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  • S2 3b fuse 27 blowing

    Evening all,

    My Dad and me are currently in the process of trying to bring a 1991 S2 3B engine back to life that has been off the road for a number of years.

    The main issue we have just discovered after replacing all the fuses in the fuse box is that fuse 27 (engine timing 1) blows as soon as you put the fuse in. We disconnected the ECU, replaced the fuse again and it does not blow.

    For info, we were told by the previous owner that it has a bigger turbo fitted and that the ECU has been chipped.

    Guessing we may need a new ECU or would you recommend we get it repaired somewhere?

    Any advice would be greatly appreciated.

    Thanks,

    Phil.

  • #2
    As I understand it S27 feeds a holding relay within the ECU which feeds power to the MAF and injectors during startup.

    Check the MAF wiring plug for shorts. Try disconnecting the MAF.
    Check the injector wiring for shorts.

    If all that checks out ok then maybe there’s a fault in the ECU but it would be unusual.
    Last edited by newsh; 21st November 2017, 21:55.

    S2 Coupe 3B Project


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    • #3

      S2 Coupe 3B Project


      Ur quattro restoration

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      Boost is the new rock and roll!
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      • #4

        S2 Coupe 3B Project


        Ur quattro restoration

        S2 Avant

        Boost is the new rock and roll!
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        • #5
          Thanks for the reply newsh,

          Will check this tomorrow and report back.

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          • #6
            Hi Newsh/anyone else interested,

            I disconnected the MAF- tried the fuse, still blows,

            Checked the injector harness for chaffing/shorts, nothing obvious found.

            So I removed the ECU and its clear someone has had it apart before. I removed the case and thought I would check for anything obvious.
            On the edge of the PCB where the ribbon cable connects on the other side of the board are the row of solder tails sticking up. I found a couple of them at the end were folded over and I think touching each other.
            I haven't tried the ECU back in the car yet but guess this could be the problem?


            ecu.jpg

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            • #7
              The only reason for someone to remove the bottom of the case would be to solder in a replacement map sensor on a 3b as all the ecu's I have seen have removable eproms (not soldered). As its apart you might as well seperate the boards and take some pics of the map sensor whch is usually a large black component unless its been replaced and the two eprom chips, one on each board. Might help support whether the ecu has been modified.
              I guess one or more stuck injectors might cause an issue. Obviously if you have removed the harness and it still blows the fuse then that can be eliminated!
              It is a pretty robust piece of electronics so without any obviously sign on a component of damage its difficult to make an educated guess.
              No harm in seperating the boards and perhaps taking a few shots and posting then in case anyone can see anything.

              Whoops! I thought I was looking at the bottom of the boards not the top! Looking at another ecu not all the pins are perfectly straight on either side of the ribbon cable.
              Last edited by twoqu; 24th November 2017, 15:04.

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              • #8
                If you are uncertain on how to seperate the boards this might help:
                http://www.quattro123.com/MPTSAudiECUupgrade2.htm

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                • #9
                  Is s27 fuse on one of the auxiliary fuse holders? If it is it’s worth checking it doesn’t short out anywhere under the fuse box.
                  Greg

                  S2Forum.com Administrator & Webmaster

                  '93 Coupe with a few tweeks

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                  • #10
                    Not sure if its of any use :
                    https://www.s2forum.com/forum/techni...onic-mysteries

                    I think I can recall a few failures like this but without any feedback about the cause. It does sound like it is being shorted somehow.

                    Here is one:
                    https://www.s2forum.com/forum/techni...-blown-fuse-27

                    Looks like some kind of over voltage protection I guess.
                    Last edited by twoqu; 25th November 2017, 14:49.

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                    • #11
                      Tried the ECU back in the car after straightening the solder tails to make sure none where shorting out on each other.- No change, fuse still blows straight away with the Ignition switch turned off.
                      I've checked under the fuse box for shorts but nothing obvious found.

                      Just to clarify my problem. With the Ignition switch off. Fuse 27 blows as soon as you try to replace it. If you disconnect the ECU it does not blow.

                      Any further suggestions would be greatly appreciated.

                      My guess is its an ECU internal fault/short as previously thought so I will separate the boards as others have suggested.

                      If anyone has a spare/ working ECU for sale please let me know.

                      Thanks for all the previous replies so far.

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                      • #12
                        Originally posted by pwilson View Post
                        Just to clarify my problem. With the Ignition switch off. Fuse 27 blows as soon as you try to replace it. If you disconnect the ECU it does not blow.
                        Any further suggestions would be greatly appreciated.
                        In a B3/B4 S2/RS2, S27 is the 5 amp fuse that is connected between the No. 30 constant (not switched) power and Pin T55/18 on the ECU. Pin T55/18 is the constant power to the ECU, the one that keeps the ECU memory hot. There is also switched power coming into the ECU via Pin T55/27 but that doesn't seem to be the issue.

                        Power from Pin T55/18 does flow to the holding relay and out to the MAF and injectors, as others have said so the short could be internal in the ECU (in general), in the holding relay in the ECU specifically, in the MAF and in the injectors. Since the MAF and injectors rarely short out and you know that the ECU has been chipped by unnamed perpetrators, you pretty much need to eliminate the ECU as the culprit by trying a virgin (or known to be good) 3B ECU in its place.







                        HOLDING RELAY INFO: http://forums.quattroworld.com/s4s6/msgs/102009.phtml



                        RS2'd 93 UrS4 5 spd sedan
                        94 UrS4 V8 6 spd manual avant

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                        • #13
                          Pin 19 on the ecu is a ground. With the ecu disconnected, check for a short on the wiring plug between pin 18 and 19. pin 17 goes to fuel injector #1
                          Mike

                          http://www.s2-audi.co.uk

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                          • #14
                            If you find a fix I have an ECU here that does the same! Lol

                            S2 Coupe 3B Project


                            Ur quattro restoration

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                            • #15
                              It does look like jump starting is a major contributory factor.
                              If that Paul N schematic is correct then there is a diode between Pin 18 (B12V+) and Pin 19 (Ground).
                              If you look at the pics in the link attached to Dave's post you can see the ecu on the right looks a bit charred in the holding relay area. So thats a good place to start looking!
                              http://forums.quattroworld.com/s4s6/msgs/102040.phtml

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