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Old 02-21-2011, 10:25 AM
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bluebee bluebee is offline
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Originally Posted by jeffstri View Post
I'd refer him to a post (if there is one or make a new one) that clearly explains the difference between fault codes and a diagnosis.
Very interesting. Very well written post. Good healthy discussion.

You sir, will change opinions. Let's see what we can do together.

Where should we refer them for diagnostic hints when they have the ABS BRAKE DSC trifecta, yet, they say "my mechanic read the codes and says it's" (a) the hydro unit, or (b) one of the three steering angle sensors?

Note: If the mechanic says it's the (c) wheel speed sensor, we can provide six diagnostic tests.
If the mechanic says it's the (d) brake pressure sensor, again, we already provide diagnostic tests.

And, why don't we ever hear anyone tell us that the fault codes directly indicate (e) the ABS control module itself?

Originally Posted by jeffstri View Post
I read all Bill's posts in this thread before I made my first post, then re-read them before I made my last post
You know, I don't believe most of what I read here (of what people did); but I "do" believe you. You are spot on.

Let's be blunt.

I don't really know what to tell them when they are told by their mechanic that the ABS BRAKE DSC trifecta is due to a fault code implicating something other than the ABS control module.

Most often the mechanic tells them the fault codes indicated the:
  • wheel speed sensor
  • brake pressure sensor
  • steering angle sensors
  • hydro unit
Rarely, if ever, do I see someone coming here who says the mechanic's fault codes indicated the ABS control module itself.

So, if it's the wheel speed sensor, I tell them to run one or more of the six tests.

If it's the brake pressure sensor, I tell them to run the flying-leads test for voltage as you press on the brake pedal.

For steering angle sensors, I point them to the MAX_VQ BMW PDF explaining the DSC system.

And I have no idea what to tell them when the fault code indicates the hydro unit.

It's important to note that most of the time, it's none of these things (time and time and time again we find that out anecdotally). With the trifecta, it's most often the wheel speed sensors or it's the ABS control module ... which nobody (even I fell into this trap) wants to believe, at first.

Originally Posted by jeffstri View Post
If there's a specific post of his which explains the difference between a fault code and making a diagnosis, why not refer Tom, Dick and Harry to it
Maybe we need a separate thread specifically on the difference between a "fault code" and "making a diagnosis" specifically for the ABS BRAKE DSC trifecta!

The intent would be to refer people to THAT post when they tell us "my mechanic said it was the < fill in the blank >".

Originally Posted by jeffstri View Post
Why not assume they can understand the difference between a fault code and making a diagnosis?
I am not sure I fully understand that, in practice myself, when it comes to the ABS trifecta.

Sure, I know that the fault code is what the computer senses isn't giving the right values - and I certainly realize it could be something else altogether - which is why you need the diagnosis ... but in the specific case of the BRAKE ABS DSC trifecta, I'm not sure what you're suggesting as the diagnosis (given the various fault codes that we know pop up) implicating the wheel speed sensors, the brake pressure sensor, the steering wheel sensors, etc. ... but almost never (if ever) the ABS control unit itself.

How do we diagnose a bad ABS control module anyway?

Originally Posted by jeffstri View Post
You don't have to re-create the explanation every time. If there's already a good post, just refer to it or copy and paste.
Bingo. I HATE re-creating the explanation every time! It's just plain silly for a whole bunch of reasons. You think like I do.

We make the explanation ONCE ... and then we refer people to THAT well-done explanation. Everyone wins. And the next person stands on our shoulders.

However ...

Given what you've said, I do not know if we actually have a good explanation.

Here is what I have been referring people to (from post #36 of the best links):
Do we have something better?

Originally Posted by jeffstri View Post
Probably because they just had fault codes scanned, just like you had initially, not a diagnosis.
Here, for example, is what I've been referring people for fault codes (from post #48 of this thread):
- carsoft.pdf (127.3 KB, 2912 views)
- peake-codes-e39.pdf (410.1 KB, 1305 views)

I realize the difference between dumb reading of a fault code and a proper diagnosis ... but ... I do not know what to tell people for a proper diagnosis when they have the ABS BRAKE DSC" trifecta.

Up until now, the only thing I can tell them is what people told me:
- If you have the ABS BRAKE DSC trifecta ...
- Test the wheel speed sensors (repair or replace any bad ones)
- If all are good, then repair or replace the ABS control module

The assumption is that the fault code (almost all of them in this particular situation) is "really" telling you there's something wrong with the ABS control module. (And which is very often fixed by repairing the ABS control module ... so ... they were right.)

But ... how do you PROVE it was the ABS control module.
I'd love to know how!

Remember, I spent a year trying to do that (and failed!).

It "was" my ABS control module ... but I couldn't "prove" it with a diagnostic test!

What you're saying, I think, is we're missing a whole slew of "diagnostic" tests, especially when the fault codes indicate various other sensors (most commonly the brake pressure sensor and the various steering wheel sensors) and not the ABS control module, per se.

I won't disagree. We are missing those diagnostic trees.

With the little that I know, if someone asks for diagnostic tests for the 20 or so inputs to the ABS, I'd refer them to this BMW PDF from Max_VQ in post #61 of this thread:
- DSC part 1.pdf (988.0 KB, 1572 views)
- DSC part 2.pdf (771.1 KB, 1711 views)

From that PDF, they can build their own diagnostic setup (just like I tried to build flying leads for my brake pressure sensor fault code).

And, I refer them to this PDF from post #48 of this thread:
- carsoft_explained.pdf (643.2 KB, 2418 views)

Which "purports" to provide diagnostic hints (but I wonder what you think of that).

Originally Posted by jeffstri View Post
I don't know how to make any clearer to you that BMW diagnostic tools such as DIS and INPA (and probably newer versions of Carsoft) do a lot more than read fault codes.
I have NEVER used any of those tools (other than the older Carsoft); and, I've tried to get information about them here but failed for the most part.
- Description of Carsoft, GT1, INPA, DIS, EDIABAS, & Peake (1) & which can modify the auto-lock car door feature (1) & what does Carsoft do anyway (1) (2) (3) (4)

In summary, we need to point people to diagnostic tests for the key fault codes that come up during a BRAKE DSC ABS trifecta.

Given that most of the time it's the ABS control module, and almost all (all?) the time the fault codes do not implicate the ABS control module per se ... the question becomes ...

Q: Where should we point people with the trifecta who have been told by others the fault codes report a bad sensor and not a bad ABS control module?

Given the most common fault codes implicate the following:
  • wheel speed sensor
  • brake pressure sensor
  • steering angle sensors
  • hydro unit
And, the most important diagnostic question of all:

Q: How do we diagnose a bad ABS control module?

Last edited by bluebee; 02-21-2011 at 10:47 AM.
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