cj.surr Posted September 30, 2019 Report Share Posted September 30, 2019 I have a smaller dry cell battery that often drops to about 8v when the starter is first engaged. For that short moment (fraction of a second), the ECU will power off and lose connection to my laptop or dash. Is there a recommended size of capacitor that I could add to the +12v power going to the ECU to prevent it from shutting down during this scenario? I would assume a diode would also be needed to prevent the capacitor from discharging backwards into the system. Thanks Quote Link to comment Share on other sites More sharing options...
Richard Hill Posted September 30, 2019 Report Share Posted September 30, 2019 I don't think the ecu is powering off at 8 volts, more likely is that the comms chip is dropping out. I know this doesn't really help but maybe a smaller capacitor would be enough. Quote Link to comment Share on other sites More sharing options...
cj Posted September 30, 2019 Report Share Posted September 30, 2019 This can also happen if the grounding to the engine isnt quite up to size. Remember that in most setups the ECU grounds to the engine block or intake manifold. If there is an undersized or corroded ground anywhere between this contact point and battery negative it can cause the voltage at the ECU to drop even if the supply side is good. Quote Link to comment Share on other sites More sharing options...
Richard Hill Posted September 30, 2019 Report Share Posted September 30, 2019 Try ecu logging batt. Voltage, this will show you if just the comms is dropping out. Quote Link to comment Share on other sites More sharing options...
cj.surr Posted October 1, 2019 Author Report Share Posted October 1, 2019 I think the ECU itself is powering off. I did an ECU log and the log didn't show anything from before the engine starting. So I think the ECU shut down and lost that part of the log. Can't be ground in this case, my ECU and engine harness is grounded at the chassis. So, to size the capacitor, I'll assume the ECU is consuming less than 2A, since I'm not switching anything on the high side. Quote Link to comment Share on other sites More sharing options...
Adamw Posted October 1, 2019 Report Share Posted October 1, 2019 What ECU do you have? The wire-in ECU's dont drop out until below about 6.5V, the ADC's start getting inaccurate about 6.8V. Some of the plug-in ECU's wont go that low as the main relay control circuit will cut power to the ecu before then - in this case it could be fixed by a small change to the wiring. You dont want a cap or any other type of filtering on the ECU batt input as that measured voltage is what your deadtimes and dwell is based on. Quote Link to comment Share on other sites More sharing options...
cj.surr Posted October 1, 2019 Author Report Share Posted October 1, 2019 5 hours ago, Adamw said: What ECU do you have? The wire-in ECU's dont drop out until below about 6.5V, the ADC's start getting inaccurate about 6.8V. Some of the plug-in ECU's wont go that low as the main relay control circuit will cut power to the ecu before then - in this case it could be fixed by a small change to the wiring. You dont want a cap or any other type of filtering on the ECU batt input as that measured voltage is what your deadtimes and dwell is based on. That's a good point, I figured there would be a reason I didn't want a cap on the supply. It's an Xtreme wire-in. I didn't think battery voltage would get down to 6.5v, I'll have to check if there's any significant voltage drops in the starting system. It may be difficult because of how short of a duration it occurs in. I have the ECU and harness powered off the starter terminal, so it is subject to the voltage drop of my starter cable, which is fairly long. I may be better off running a separate power supply from the battery to the engine harness. Quote Link to comment Share on other sites More sharing options...
barge Posted October 1, 2019 Report Share Posted October 1, 2019 I had this same issue and pretty much asked the same question. The issue turned about to be my power wire routing which was set up just like you did. I wired from the trunk battery, to the starter, to the power distribution. I separated the feeds (you shouldn't need nearly as big a wire) over the winter when i did a bunch of wiring changes and it resolved any dropping issues during startup (I think I was also having starting issues in cold weather as well due to the same issue). FYI I'm running a tiny lithium battery in my car. Quote Link to comment Share on other sites More sharing options...
Richard Hill Posted October 1, 2019 Report Share Posted October 1, 2019 I have tried powering the ecu from the starter motor terminal, and never had any success. It has always resulted in a big voltage drop caused by the huge transient currents drawn by the starter. Running separate feeds for the ecu and fuel pump should minimise the voltage drops. The situation is often compounded by small, race batteries which in my experience only add to the difficulties. Quote Link to comment Share on other sites More sharing options...
Greg W Posted October 1, 2019 Report Share Posted October 1, 2019 We had an alternator fault on one of the Salt racebikes this year , loosing voltage over the whole run and ride back to the pits it shut off and log cutout at 6.8v at the ecu Quote Link to comment Share on other sites More sharing options...
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