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  1. #1
    Saab Nut
    Join Date
    10 Apr 2017
    Location
    Denham, England
    Posts
    460
    Saab(s)
    2011 2.0 9-5 Aero
    Thumbs Up:   66

    Defect diagnosis

    Some time back I mentioned that it is entirely possible to diagnose many defects without the aid of any diagnostic tools except system knowledge. What I didn't say was that there will be times when the diagnostic tools will lead you in the wrong direction and others when they will let you believe that a fault doesn't exist.

    I am not saying that you shouldn't buy an OBDII* reader or a Tech II or other computer diagnostic tool. What I am saying is that you need to understand what the tool is telling you.

    A key component in making the best of a code reader or other tool is WIS http://saabworld.net/showthread.php?t=34679
    http://saabworld.net/showthread.php?t=33310
    which has troubleshooting instructions and details of what can cause the various codes. If you don't have a Windows computer you will probably be thinking that it isn't practical to download WIS but you don't need a particularly powerful machine so a used laptop will do very nicely (I have a copy on a 2005 Philips device with 2MB of RAM). Having a copy of WIS may well save you more than the cost of a computer.

    Onboard diagnostics has come a long way but it remains only as good as the system design allows it to be. Allow me a story from 1989.
    The Airbus A320 was one of the first aircraft with what is now called a central maintenance computer. Airbus called it the centralised fault data interface unit (CFDIU). One problem we soon noticed was that the air conditioning ram air inlet doors were failing. The CFDIU was interrogated and tests were performed which indicated that the actuator had failed. Great, we'll change the actuator (easier said than done) except that it didn't fix the fault. The problem was the computer that drove the actuator. The designer had decided that reading the feedback signal from the actuator was sufficient. It wasn't there were two reasons why there may be no feed back, a failed actuator or no output from the computer to move the actuator. The actuator was more reliable than the output transistors.

    What has that to do with my car then? Simple, if you are relying on feedback from a control to tell you there is a fault, think 9-5 air distribution, don't forget that the output to the motors and the linkage from the motor to the flap it moves can cause the same symptoms as a failed motor, with the same fault code!

    The other thing I mentioned is the possibility that the diagnostic tool may tell you that a fault doesn't exist. Aircraft maintenance engineers, the good ones at least, learn very quickly that talking to the crew is as important as interrogating the box. Many a car mechanic would do well to talk to the driver before starting work. I have had my car since 2017 and for much of that time the use of full right lock, in hot weather, caused the Service ESP message to be displayed. A garage with the appropriate tools failed to find the problem, more than once. Only by thinking outside the box was the problem finally found. The car was lifted, the front wheels removed, and the test equipment connected. With the engine running and the car in 1st gear, to get the wheels turning, the cables to the wheel speed sensors were manipulated! Bingo, the message appeared and the diagnostic tool displayed the failure. Both front wheel speed sensors were replaced. Later testing of the removed parts revealed broken wires on both sides.

    I hope this essay has served to put diagnostic tools in their place. They aren't infallible nor are they a substitute for knowledge and experience. You need both the tools and the knowledge of how to use them; coupled with the documentation produced by the manufacturer.

    *OBDII is On Board Diagnostics II (two).

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  2. #2
    Saab Nut
    Join Date
    10 Apr 2017
    Location
    Denham, England
    Posts
    460
    Saab(s)
    2011 2.0 9-5 Aero
    Thumbs Up:   66
    In the previous post I covered defect diagnosis using test equipment this post will give you some ideas as to how to diagnose defects without test equipment.

    Soon after I bought my current car in 2017 I discovered that in hot weather using full right lock would cause the Service ESP message to be displayed and the ABS and TCS lights to illuminate. It didn’t do it all the time and the Saab dealer’s test equipment showed a fault with a wheel speed sensor (they didn’t tell me which one) but it didn’t come back after being cleared.

    I lived with it until recently when it started doing it at any temperature. The first thing to realise is that the fault was connected with steering angle and ABS/TCS so reasonable to assume that it was connected with the front wheels. Also reasonable to assume that it was connected with ABS. So, knowing that ABS uses a sensor on each wheel and that there were no problems with ABS when driving normally I was able to determine that it probably wasn’t a reluctor issue because a broken reluctor ring is broken all the time, not just at full lock. This pointed me towards the sensor wiring.

    To try to eliminate a chafed wire I used spiral wrapping on the left wheel speed sensor wiring. Just because it only happened on full right lock didn’t mean it was the right sensor. What happened next was unexpected; I got the message and light simply by driving over speed bumps. Later I got the red Brake warning as well and the park brake stopped working. I rapidly removed the wrapping and the problem reverted to occurring just on full right lock.

    At this point I was fairly convinced that the problem was with both sensors but I took the car to an independent specialist who, listening to what I had done, raised the car and connected his computer, we manipulated the right front wheel speed sensor wiring and the message appeared. Defect isolated, both front wheel speed sensors, they were changed.

    If you are trying to diagnose a defect without recourse to onboard diagnostics the things to note from the above experience are:
    1. Know how the system is supposed to work, WIS can give a good idea of how things work so, again, I recommend getting a copy.
    2. Take the time to work out when the problem occurs, obviously if it is a hard fault that isn’t an issue but intermittent faults are the hard ones to find.
    3. Work in sequence; change one thing at a time, don’t just throw parts at the problem because you will never know which one fixed it. You will also end up paying more that way.
    4. This one may be controversial but I’ll say it anyway, use new parts if at all possible. I have seen reused parts fitted and then weeks of troubleshooting and other parts changed only to find that the reused part was the problem all along. Unless you can test a part from a scrap yard consider it suspect.
    5. Take advice from people who have had the same problem, there are some parts that are known to fail and cause problems. In most cases they are well documented, learn from other people’s mistakes, you don’t have time to make them all yourself.
    6. Use the documentation, WIS again, following the correct process to change the part is usually easier than trying to find your own way.
    7. You can’t have too much information.
    Finally, where WIS gives a torque figure for tightening fasteners, use it. Over tightening of fasteners causes many a problem and a good torque wrench is much cheaper than having to repeat the process of changing a broken part.

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  3. #3
    Saab Nut
    Join Date
    10 Apr 2017
    Location
    Denham, England
    Posts
    460
    Saab(s)
    2011 2.0 9-5 Aero
    Thumbs Up:   66
    After considering OBD II and some troubleshooting, let’s have a look at electrical faults. By this I mean, “It doesn’t work when I operate the switch”.

    If you have got to this third part of the thread you will know that I strongly recommend a copy of WIS the Saab Workshop Information System.

    At this point a note of caution: The car electrical system has enough energy to start a fire or cause considerable damage. If you aren't confident in your ability to work with the system live get a specialist in. There are some things you can do safely before doing that.

    The first thing to do when something electrical doesn’t work is to check whether you have met the conditions for it to work. Ignition on, doors closed etc. I spent a few months convinced that my headlamp washers weren’t working. When I had the right conditions they worked fine. Read the documentation, better to find out that you made a mistake than to rip the car apart first.

    If the conditions are right and it still doesn’t work, check whether other things that should also be working are in fact working. Saab uses a relay in parallel with the ignition switch on some models, if it fails so do a lot of other things.

    At this stage you haven’t needed any tools or test equipment. If you are confident enough you can move to the next stage but remember, stop if you aren't happy with what you are doing.

    To go any further a simple test lamp would be a good idea. Take a small (5W) 12V bulb and holder attach a wire to each contact of the holder and a crocodile clip to the other end of each wire. Connect one wire to a good earth (ground) and then use the other to check for power, if the lamp lights you have power. Most of the time this will be all you need.

    You can now use this test lamp to check whether there is power reaching the item that doesn’t work. If you have power you need to check whether the earth is OK, to do this connect one wire of the test lamp to the power supply and with the other touch the earth pin in the connector. If the lamp lights the earth is good.

    Having confirmed that both power and earth are available you will need to refer to the wiring diagram to see whether anything else is involved. If you aren’t comfortable reading a wiring diagram it might be a good idea to stop here. You may need/want to get a specialist involved but replacing the component might be all that is necessary.

    Bear in mind that this will work for a simple circuit but many circuits on a car aren't simple.

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    Last edited by GeoffR; 1 Week Ago at 10:06.

 

 

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