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Summary of change:
'''ECU needs power supply to operate... But how was the exact install ? If no supply after 20 seconds, how did that happen ?''' '''Warning''': this page is based on ''heresay'', because no wiring or install docs of any kind was made available (or made at all). May, or may not reflect the exact situation. However, might ring a bell, perhaps someone might have seen sg. similar or recommend best practices to make the install smoother (or simpler to understand, diagnose), less likely to cause surprise. ---- '''Inductive loads''' are best driven from * injector output * IGBT ignout * stepper P259 only if none of the above are available. Even fuel pump relay (except if driven from low current, logiclevel signal). ---- '''The main relay can be special, eg. if the driven load is powered (eg. main relay) even when EC36/23 (injector +supply) is not''' P259 outputs have added low voltage (SMB diode) flyback (inside VEMS ECU-s) since 2015 (v3.8 and v3.8b). This provides added current protection, but an added diode (external, in the harness, or internal, in the ECU on request) might be necessary '''if the driven load is powered (eg. main relay) even when EC36/23 (injector +supply) is not''' to prevent unintentional turnon, or "prelling". ---- '''Tricks to drive main relay''' Some installers configured (MISC1 or) MISC2 output to drive the main relay always (0..25500 RPM 0..510kPa, etc...) when ECU is powered up. This may, or may not be a good idea. We review the install/wiring plans of any such install. ---- An ECU was sent for inspection (without harness, or car), with symptom * if engine is started after ign on, it starts and runs * if engine not started soon after ign on, after about 20seconds ** EC36/15 pulls down, and fuel pump relay is activated, fuel pump turns on ** no ECU communication ** engine cannot be started ** now almost certain, that these are '''simply caused be ECU NOT getting supply voltage at this point''' The ECU worked perfectly, tested with +12V supply voltage on EC36/25 * ECU operations are never tested without supply voltage (because ECU needs supply voltage to work). Of course, without any (otherwise obligatory) install docs (or even just minimal wiring docs, still not provided, so reproducing the setup cannot be done, we can do some guesswork instead), it was not clear * why the ECU works perfectly (with supply voltage) when tested, * and not under some circumstances (in the field, without supply voltage, as it seems now). The same ECU worked perfectly when fuel pump relay was driven (as recommended) from ignition IGBT instead of p259. ---- Perhaps somehow the "self holding" MISC output can release (perhaps just for a quick moment) after about 20 seconds '''instead of being always active''' ? * 1.2.31 (or 1.2.38 ?) * was the main relay driven by injector output ? or IGBT ? Or p259 ? ---- [http://vems.hu/files/MembersPage/PoweredM/BMW_E30_M20_2.7L/ intended and actual config in ECU ar very different] ---- May, or may not be related to MembersPage/BmwMFiftyTwoMainRelay
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