macambra Posted February 2, 2012 Share Posted February 2, 2012 So, I am building the wiring harness for my MS install and I think I have a discrepancy between injector current draw and what the megamanual calls out on the external wiring schematic. I have factory injectors(aprox 2.5 ohms), battery is 12V, and E=IR. So, 12/2.5= 4.8 amps per injector. Three injectors per leg-I ccalculate aprox 15 amp draw. The megamanual calls out for a 5 amp fuse per leg. Am I missing something here? I know that the frequency will absorb some of that due to duration, but I should see the peak current at some of the higher RPMs. I spent alot of time building this I don't want to blow it up on something stupid. FYI on my background: Automechanic 20+ years (career for 10,hobbie for the last ten), electrician 10 years. I took this on to learn about electronics. I never had to concern myself with this stuff working on OEM equipment. Quote Link to comment Share on other sites More sharing options...
Matt Cramer Posted February 2, 2012 Share Posted February 2, 2012 What MS variant were you planning to use with this? Low impedance injectors should not be driven full current full time, or they'll overheat. You either add current limiting resistors or hit them with a very short shot of full current and then switch to current limiting. Quote Link to comment Share on other sites More sharing options...
mobythevan Posted February 2, 2012 Share Posted February 2, 2012 Depending on your install you may have three options with low impedance injectors: Use a large wattage low ohm resistor like 6ohm 25 watt in series with each injector to limit current Use PWM (pulse width modulation) feature in megasquirt to limit current Or purchase a peak and hold driver board/box that goes between megasquirt and the injectors to drive them correctly. This is what Matt was referring to that hits them with a large current and then backs off. Quote Link to comment Share on other sites More sharing options...
macambra Posted February 3, 2012 Author Share Posted February 3, 2012 (edited) I have built it per the MS2 extra specs. What is the recommended setting for the stock injectors? The screen shot here is the default. I haven't adjusted anything yet. Edited February 3, 2012 by macambra Quote Link to comment Share on other sites More sharing options...
kaito Posted February 3, 2012 Share Posted February 3, 2012 Im sorry man but you have not read the megamanual enough. It is definitely in there. There is also a video on DiyAutoTune.com that goes through setting up a MS2 for a fuel only application that touches on the subject. I believe its on their nova install article. Dont forget the extra manual too. You are probably not running extra code but the extra manual in my opinion has more of the "meaty info". Good luck. Quote Link to comment Share on other sites More sharing options...
FricFrac Posted February 7, 2012 Share Posted February 7, 2012 So, I am building the wiring harness for my MS install and I think I have a discrepancy between injector current draw and what the megamanual calls out on the external wiring schematic. I have factory injectors(aprox 2.5 ohms), battery is 12V, and E=IR. So, 12/2.5= 4.8 amps per injector. Three injectors per leg-I ccalculate aprox 15 amp draw. The megamanual calls out for a 5 amp fuse per leg. Am I missing something here? I know that the frequency will absorb some of that due to duration, but I should see the peak current at some of the higher RPMs. I spent alot of time building this I don't want to blow it up on something stupid. FYI on my background: Automechanic 20+ years (career for 10,hobbie for the last ten), electrician 10 years. I took this on to learn about electronics. I never had to concern myself with this stuff working on OEM equipment. Factory injectors are low impedance or 2.5ohms. IIRC the '81 has a current limiting resistor of around 11 ohms (?). If that's the case then you are drawing roughly 1 amp per injector so with three injectors you'll pull 3 amps (ish) per bank. With the MS you can run PWM and you don't need to run a resistor in series Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted March 29, 2012 Share Posted March 29, 2012 (edited) Just to be perfectly clear, with 6 factory injectors (or any low impedance injector), the MS can drive 3 in each bank directly without current limiting, then apply PWM to keep them on. I'd rather not wire resistors in series with low impedance injectors. MS-3 without the expansion. Edited March 29, 2012 by SleeperZ Quote Link to comment Share on other sites More sharing options...
FricFrac Posted March 30, 2012 Share Posted March 30, 2012 Just to be perfectly clear, with 6 factory injectors (or any low impedance injector), the MS can drive 3 in each bank directly without current limiting, then apply PWM to keep them on. I'd rather not wire resistors in series with low impedance injectors. MS-3 without the expansion. Yes that's what I said - use a resistor or use PWM with low impedance injectors.... Quote Link to comment Share on other sites More sharing options...
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