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Old 09-18-2010, 08:17 AM
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540iman 540iman is offline
resident, old fart
Location: N.W. Indiana
 
Join Date: Sep 2008
Posts: 1,525
Mein Auto: 2002 540i sport
Welcome! 1st post or 1000th, we are all here to share opinions and experiences. I agree with a lot of what you stated and disagree with a little of what you stated. First, I don't believe anyone (mainly me, I guess) took issue with the way Agent15 went about his diagnostics. I think that a lot of the problem is that Carsoft and others don't, as you say tell you that the patient has a low white blood cell count and stop there. They often "attempt" to go farther and diagnose the patient as having cancer. The last ABS system I repaired was interesting because all I have in the way of BMW-specific diagnostic equipment is Carsoft. The buddy I was working with had INPA, progman, and all the other stuff you mentioned. Both our diagnostics lead to a faulty yaw sensor and NEITHER were correct. Carsoft also said a wheel speed sensor was bad which also was wrong. I determined it was wrong with a diode test. I don't agree with your statement that the diode or resistance test is not conclusive. If you test at the module connector, it is less conclusive then if you remove the suspect wheel sensor-have it and it alone in your hand and then test it with a VOM. You need to make sure that the sensor does not have metal shavings stuck to the end of the sensor and that the wiring to the sensor (between the module and each sensor) is not also what is giving you an open or short.

I think what was being conveyed to Agent15 is to not be surprised if the diagnosis, as given by some equipment, will be wrong and may lead to replacement of parts and the problem will still be there. You must, as you say, get more tests that will further define the problem. Initially, I think I was of the opinion that 90% of the time, problems with the trifecta lights will ultimately be a bad module and 10% a bad sensor. I now think it is more like 70-30. I have found more bad sensors lately, but have NEVER found one that tested bad and was really good or vice verse. I would bet a paycheck that if Agent15 or anyone for that matter has the sensor in his hand and can get meter leads deep into the sensor such that he can test either in ohms or voltage bias forward and reverse (diode test) and the sensor tests bad, that it IS bad. Conversely, if it tests "ok" meaning there is a definite difference when meter leads are reversed and sensor does not test open or shorted BOTH ways- it is NOT the fault.

The main point is that any diagnostic equipment that does not test by going between the sensors and the module simply can not definitively see a bad module from a bad sensor. It is as simple as stating that a blood pressure cuff can not diagnose whether high blood pressure reading at a cuff is because of a faulty heart or a clotted artery. You must somehow get readings I suspect (certainly not a Dr.!) between the heart valves or whatever you do to isolate. If your Indy or dealer hooks up to your OBDII port or your 20 pin and definitively tells you that you have a bad wheel speed sensor, that person is full of doo-doo. Only thing you can diagnose correctly 100% of the time from either of these two points is a bad module due to a communication error or similar. They can not tell a bad input (sensor) from a bad module at these test points.
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