Bleeding the ABS pump?

#1

Hi all,

I thought I’d turn here to hopefully get some help. My ABS stopped working after working on the car all winter. I was at VIR this past weekend (HPDE group 3) and would have liked the ABS.

Situation:

I replaced my SS lines with new ones and in the process damaged the threads to the LF hard line. I replaced this hard line and probably introduced air into the ABS pump.

When I am rolling to a stop at very slow speed and lightly press the brakes, the pedal pulses like ABS and there is a strange sound like a motor running.

I have had all the wheel sensors tested, and they are good. I have also looked for the ABS relay which is near the ABS computer, but it is not there. I have also tried a replacement ABS computer and have not had any luck.

My car is an 89 325i sedan, and I was told the ABS relay may have been changed to an internal relay in the ABS computer… but I’m not sure if this is true.

Sean Thompson with NASA told me there is probably air in the ABS pump and there is a process to jump pins on the ECU and manually activate the ABS pump, allowing you to bleed it. Has anyone heard of this, and is there a process in doing it??

Thanks for any help, this has been a pain to deal with.

#2

Chris,

I am in the same boat (damaged a hard line that connected to the ABS pump) and plan on working on it this weekend. I have been researching this and opinions are mixed. Several forum posts (like this one http://www.318ti.org/forum/archive/index.php/t-14322.html) suggest that you just bleed the system, go out and activate the pump, and re-bleed. I will be discussing with my local BMW master tech and get back with any further suggestions. Best of luck,
Ed

#3

When I changed my front brake lines to SS, I had problems with bleeding the brakes. I bled them and they seemed firm, but when I drove the car, the pedal went to the floor and had to be pumped to get the brakes to work.

During a second bleeding, I got a lot of air out of the left front. The pedal felt better, but when I drove the car it was still soft and had to be pumped to get the brakes to work. I then bled the brakes again. I noted that the Bentley manual said to pump the brakes at least 12 times on an ABS car. I did that and after a couple of times cracking the right front bleeder valve, got a huge gush of air out of that bleeder. That took care of the problem with the pedal firmness.

Don’t know if my problems were because I got air in the ABS pump or what, but you might try bleeding the system as described in the Bentley manual to see if that helps.

Sadly, I have some other issue that is keeping my ABS from working, so I’ll be going through the process of trying to track down the cause. On that note, how did you verify that your wheel sensors are working?

#4

Thanks for the info, I’ll try pumping the brakes a lot more when bleeding and see if that helps.

I had a shop test the sensors, so I’m going on their word that they are ok. If I make any progress on this I’ll let you guys know.

#5

I have a different bleeding issue. The previous owner of the car changed all the brake lines and the car sat for two years with no fluid. We added fluid and tried to pump the brakes with the pump and hold method, but the pedal is unnaturally solid and will not pump, even with a bleeder open. We tried this with the car on, the car off, the front brake bleeder open, the rear brake bleeder open, nothing changes. Any idea what causes this? When the car had no fluid in it, the pedal predictably dropped straight to the floor, but now with fluid it just sits there rock solid. I must be doing something wrong with the ABS pump or something. If I just put pressure on the master cylinder cap, would that necessarily push fluid through the ABS pump, or is there something that I am badly missing? It is entirely possible that I just don’t know what I am doing bleeding on this car. Please advise.

-Scott

#6

Randy87is wrote:

I just went through this too. From all the information I could gather, take a multimeter and read the ohms on each sensor. The front ones disconnect on the wheel wells inside the engine bay and the rears’ disconnects are where the rear seat used to be. They should read between 800 and 1600 ohms. Mine all read around 1000. Then, with the meter still connected, spin the hub. The ohms should jump pretty erratically. When I did this, the two sensors on the left side made the meter beep like it had continuity but it wasn’t constant, more like a fast pulse. The two on the right side made no sound whatsoever. I have no idea which is supposed to be correct, silent or beeping. Also, make sure you take the sensors out and clean them really well. You might also try sticking a rag down in the hole and cleaning the teeth of the abs ring on the hub.

However, after all of my sensors tested properly and my abs light still came on a couple minutes after I started driving, I got frustrated and just replaced the two front sensors with some junkyard ones and it now works. It’s worth testing the sensors but just be aware that if they all test well, you may still have a broken sensor. As the saying goes, ask me how I know. I’ve also heard of using an oscilloscope but I don’t have one so I’m not sure on that procedure. Typically, if the abs light comes on after you drive for a mile or so, it’s a sensor, if it comes on immediately, it’s the relay or the computer.

Re: the abs pump, the only thing I know about this is that the aforementioned 12 pump theory helped mine immensely.

#7

The R. Patton technique for finding bad wheel speed sensors is to disconnect all but one sensor, then drive the car. If the ABS light comes on that sensor is generating a speed signal and is good. Repeat for each sensor until you find one that doesn’t turn on the ABS light.

#8

BigKeyserSoze wrote:

[quote]I have a different bleeding issue. The previous owner of the car changed all the brake lines and the car sat for two years with no fluid. We added fluid and tried to pump the brakes with the pump and hold method, but the pedal is unnaturally solid and will not pump, even with a bleeder open. We tried this with the car on, the car off, the front brake bleeder open, the rear brake bleeder open, nothing changes. Any idea what causes this? When the car had no fluid in it, the pedal predictably dropped straight to the floor, but now with fluid it just sits there rock solid. I must be doing something wrong with the ABS pump or something. If I just put pressure on the master cylinder cap, would that necessarily push fluid through the ABS pump, or is there something that I am badly missing? It is entirely possible that I just don’t know what I am doing bleeding on this car. Please advise.

-Scott[/quote]

MC’s can easily get damaged if there’s no fluid in them. They either rust internally, or the piston gets pushed too far by the pedal and seals get damaged.

That aside, start removing brake lines and figure out where the fluid is stopping. Start at the ABS unit. Identify which brake line going in/out of the ABS unit does what. Then remove one of the in lines and watch for fluid to spurt when you press the pedal. Do the same with the out lines.

The ABS unit as 6 lines I think. LF in and out. RF in and out. Rear in and out.

Greg Moberg’s Lemons car had similar symptoms. The ABS unit ended up being bad. We figured this out when we couldn’t get brake fluid to come out of one of the ABS’s “out” lines.

#9

Was the ABS unit bad, or one of the ABS relays.

I find it maddening that you can’t just remove the entire ABS system on these cars and still be legal. I see so many posts about wheel sensors, abs pumps, abs relays, and everything else, it seems there is far more money being spent keeping ABS working than if we just yanked it off and modulated lockup the old fashioned way, with the driver.

-Scott

#10

I still can’t get this thing to bleed. With the car on jacks and running, you can pump the pedal a few times and get a little squirt out of a bleeder, then the pedal gets rock hard and doesn’t want to do anything. Frustrating. Is there a way to cycle the ABS so that the pump stays running, or what do I need to check on the ABS relay or whatever the little part is that seems to go bad on these cars?

This is the last issue keeping this car from going on-track.

-Scott

#11

Take power and control signals off the ABS unit by disconnecting the plug. If the brakes then behave normally the ABS unit may be okay and the problem lies in the ABS module. If that makes no difference, identify the line that runs to the brake you can’t bleed and loosen it where it connects to the ABS unit. If fluid flows freely there and the pedal doesn’t become hard, there’s an obstruction in the brake circuit after the ABS unit. If fluid won’t flow freely the ABS unit is bad and needs to be replaced.

You don’t have to have a working ABS system on the car. You just can’t remove the components.

#12

I took the plug on and off, no difference. After a while of standard bleeding, I got the front left to work ok. Then I got the front right, after pushing a lot of crud colored fluid out, to work ok, smooth pedal travel all the way to the floor. Then I went to the back left and it only went halfway down, I worked it for a bit, got a little better. Then I went to the back right and got some crud and air out until it went smooth, then I went to the back left and finished it up.

It strikes me that I likely bled the entire system out of order. Is there a published order?

There is still some air in the system, but I finally at least have a smooth pedal.

Can these cars be gravity bled wheel by wheel, or does the ABS or other system on the car prevent a gravity bleed? My thought is to put the car level, start at the back right, pull the bleeder out, and just let it drip in a pan for a few hours, then move around the car. Not sure if it will work or not. Thoughts?

#13

Bleed sequence RR, LR, RF, LF.

RR will take a long time. But LR and RR brake lines don’t actually separate until the diff area so you don’t have to push much fluid out LR.

Never heard of a gravity bleed. Wouldn’t you end up with a bunch of air in the lines?

#14

According to the old timers, gravity bleeding is the only way to ever go. As long as the master cylinder is higher than the calipers, if you open the bleeder, the fluid should slowly drip out of the caliper following gravity, letting the air out with it. It has worked on all of my other race cars well, but I wasn’t sure if the ABS pump would impede this.

-Scott

#15

Scott, if you open a bleeder screw and just let it drip you aren’t letting any air into the system. If you push the brake pedal while this bleeder screw is open you will see lots of fluid come out, but it’s when you let off the pedal while the bleeder is open that air enters the system. That is why when you bleed the system manually you want to keep the bleed hose in a cup of fluid.

I have replaced the ABS pump in both my Spec E30 car and the lemons car and know for a fact there is no special bleeding procedure. Passenger rear then driver rear then passenger front then driver front. If you have a pedal you are good to go. Basically you start from furthest away from the master and work your way forward. Order really doesn’t matter that is just how I learned. As long as you bleed the back together as they share the same line. The front is much easier say if you need to change a stuck caliper in the paddock you only need to bleed that brake line.

BigKeyserSoze: you are dealing with a system that was left dry for 2 years. The Master Cylinder, ABS pump will probably need to be replaced. Hopefully the hard lines are still good. The master does not like to be completely dry!! I had to let my system sit for 4 days when I replaced the ABS pump on the Spec E30. In that time I never touched the brake pedal. Once the ABS pump was installed and the brakes were bled I didn’t have a problem for 2 years. Move forward to last Oct. I stole the calipers off my spec E30 and swapped them to the lemons car. About a week before the IFU I reinstalled my calipers and went to bleed the brakes. I filled the system reattached all the brake lines but after a few quick bursts of fluid I was getting nothing from the RR. Walk up to the reservoir and its full WTF. I try it two more times before I realized I left that stupid cup with the tiny hole in the bottom in the reservoir. A couple times pushing the pedal had removed all the fluid from the reservoir and master even though lots of fluid was still in that dumb cup. I removed the cup and tried to re-bleed the system with no luck. Moral of the story the master only has to be pumped while dry one time. $300 later I was back in business.

#16

This is so frustrating.

Recall that this car sat with no brake fluid in the system for two years.

Here is what I did today:

I modified the master cylinder cap with a male quick disconnect fitting so I could easily power bleed the system by hooking my air compressor up to it on low pressure. I took the master cylinder resevoir off, cleaned it out the best I could (it was nasty), filled it up, pressurized the system to 12psi on my air compressor hose, and removed the rear passenger side bleeder and let it flow. The car was off for all of these bleeds, and I removed the bleeder screw completely for all bleeds. The flow on the passenger rear was not great, but I put an entire resevoir of fluid through it and it was flowing clean, but not too fast. Flow on the other three was ok. Went to the rear drivers side, pressurized more fluid through that, looked good. Then did the front passenger side. Then the front driver side. buttoned it all up, took it for a drive, and still virtually no brakes. The first 3/4 of the pedal travel it just drops. The last 1/4 or less will lock the passenger rear side, but there really was virtually no brakes. It is as though the front brakes are doing nothing, and the driver rear isn’t doing much of anything either.

All in total I probably put a quart of fluid through the system.

If you get in the car, no matter what you do, you cannot get a firm pedal. It will not pump up at all. Usually with a bad master cylinder you can pump up a firm pedal.

How do I diagnose what failed? The prior posts seem to indicate that the master cylinder and/or the ABS device may have died, but how do I narrow that down? Also, is there a manufacturer that makes good, new master cylinders? I have had poor experiences with rebuilds over time. Also, as noted above, even with a mad master cylidner, you can usually pump it up to get a firm pedal. Not so with mine.

Any thoughts on what I can do here? I am running out of time to get this thing on track and I really want to run it. Any help would be appreciated.

-Scott

#17

A reservior of fluid won’t flush air out of the rear brakes with any certainty. You could have a bunch of air in your rear lines. That would fit the symptoms near as I can tell, especially if you have more air in driver’s rear then passenger rear. Put 750cc thru your passenger rear and then 250-350cc thru your driver’s rear. Then you’ll know you got all the air out.

#18

You can’t be serious. 750cc through the passenger rear? Good thing I buy a gallon of cheap fluid to flush!

-Scott

#19

What about the fact that the fronts do nothing?

#20

BigKeyserSoze wrote:

Having a big pocket of air in the rear brake lines would explain why the fronts are doing nothing. Nothing is sure but death and taxes, but my theory fits the symptoms. Anything soft in the system makes the entire system soft. That’s why a single pad worn at a slant, like rear pad due to no anti-rattle clip, causes the whole system to be soft.

The “T” for the rear brake lines is near the diff. So once you put a bunch of fluid thru the passenger rear, you don’t have to do much more for the driver rear.

Sure 750cc is a lot for the passenger rear, but 500cc might not be enough. So we might as well go overkill and that way we can be absolutely sure that we’ve done a complete flush of the longest brake line. Doesn’t hurt to use a different color of fluid too.