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Yah the normally closed nature of relay 3 is a pain to deal with
I just realized why Mitsubishi reversed the operation of this relay to Normally Closed instead of Normally Open!
It is safety so if relay fails, it will bypass the 8V supply!!! And stay 12V full time!
SMART!!
Last edited by 2006EvoIXer; Feb 24, 2018 at 11:48 PM.
The contacts burn up due to the back emf of the pump motor and the spark when the contacts open.
The fix for that is to put a diode across the pump.
Are you taking about this?
Originally Posted by mrfred
This is the switching signal circuit. Its necessary to use terminal 85 for the positive terminal because some relays may have a flyback diode, and if 85 and 86 are reversed, then it creates a short circuit.
Last edited by 2006EvoIXer; Feb 25, 2018 at 06:33 AM.
2 different things but basically the same concept. Some Relays come with a flyback diode across the coil driver of the relay to protect the control circuitry from spikes when the path is blocked (I mentioned maybe using one to you). Well the fuel pump is a massive coil (inductive load) and when the relay breaks contact it causes a HUGE voltage spike that can arc across the contacts within the relay and damage it. Just like some relays come with the flyback diode you could add a beefy flyback diode across the pump. Im in no position to say if its necessary or not in our application, but most people, including the OEM's don't seem to bother and for the most part it seems to work out fine.
But on fuel pump, how severe is 8V since no one should be cutting power when pump is seeing 14V? Now if Relay #3 fails, then it will be 14V cut since it will be at full voltage at idle.
As scary as it sounds, spark inside fuel pump shouldn't be a problem since there isn't enough oxygen to cause explosion (thanks to volatility of gasoline, even at 15%).
Originally Posted by Biggiesacks
2 different things but basically the same concept. Some Relays come with a flyback diode across the coil driver of the relay to protect the control circuitry from spikes when the path is blocked (I mentioned maybe using one to you). Well the fuel pump is a massive coil (inductive load) and when the relay breaks contact it causes a HUGE voltage spike that can arc across the contacts within the relay and damage it. Just like some relays come with the flyback diode you could add a beefy flyback diode across the pump. Im in no position to say if its necessary or not in our application, but most people, including the OEM's don't seem to bother and for the most part it seems to work out fine.
every time that relay pulls open its cutting some power, and the arcing wouldn't happen in the pump it would happen inside the relay at the contacts. How severe is highly dependent on the variables of the circuit. Which is why I wouldn't even begin to speculate. As far as i know though its a non-problem in this application.
Last edited by Biggiesacks; Feb 25, 2018 at 10:13 AM.
Reason: edited the wrong post
Arcing in relay won't be of any concern other than the buildup. Thanks for clarifying.
Originally Posted by Biggiesacks
every time that relay pulls open its cutting some power, and the arcing wouldn't happen in the pump it would happen inside the relay at the contacts. How severe is highly dependent on the variables of the circuit. Which is why I wouldn't even begin to speculate. As far as i know though its a non-problem in this application.
So i went to wire this up today and discovered that things where not what they may have seemed, at least to me. When i tested the stock circuit it wasn't behaving like i expected, it seemed to be energizing the coil all the time. This couldn't be right i was thinking, perhaps i had tapped the wrong wire. I quadruple checked the diagrams and the pinouts and everything was right. Then i realized my mistake, after a closer look at the wiring diagram I found that the bypass relay is normally closed! This means the ECU energizes the coil to open the relay and force current through the resistor. This is the opposite behavior I had expected. Maybe your thinking just wire the bypass relay to be normally closed too, but the problem with that is the bypass relay is wired straight to the battery, which means if its normally closed then the fuel pump will get power all the time when the car isn't running. The simple and obvious thing to do is just add another relay to invert this behavior, but I'm gonna try see if there is something else I can do first. I'm open to suggestions if anyone has any.
ok, so if I understand this correctly, if pin 39 was used it would activate the relay and give full power to the pump when in low voltage mode, instead of high, so its backwards?
I'm just fitting a Link G4+, so an option is to drive the relay from another output from the ecu, but worth finding out if I could have used Pin 39. I'll just set it up on another Pin.
ok, so if I understand this correctly, if pin 39 was used it would activate the relay and give full power to the pump when in low voltage mode, instead of high, so its backwards?
I'm just fitting a Link G4+, so an option is to drive the relay from another output from the ecu, but worth finding out if I could have used Pin 39. I'll just set it up on another Pin.
Basically the ecu sends active low to enable low voltage operation of the pump. The default state is full voltage to the pump.
Whats the best way to splice into the wires at the pump?
I use wire cutters and nibble away at wire coating being careful not to cut any copper strands. Then I do the same with my extra wire and wrap the 2 together. Then solder the wires together. Then shrink wrap them.
Multiple shrink wraps since this is power and I don't want any short circuits near fuel tank.
Feb 24, 2018, 10:59 PM
