rj Posted July 6, 2019 Share Posted July 6, 2019 Link to comment Share on other sites More sharing options...
revilla Posted July 6, 2019 Share Posted July 6, 2019 OK the latest installment in this epic tale of one man failing to find something with an oscilloscope ...So if an effort to work out whether it was the voltage falling on the ramps or the positive back emf spikes causing the issue I clamped a hefty diode across from the ECU supply line to the positive battery in such a way as to clamp the spikes off at about +0.7V but do nothing to the negative ramps.It still misfired.So to check my diode clamp was working I hooked up the scope to the main relay switched supply and it was going mental! Quite why I hadn't seen this on previous looks I have no idea, maybe I wasn't looking at this particular line whilst actually inducing a misfire, or maybe it's just because I was using a much longer timebase here and the scope was previously triggering on the good bits (most likely), but anyway here is what it was doing. This is at 400ms per division so you can see a few seconds of running with the engine cutting in and out:So on this long timebase the "fuzzy" bits a volt or two below the battery voltage are the compressed view of the ramping waveform I was seeing before. The bits where it jumps up to the full battery voltage are the engine cutouts, so during the cutouts everything shuts down, no ignition, no injection, no fuel pump, so virtually no voltage drop. But there are big negative transients spiking down to basically 0V for very short periods. And if you look carefully, every single engine cutout immediately follows one of these fast supply dropouts. So it's quite clear that these are the cause.So it's not inductive coupling of ignition back emfs, its not the resistance leading to voltage drops, there's something that is causing the supply voltage to spike down to zero suddenly above a certain RPM.Loose connection vibrating somewhere?Odd that it does come on so suddenly at a particular RPM but unless something is actively crowbarring the supply I can't think of much else. At least that's really clear and obvious to see now I know what I'm looking for so I should be able to trace it back around the loom to see where the supply goes from being reliable (at the battery) to unreliable (at the engine loom).At least now I know both which wire is the issue and what the issue is on that wire. Just got to find the source now. Link to comment Share on other sites More sharing options...
revilla Posted July 6, 2019 Share Posted July 6, 2019 Update ...So it seemed to me that the most likely place to find a loose connection that would vibrate with the engine would be the main starter motor terminal.As the car has a master switch, everything that would normally be taken off the battery +ve is taken off this terminal to put it downstream of the master switch.I checked the nut and it was secure. But a bit of careful wiggling with my fingers and damn sure I've found a fractured cable!There's one big wire that is attached to a large ring terminal which bends sharply backwards in exit. The joint between the wire and the ring termimal is covered in heat shrink, but I can feel the wire moving relative to the terminal through the heatshrink. And the wire end feels rigid. I think it's only the heatshrink holding the two ends together.My guess ... That ring terminal is not crimped on but soldered, and the solder has wicked into the strands of the cable where it makes the sharp turn and made it rigid and brittle, and it's then fractured with vibration. And now at a certain frequency it resonates and vibrates enough to intermittently break the connection, cutting the ECU supply.I need to get it up in axle.stands and take all those wires off for a proper look but I'm fairly sure one of those cables is going to need remaking.After all the fancy theories it's going to come down to a good old broken wire. It's always something simple in the end. When you find it! Link to comment Share on other sites More sharing options...
revilla Posted July 6, 2019 Share Posted July 6, 2019 Took all those wires off for a look. The suspect wire wasn't soldered, it was crimped ... sort of! The terminal was rattling around loose on the end of the two cables inserted into it, it was only the heat shrink holding it together. The terminal just fell off as I cut off the heat shrink and the ends of the wires were greasy and dirty.A continuity check with a multimeter also confirmed that one of these wires was indeed the feed back to the ECU.So I cleaned everything up and crimped on two new, separate ring terminals and sealed them up with adhesive-lines heat shrink.One of the other ring terminals on that post was soldered so I cut that off and put a decent sealed crimp ring terminal on that too.Everything around the starter terminals rewired.Too late to rev it up tonight, I've tested that it still starts so far but I'm really hopeful that was the last issue and it will run fine tomorrow morning. It certainly can't have been helping and fully explains the symptoms.Poor connection held in contact by heat shrink.At certain RPM the vibration in the wire resonated, vibration caused brief failures of the connection, seen as the brief spikes to 0V on the waveform.These cause very brief supply interruptions to the ECU supply, which cause it to reset, seen as periods of little or no current draw on the waveform immediately following a visible supply interruption.ECU resets result in a brief period with no ignition pulses followed by an IACV recycle.If it doesn't work in the morning I'm going to scream! Link to comment Share on other sites More sharing options...
John Vine Posted July 6, 2019 Share Posted July 6, 2019 Well done, Monsieur Andrew "Maigret" Revill! A very fine piece of detective work.JV Link to comment Share on other sites More sharing options...
ECR Posted July 7, 2019 Share Posted July 7, 2019 A very addictive thread....ill wait till the final scene where the real culprit is unmasked before offering premature congratulations ( but I’m sure you’re right) Link to comment Share on other sites More sharing options...
Area Representative Piers300 Posted July 7, 2019 Area Representative Share Posted July 7, 2019 Very well done Andrew. Piers Link to comment Share on other sites More sharing options...
revilla Posted July 7, 2019 Share Posted July 7, 2019 Yes! The patient is free to go home. Link to comment Share on other sites More sharing options...
SM25T Posted July 7, 2019 Share Posted July 7, 2019 Top marks for perseverance ! Link to comment Share on other sites More sharing options...
Ian Mackenzie Posted July 7, 2019 Share Posted July 7, 2019 Excellent.... Link to comment Share on other sites More sharing options...
ECR Posted July 7, 2019 Share Posted July 7, 2019 Superb job Andrew, really superb ..... Link to comment Share on other sites More sharing options...
chris whitlow Posted July 7, 2019 Author Share Posted July 7, 2019 Fantastic job Andrew! Thankyou for all your time and effort, much appreciated. Link to comment Share on other sites More sharing options...
revilla Posted July 7, 2019 Share Posted July 7, 2019 Chris, you're welcome. I wasn't going to let it beat me! Link to comment Share on other sites More sharing options...
grumpy the 7th Posted July 30, 2019 Share Posted July 30, 2019 Excellent work! "That's a full ten pint fault" as we used to say at work after a "ba$t**d" of a fault.But then again, we used to say ....................... "It's not worth getting thee' co-at off for less than a gallon"Well done that man. Link to comment Share on other sites More sharing options...
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