+12V or Ground source from DME while running?
10-19-2014, 11:48 PM
#1
Race Car
Thread Starter
+12V or Ground source from DME while running?
May be a long shot. I've eliminated my fusebox, and have minimal wiring. I have the fuel pump wired into the same switch that turns on the DME. I'd like to have it cut off in the event of a stall for safety purposes. I had thought pin 20, which controls the ground for the fuel pump side of the DME relay, would do it, but it is always giving ground when the switch is on, regardless of if engine is running or not.
Anyone know of a source? Could be positive or ground, but I'd like it to only work when teh car is running.
Anyone know of a source? Could be positive or ground, but I'd like it to only work when teh car is running.
10-20-2014, 12:28 AM
#2
Rainman
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howabout a relay triggered by the ign. coil?
tap one of the wires on the coil to activate a relay whose power poles go to the fuel pump.
switched power, if the DME doesnt see the engine running then it doesnt make a spark and that would make it not run fuel.
although the coil is pulsed it seems a consistent enough 12v source to at least run a MAF off of.
tap one of the wires on the coil to activate a relay whose power poles go to the fuel pump.
switched power, if the DME doesnt see the engine running then it doesnt make a spark and that would make it not run fuel.
although the coil is pulsed it seems a consistent enough 12v source to at least run a MAF off of.
10-20-2014, 08:05 AM
#4
Race Car
Thread Starter
Spencer, the coil is always hot when the key (in my case, switch) is on, so that would give me the exact same results (and I actually have the coil tied into the same source as the fuel pump anyway). So that won't accomplish what I'm trying to do.
Joshua, the wire to 85b is the one I referred to, goes to pin 20 of the DME. It gives ground whenever the key is on.
Maybe if I phrase it like this - how is the fuel pump turned off in the case of a stall? What triggers what (presumably relay) to cut power to the fuel pump (presumably the ground is cut......but is the whole DME, and if so, what signal triggers what to do that)?
Joshua, the wire to 85b is the one I referred to, goes to pin 20 of the DME. It gives ground whenever the key is on.
Maybe if I phrase it like this - how is the fuel pump turned off in the case of a stall? What triggers what (presumably relay) to cut power to the fuel pump (presumably the ground is cut......but is the whole DME, and if so, what signal triggers what to do that)?
10-20-2014, 12:29 PM
#5
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On a stock system the fuel pump is shut off if the DME senses that the rpm has dropped below 200 RPM. I'd guess that the speed and reference sensors tell the DME what the RPM is.
10-20-2014, 02:19 PM
#6
Race Car
Thread Starter
Ah, okay, so I need to intentionally stall the car to verify this. I had assumed the ground would break unless the engine were running, since the fuel pump only runs for a couple of seconds at first key-up (I'd assumed that 2 second strategy was in the DME as well). Will try that, again, and if it works, will add the relay in-line as planned.
10-20-2014, 04:00 PM
#7
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King, it sounds like there is something amiss...
When first key-on, there should be no fuel pump priming. The fuel pump turns on when the key is in the 'cranking' position (physical switched operation). When RPMs are >=160, the DME turns on the pump via pin 20.
If RPMs drop below 160, the DME turns off the pump. (assuming the key isn't in cranking position)
When first key-on, there should be no fuel pump priming. The fuel pump turns on when the key is in the 'cranking' position (physical switched operation). When RPMs are >=160, the DME turns on the pump via pin 20.
If RPMs drop below 160, the DME turns off the pump. (assuming the key isn't in cranking position)
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10-20-2014, 04:28 PM
#8
Rennlist Member
You can go to summit or other racing supply place and get fuel pump cut off switches that kill power when you lose oil pressure (like when engine dies) or when you have a hard impact. They're cheap too and designed for safety...
10-20-2014, 05:10 PM
#9
Race Car
Thread Starter
King, it sounds like there is something amiss...
When first key-on, there should be no fuel pump priming. The fuel pump turns on when the key is in the 'cranking' position (physical switched operation). When RPMs are >=160, the DME turns on the pump via pin 20.
If RPMs drop below 160, the DME turns off the pump. (assuming the key isn't in cranking position)
When first key-on, there should be no fuel pump priming. The fuel pump turns on when the key is in the 'cranking' position (physical switched operation). When RPMs are >=160, the DME turns on the pump via pin 20.
If RPMs drop below 160, the DME turns off the pump. (assuming the key isn't in cranking position)
I initially tried to use the original DME relay. but I couldn't get it to be clean, since there aren't any sockets for a 9 pin relay out there, so I isolated the two.
Anyway, now when I key up, pin 20 grounds. KOEO (Key On Engine Off). FWIW, I'm just using a test light hooked to the battery.
Tom....thanks for heads-up. Will check into this if I can't figure out what is going on with the pin 20 ground. I had thought I may wire in an intetia switch, but I don't think it would be as safe given how rough some gators, and even tracks (Sebring) can be.
10-20-2014, 11:08 PM
#12
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