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Potential solutions to why your AAN-powered UrS isn't starting

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  • Potential solutions to why your AAN-powered UrS isn't starting

    Okay, your AAN-powered UrS won't start. Not to panic. There is a solution. However, to get there, you will need to go through some diagnostics/checks to help pin-point the problem.

    This post is based on a thread started by Paul Nugent (S2Central.net) on this forum. Paul's post was S2/RS2 specific, particularly on the electrics.

    Since "my car won't start" posts are common on my AAN-based home forum (quattroworld), I thought that I would build on Paul's post, using most of his text, and adding the specific AAN-related info that Paul didn't include. I have also added hyperlinks to more info on the potential source(s) of the problem. Once I got finished with that (in Kawf html format), I converted it to vBulletin format for the Audiworld UrS forum and once that was done, it was an easy cut and paste to bring it full circle back to the S2Forum. Aimed primary at the AAN-powered UrS4 and UrS6s but the device info links will work well for the ABY and ADU S2/RS2 vehicles. Hopefully this will help somebody here.

    Prologue: Sudden failures / no start situations will almost certainly be due to an electrical fault, but there is always a chance that something like a slipped timing belt or whatever can cause such problems. The G6 Fuel pump can also fail with little or no noticeable warning. Same with the G40 Cam Sensors. More below:

    Take a deep breath. Here we go:

    1. Immobilizers and Alarms: If you have any doubts about the cars security and/or aftermarket fitments then seek out problems in those areas first. They can be wired in all sorts of wonderful (and stupid) ways to interfere with the starter, fuel pump, coil or ECU wiring.

    2. Fuse Checks: Sounds stupid, but check all the fuses and robustness of 12V straps around the battery, alternator, starter and ignition switch. Check ground straps too. It could just be power not getting to the starter. Check the fuses that power the ECU and the injectors Battery info

    3. Starter System: If the car does not turn over (aka "crank") when you turn the key - then suspects are the ignition switch, the starter motor or security (alarm system). A starter motor fault is commonly the solenoid that activates the main feed. If you hear it clicking, then the solenoid is trying to engage the starter. The battery needs to supply something like 120-200 Amps during cold cranking so this is quite a test for old batteries. A weak battery will not start an AAN (see 4.3 below). It can also be a bad connection on the starter.

    Audipete suggests that "Bosch decided to not put an insulated wire from the solenoid to the starter motor on many of their starters. In your picture it's covered with a white sleeve. Often its just a bare copper braid and folks assume it's ground - when in fact its the direct opposite -it carries the current to the starter motor field winding. Often that braid will corrode and even disintegrate - and I've had it happen between starts, with a tell tale cloud of acrid smoke...twice my wife has witnessed this, seen and smelled the smoke and said "replace the starter". the other tell tale sign of a failure here is that when you turn the key, the voltmeter drops to 8 volts and nothing happens - no click, nothing."



    4. - Starter System Good: If the car spins on the starter, i.e. it cranks, but still doesn't start, then you need to find out which one (or more) of the five main things is improper cam/crank timing, airflow signal, fuel, spark and/or compression.

    4.1 - Cam Timing: Check that the alignment marks on the crankshaft pulley (or flywheel via the bell housing window) and the camshaft pulley are perfectly aligned as shown in this Cam Timing post. On an early (pre-mid Feb. 92) AAN with 'tuna can' dizzy - check that the rotor arm (or sensor) alignment is also visually correct. Hand crank the car to be certain that everything moves as it should.... Remember two crank revolutions to one camshaft rotation.

    4.2 - Airflow signal: Crazy as it might sound, if the engine is cranking but not starting, a simple check is to disconnect the G70 MAF and try again. A bad MAF shouldn't stop an engine from starting but there anecdotal stories of just that: MAF connected = no start, MAF disconnected = start. In one such case, it was a leaf stuck in the MAF (so many reasons why that should not have happened - but it had and the MAF problem was the no start problem).

    4.3 - a) Fuel - Power at the injectors: Check also that 12V is present at one side of the fuel injectors (in the black wire with the red stripe) while the ignition is on - this tells you the ECU is correctly passing power thru an internal Holding relay and that the injector fuse is OK. If not, check the fuses above the ECU

    4.3 - b) Fuel - Fuel At the Injectors: The ECU only activates the fuel pump via the J17 Fuel Pump Relay when it thinks the engine is spinning faster than ~13rpm based on a signal from the G28 Engine Speed Sensor. - This is primarily a safety feature that will shut off the fuel pump in a serious accident. Unfortunately, this safety feature also means that a weak battery that is unable to spin the engine during starting to more than 13 RPM will result in a non-start. Charging or replacing the battery can be the quick and dirty solution.

    To ascertain if the fuel rail is receiving fuel you need to split the fuel line at where it enters or leaves the fuel rail. You can follow this fuel pump testing (and replacement) procedure Expect some spillage when working here and do take ALL sensible precaution not to set anything (you, your car, the forest) on fire. You likely need an assistant to try and start the car while monitoring the fuel supply. If you see fuel, then the ECU is definitely powered up and definitely seeing the RPM signal from the G28 sensor. Also it verifies that the fuel pump and relay are active. If you wish to test the fuel supply rate then do so safely with a measured container and operate the pump for 15 seconds - you should get well in excess of ~300cc of fuel in that time. One way to TEMPORARILY force fuel pump on is to remove the cover of the relay and close the contacts with insulating tape... refit the cover and plug it back in. This is for TESTS ONLY for obvious reasons. (This was Paul's suggestion, not mine). More fuel pump info HERE

    4.4 - Spark: The ECU only activates spark when it gets good signals from both the G4 crank position sensor and the G40 camshaft position sensor at the expected times. A no spark condition is most commonly caused by a fault G40 sensor - sometime pouring cold water on it will get you home (but that is just putting off the inevitable need to replace the G40 before it totally dies on you). The G4 crank sensor is much more robust but it is worth checking that the G4 reference pin on the flywheel is still intact - can be seen or felt (carefully) through the bell housing window. Next source of failure is likely to be in relation to wiring faults. A common one on ABY/AAN is a bad ground from coil packs to the head and/or intake manifold.

    4.5 - Compression Testing: Remove the Fuel pump fuse (Fuse 17) and disconnect the injectors from their harness connection. With a compression gauge screwed into each cylinder in turn - spinning the starter with throttle pedal fully depressed - do this for about 4 or 5 rotations and record the cylinder pressure. They should all be approximately the same and all well above ~10 Bar (145 psi). More compression testing info HERE .

    Refit spark plugs (after checking them physically) and tighten no more than 22 lb-ft (30 Nm). Then reinstall the FP fuse and the injector connectors.

    5. - ECU Power If the car has 'no fuel' or 'no spark' faults then its wise to check the ECU has good 12V connections and red holder fuse above the ECU is good. If you can retrieve fault codes via OBD 1 (blink codes) then it is a good sign obviously, but it doesn't tell you everything.

    6 - Final Checks: Faults in MAF, throttle pot and every other sensor on the engine should not cause a no start condition... If the engine is as follows -

    - correctly timed mechanically
    - makes good compression (broadly verifies condition of rings, valves and gaskets)
    - turns over on the starter faster than ~25rpm (weak batteries may not permit this)
    - ECU powered up, earthed and fused correctly (more to come on this one)
    - Injectors are receiving nominal 12V supply (key ON)
    - Coil or coilpacks are receiving nominal 12V supply (key ON)
    - G4, G28 and G40 are wired and working properly
    - The fuel pump is good (NOTE - It does NOT prime when you turn the key on)
    - There is good fuel in the tank (not 5yr old watered down filth)
    - The fuel pump relay is good
    - No aftermarket security wiring or hidden switches are messing things up
    - Then it simply WILL start... it might run like crap if other sensors are missing

    Thanks again to Paul Nugent for the majority of the text. I have added and/or rearranged bits to better suit the AAN and likelihoods (e.g. I moved compression testing down the order and added the MAF to the list)

    REFERENCE: Paul N's original post
    Last edited by UrS4boy; 30 January 2013, 19:45.
    RS2'd 93 UrS4 5 spd sedan
    94 UrS4 V8 6 spd manual avant

  • #2
    Please feel free to suggest comments or changes. I don't consider this to be a static document. Paul was very throrugh and my additions help a bit more. Not to say that there aren't other things, including a better order of doing the checks.
    RS2'd 93 UrS4 5 spd sedan
    94 UrS4 V8 6 spd manual avant

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    • #3
      Bump

      Mods, this should be a sticky

      S2 Coupe 3B Project


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      • #4
        Originally posted by newsh View Post
        Bump

        Mods, this should be a sticky
        Bttt

        S2 Coupe 3B Project


        Ur quattro restoration

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        • #5
          Added as a sticky newsh
          Greg

          S2Forum.com Administrator & Webmaster

          '93 Coupe with a few tweeks

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