Guide to Diagnostics

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  1. Guide to Diagnostics 
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    Hi, this is the first thread i'm starting, and i'd like to give some info about diagnostics, the pros and cons and what to do with the info.

    What you have

    There are plenty of different diag types, most ecus from the 80's have inbuilt diagnostics, called On Board Diagnostics. This was self checking the sensors, and also making sure the sensor values were plausible. This system was active in the US, but in most cases, wasn't fully active in Europe. This first system in the US displayed the CHECK ENGINE light, and would flash simple engine related codes which could be decoded with a reference book.

    This is inside the Euro ECUs but the light isn't connected up. That's why the "Stomp codes" won't work in the UK. The check engine light only became active from the full OBD2 era onwards.

    To read codes from these, you will need a BMW specific code reader, Peake, Carsoft, or the smarter stuff like DIS/GT1, Autologic etc. ELM327 obd2 stuff won't work.

    These BMW specific readers have the protocols installed to talk to the majority of modules while the stomp code just communicates to the ECU.

    These are the readers that will diag ABS and airbag problems, as well as any other module installed.

    In 96, OBD2 was introduced. OBD2 has features such as a flash memory for reprogramming, adaptation (which is self adjusting it's own maps to match the targets it is programmed with) and the easy OBD2 protocol.

    As with before, Europe didn't get this fully working, the software was there, but the easy protocol was deactivated.
    The idea was a simple cheap diag system for every car that any garage could use for emission based faults so you didn't have to go to the dealer.
    That was the law in the US, but it wasn't in Europe, we still had to wait until 2001 for petrol, and 2004 for diesel.

    It was simpler to do a global update for every ecu, but euro stuff was still on "dealer only" as they could get away with it.

    You will find some euro cars from '96 that were OBD2 compliant, which was nice of them, but BMW stood back from this.

    **this means '96 on Euro BMWs WILL NOT WORK WITH OBD2 READERS**

    Yet again, you'll need to use Carsoft etc to communicate.

    From 2001 onwards, the familiar OBD2 16Pin plug near the dash started to appear in e39s, e46s etc, and OBD2 was fully switched on.

    Remember, the OBD2 system was only concerned with emissions and efficiency, so you won't communicate to the ABS, Airbags etc.



    Getting the codes

    Once plugged in with the ignition on, follow whatever instructions to pull the codes out. If someone else is reading them, tell them everything you did, whether you unplugged stuff, swapped stuff, the last time the codes were cleared etc. This can be vital information to the diagnoser.
    If you are using your own reader, record all the details, including and reference numbers for the code.

    DO NOT JUST READ THE FIRST WORD YOU RECOGNISE AND SWITCH OFF!

    A common problem is not reading the code description, just taking "lambda" or "MAF" and then going straight out for a new one. If there is no description, still take down the code number and it might be possible to find a description somewhere else.


    Understanding the codes and good diagnostics

    The diag system checks the sensors in a few different ways, firstly, during start up, it will check the resistances and start up values to make sure everything is still there. Once happy, it'll monitor the readings to make sure everything matches up with it's own criteria.

    For example, if you pull an ABS sensor out of the hub and still leave it connected, you won't get any codes after starting. The system has verified the sensor is connected, and the system remains happy.
    Once you move off, and get to about 5mph, the ABS system realises it isn't getting the pulses from the loose sensor, and the light comes on.
    That would be what's known as a plausibility error.

    In other words, there should be no way you'd be driving with one wheel not moving, and the system deactivates.

    Switching off and on again, the system will recheck the sensors are connected, the light will go off, and come on again once moving.

    For Airbags, the system is smarter. ANY fault whether current or just detected for a fraction of a second will put the light on, and it'll stay on. The system will remain active, but it will decide using the knowledge that one input was known to be bad so don't think that when the light is on, the airbags are all deactivated.

    For each of the systems, and the codes recorded, there will be information on what it noticed was wrong. This is why it's important to get the full description of the code.
    For most sensors, it will note if it is disconnected, unresponsive, or has a short circuit on it.
    An unresponsive ABS sensor could be due to it being loose, or the ABS ring gummed up. Cleaning might fix this, but disconnected could mean either the plug is adrift or the wires are broken internally (very common).

    A short to ground or resistance could mean a damp plug.

    All of these errors have very different causes and fixes, so it's important to get as much information as possible.



    Shooting the messenger

    On of the common problems is misreading a plausibility code, and swapping the sensor. It can and does happen that the sensor is bad, but in a lot of cases, the sensor is fine, and is just reporting a condition somewhere else.

    A major example would be a vacuum leak.

    Take for example the split rubber boot on a MAF. When this occurs, the engine draws in air behind the MAF, which isn't recorded.

    Measurements from the lambda sensors will show more O2 passing through the system than expected. Fuel trims will be set higher, and the system will survive.
    You may get a code here saying Lambda out of limit, which means there is far too much O2 passing through (lean), adaptation will show the fuel trims are sky high and it might recognise the true cause is the air measurement is wrong.
    That would store a MAF code.

    So-
    air leak can give lambda codes, MAF codes, and adaptation limits reached.

    A lot of people here will get a new MAF, or change the lambda sensor, and each attempt is £100+ down the drain from not reading the codes correctly.

    Disconnecting the MAF makes matters worse. When that is tried, the ECU recognises the signal is missing, and swaps the an Alpha N system, and guesses the expected airflow from the rpms and Throttle angle.
    It'll also set the system to be over rich, as a lean condition could go un-noticed and destroy the engine.

    Unplugging the MAF and using the default A.N can often make the problems go away, as it is quite an accurate system.
    This can give a "false positive" that the MAF was the cause as plugging it in makes the car run badly.

    Still, the airleak hasn't been fixed!

    If the system then gets diagnosed, it'll have a MAF disconnected , or often "MAF System- General" code recorded, and it'll look like the problem has been found, when it just recorded your attempts to narrow down the cause.

    The OBD is often clever enough to spot a leak, and may record the fault in perfect detail, but don't always expect it.

    In an exhaust system, a leak in a join will introduce air into the system. As the pulses of exhaust gas pass the leak, they will PULL IN fresh air, and the Lambda sensor will read high O2. The ECU will respond by adding more fuel in an attempt to fix it, but it won't work, as the fuel can't burn the air.

    Yet again, there is a lambda fault code registered, but no problem with the lambda sensor. Getting a new sensor is throwing money away again.


    Think like an ECU

    Always look at the full code reading and description, and think what the code means, and what can cause it. Not always is it the sensor's fault. It can also be something else that the sensor is reading, but doesn't make sense.
    Keep up to date with your codes, you may need to clear the fault memory and drive again to get a clean set of codes.
    The codes are recorded in non-volatile RAM, so they won't be cleared by disconnecting the battery. Old codes, or stuff created when trying to diag a system can muddy the waters and guide you up the wrong path.

    Always try to consider what other possibilities can cause the problem, and try swapping the sensors with known good ones before buying something new
     
     

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    Many thanks for sharing your knowledge via this thread

    MEMBERS - If you have found this useful, press the thanks button to show your appreciation.
    2013, Porsche 911 Carrera 4S, PDK, Sports Exhaust, Chrono, etc, etc

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    Very useful, thanks. Never realised how much I'd miss Vag-Com, lol.
     
     

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  8. Connection Issues 
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    One problem I have consistently come across is the inability to read all diagnostic code consistently from the under the hood round diag connector and the under dash connector. The under the hood connector often suffers from ingress of fluid from the power flush bottle which can coat and partially corrode the connector pins. This can be cleaned by using a good switch and connector cleani9ng product such s Servisol Switch Cleaner and spraying the sockets and pins in the cap and then replacing and removing the cap several times. Do not use any abrasives to clean the pins or terminals as you can encourage corrosion and make the pins loose in the connector.
     
     

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    Really handy guide mate, cheers
     
     

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    What A Great Post!!





    Thankyou 'TheEnd'
     
     

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    Nice thread, so true and sadly too many garages including BMW who should know better, just read the headline and go on a replacement spending spree, the MAP sensor on Derv's remain my favourite, throws up all sorts of faults and none of them say "MAP sensor" seen lots of money spent here replacing bits when a little squirt of electrical cleaner spray would have fixed the issue
     
     

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    So to expand this thread, particularly for those of us purchasing new models ('93 525i to 2004 530i):

    I've got a rough cranking issue, so as i just picked up my new ride yesterday, I'm researching code readers.
    I first looked at Bavarian Autosports, but then after googling the models to read reviews and do price checks, not only did I find other models much cheaper, i found a myriad of readers, usb readers, and numerous posts on the quality, etc.

    I'm looking at a USB Reader, and alot of folks ODB Pros was high quality, genuine chips, and great customer service.

    SO: for the forum pros: What's the most recommended code reader to buy?

    Price is a factor, so what's the bang for the buck for a good reader for a do-it-yourself owner?

    Standalones, as well as USB models???

    Thanks,

    Brett
     
     

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    Early cars, pretty much everything up to about 1995 (the OBD1 era) will be looking for an ADS connection.
    Carsoft can work on these in some cases, and an INPA kkl lead (ie, the cheap vag-com type) will get through in some cases, but it can be a bit hit and miss.

    The ADS interface will need a true serial port, and not a serial to USB converter.
    After 1996, the 328's etc, KKL leads work OK, and you can use USB leads.



    ADS- most chip-type ECUs to about 1995-ish, although sometimes later on some models.
    KKL- Some E36s, E38's onwards all should be fine on KKL, and these should work all the way through to about 2006 when CAN came in
    K+DCAN - this covers 96-ish onwards to present day


    1985----[ADS]--1996---[K-line]---2006---[CAN]---present
    __^----------ADS/KKL----------------^_____________
    _____________^--------------K+DCAN--------------^
    _____________^--------KKL------^


    The Peake tool is a standalone which should do the ADS/KKL range
    The ADS/KKL INPA lead has to be on a serial port, the KKL can be USB.

    For your range, the ADS interface is the best option, with INPA if you have a serial port-
    BMW INPA / Ediabas OBD & ADS Interface | eBay
    If that link is dead, search "ADS INPA"

    If you don't have a serial port, that the Peake tool is probably the next best thing to cover your range completely.
     
     

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    OK, thanks.

    Will the ADS interface and/or Peake tool also work with American vehicles as well as Toyaota?

    I own a GMC Yukon and Toyota Tacoma as well, so would prefer something that can work between all three.

    thanks again
     
     

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