

How would you say the sensor may have gotten wet? Have you played with the hood vents or cabin filter area at all? Or were you 'assuming' it got wet and that wasn't the actual diag?

Oddly, after my car being out (parked, not driving) in a rainstorm overnight, I threw the same code the next morning when I was driving. Since then and after clearing the codes, the car has been driving fine. Although I do hear the pump priming more often (if that means anything).

That specific post was paraphrased in context. when I said "wet" there I was talking about wet from brake fluid

M3MPH1S wrote

That specific post was paraphrased in context. when I said "wet" there I was talking about wet from brake fluid

Which is the most common reason for the sensor to malfunction I've read. Have you done any brake work to warrant this assumption?

JRob23 wrote  

M3MPH1S wrote

That specific post was paraphrased in context. when I said "wet" there I was talking about wet from brake fluid

Which is the most common reason for the sensor to malfunction I've read. Have you done any brake work to warrant this assumption?

Yes, I had just recently swapped brake pads from very very worn to big fat brand new ones and had to spread my calipers a lot. I actually noticed brake fluid on the ground during the change, but didn't know about this issue so just brushed it off

This thread will help many of us to avoid this problem in the future while doing the brake job.
I knew that we should be careful not to spill the brake fluid, because it can cause damage to the car paint, but wasn't aware of possibility to short that vacuum sensor which seems to be a big PITA to replace.

Yesterday I replaced the sensor. I also unbolted the ABS module and shifted it a tad, don't know if it made any difference. My saviour was LONG needle nose pliers, and I think just blind luck positioning the new sensor.

Unfortunately all back together and I still get the 27E4 code and limp mode. When I pulled the old sensor out it was still under vacuum so I don't believe the hose is compromised. What is next for me to check??? It's been about 7 months now my car is half useless...

Cheers