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3B Motor fuses 27 & 28 blown

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  • 3B Motor fuses 27 & 28 blown

    Alright gents,

    My 3B S2 left me stranded today, thankfully not too far from home. Driving along merrily feeling for driveline vibrations (an ongoing issue) and the car cut out. Coasted downhill and pulled in a siding.

    Quick investigation revealed both motor fuses blown, replaced them both however the left one keeps blowing now when the ignition is turned on.

    The left one is fuse 27 i believe which is the ECU 12v feed, I've removed the ECU for inspection and not seeing anything untoward or any nasty smells of shorting/blown resistors etc.

    One thing to note is I have the glovebox and shelf under the steering wheel off as have been drying out my carpet (water ingress, another on going issue ) so there is a mass of wires loosely hanging down. I've had a quick look at these and not seeing anything obvious, certainly not the wires that were close to my throttle foot so goodness knows what's caused these fuses to blow.

    Also why would both fuses be blown on inspection but only the left one keep blowing now.

    Any help with this greatly appreciated, I hate electrical gremlins with a passion!!!

    Cheers lads

  • #2
    Sorry I should add that I converted to ABY ECU a number of years ago so that's possibly going to change what fuse 27 powers?


    • #3
      That's the 12v battery feed to the ecu. pin 18 of the ecu goes to fuse 27 via a red / white wire. fuse 27 goes to point L30 in the fuse box via a red wire. If you've disturbed that nest of wiring, it's possible the holder for fuse 27 has dropped down and is touching something below


      • #4
        Hi Mike, I've been trying to educate myself all afternoon on this issue. Going by previous issues people have had with this fuse blowing the holding relay diode between pin 18 & 19 was goosed so I've checked that with my multimeter.

        Not quite sure how to determine the readings, when I put my multimeter on the 2000k resistance range and connect to pin 18&19 on the ECU the numbers go up and off the scale, this is the same if I connect the leads in the opposite direction. Am I right in saying that is not a short as it would go to Zero if the diode was shorting? Not even sure how I'm testing this diode in situ is correct?

        I've disconnected MAF and injector plugs, however with the ECU connected it still blows fuse 27 straight away. With ignition off I can fit fuse 27 and it doesn't blow.

        What is point L30 in the fuse box? I'll check that out.

        Last edited by gmac78; 14 May 2021, 18:07.


        • #5
          off the scale means not a short. L30 is underneath - a torch will help - the wire will be red at that end. are you measuring for a short on the pins or the harness ? if you disconnect the ecu and the short is in the harness, your ecu is likely good. Make sure ignition is off when removing the ecu


          • #6
            I measured for a short on pins 18 &19 off the ecu with it disconnected, I'll need to check the harness side too. I'll wait until it get darker for an inspection under the dash, it will be easier to see with a torch then.


            • #7
              Also, given it is the ecu fuse, I'd be checking the engine bay harness for damage.


              • #8
                20210608_114158.jpg Alright gents, that's me back from working away so straight into trying to fix my ECU issue. I've taken som ​​ e pics of my ECU internals to see if someone can point out what component normally fails when fuse 27 blows.

                You will notice on the pics that I have a new map sensor fitted and R201 resistor mod, however that was a good few years back when this work was done and never had any issues. I've checked for a short to ground between the battery positive and fuse 27 and all checks out ok.
                Last edited by gmac78; 8 June 2021, 12:14.


                • #9
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                  • #10
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                    • #11
                      Uploading these photos did not go well!!!!!!


                      • #12
                        So I'm trying to figure out the correct way to test this over voltage diode ( D901 ) that has failed in other ECU's. Possibly even mine??

                        I've been trying the diode function on my multimeter and get no reading in either direction. However, when I try it on the 2000K resistance measurement, I get a figure that starts small and increases then goes out of range. Same happens in either direction. Does anyone know if that is a sign of a good or bad diode? Is it even the right way to this diode since it's already within a circuit?

                        I simply know nothing about electronics so keen to learn!!!


                        • #13
                          I just realised I asked a similar question a few weeks back so excuse me. Still, some further chat on this matter would be good if any electronic type chappies out there!


                          • #14
                            More questions gents, PRJ wrote on another thread-

                            If ECU is blowing fuse every time it's connected most likely the car has issues with the alternator and was overvolting the ECU.
                            The ECU IIRC at 18V collapses a diode and forces ground and power together, to prevent further damage to itself.

                            Measure the resistance on the ECU inputs between:
                            PIN 18 and PIN 24
                            PIN 27 and PIN 24

                            If you read 0 ohm on any of those, then your alternator's voltage regulator is shot, causing an overvoltage and the ECU frying the diode in itself.
                            The ECU then needs repair - the diode needs to be desoldered and replaced.

                            If you have resistance between those pins and they are not shorted together, then at least that part of the ECU is intact.


                            • #15
                              Does this mean every time the ecu is connected to it's wiring plug or every time the ignition is turned on? As mine only blows fuse 27 with the ignition on.

                              I've just measured resistance between the above pins on the ECU itself and get no reading between pin 24 & 27, but do get an increasing resistance across pin 18 & 24?

                              If my issue lies between pin 24 & 27, is this still the same over voltage diode that blows (D901)?

                              Last edited by gmac78; 9 June 2021, 09:52.