CLightning

...in addition to a car alarm and an Autolock on my clutch, I decided to up the security by adding a hidden kill switch for the fuel pump on my 93 SC300. I made the splice on the solid green power wire somewhere between fuel pump ecu and the fuel pump.

Before I test this switch, I wanted to ask if there would be any adverse effects (damage) to the car if I attempt to:

1. Start the car with the kill switch engaged (no power to the fuel pump)?

2. Engaging the kill switch (cut power to the pump) while the engine is running?

Thought I'd check with you guys before I screw something up. Thanks.

Mike

gerrb

iTrader: (34)

go for it ... none to all your questions

CLightning

The kill switch /relay worked like a charm thank you! Highly recommended for those who need extra security.

Mike

KahnBB6

iTrader: (5)

^^ I've been wondering about this myself. Great bit of info!

Ali SC3

iTrader: (10)

I don't think its the best idea in the world to place a regular switch inline on the power side of a circuit that has a current draw. it will work most of the time but its not textbook in a direct current circuit.

Solution is to introduce a relay into the system and put the switch on the ground side of the relay.

for example you could wire 12V input from the battery to the relay, and then output power from the relay to the fuel pump. this keeps the safety features of the fuel pump ecu and gives you 12V all the time, like the 12v fuel mod with a relay.

the difference is in the activating side of the relay, you wire up the stock fuel pump ecu wire (tells pump to turn on and off) to the + side, and then the other side would normally just connect to ground, but if you want to have a kill switch you insert your switch between the relay and ground. this way when you operate the switch, the relay is going to internally disconnect or connect the circuit, which is what it is designed to do, so there is less potential for switching currents, etc.. and if anything fails it would be an easy to replace relay and not your fuel pump ecu.

looks like this but if you have a fuel pump ecu the left side would be from the fuel pump ecu and not the ignition.
Keep the fuel pump ecu in the loop if its still working as it has safety cutoff features.

CLightning

Ali SC3, this is my current setup which is similar to your diagram (I think). Please excuse my novicity in regards to relays, but I'm still having a hard time understanding how placing the switch between the relay(85) and the ground compared to placing a switch between the relay(86) and a +12V ignition source would make any difference? Thanks.

Mike

ronsinsc

Excellent do-it-yourself above. The only thing I would add is don't run the pump dry the fuel that runs through the pump is used to both cool and lubricate the pump at least that's the way it used to be in the old days.

Ali SC3

iTrader: (10)

Hey Mike, your diagram may look similar but its wired in non conventional way.
Relays take some getting used to so I can help you work through it.

The way you have it connected now, the fuel pump is supplied power by the fuel pump ecu (9v low and 12v high speed operation) when the kill switch is on. Also your kill switch is on the power side.

The reason to put the kill switch on the ground side of the relay, is that its isolated by the relay then and some "switches" introduce alot of noise into an electrical system when they open and close like for example a rocker switch, sort of depends on the design and quality. the speed of the switch opening and closing and other factors affects how much they mess with the electrical system. You can guess that there are lots of sensitive components on your ignition line, and when you flip that switch you are affecting them essentially. When you put the switch on the ground side, you are letting the relay do its job by switching the power off much better than a rocker switch can.
Also if you had it on the plus side, and it somehow managed to ever touch a ground which in a switch is a possibility if it touches something or where its mounted, you would short the entire ignition system connected like that and blow the fuses at the least. if you grounded out a switch on the ground side, well the fuel pump would just turn on.

The other change I mentioned is that the fuel pump ecu is what you use to turn the relay on with pin 86, in your diagram you have the ignition power doing it, which means your relay is on as long as the car is on and the kill switch isn't on. since the fuel pump ecu is still supplying the pump with power when the relay is on, you keep the safety features of the fuel pump ecu even though you have it wired oddly. its odd because the relay is energized but the power source is not always live (fuel pump ecu only outputs power when it sees engine rpm's). since you generally want the relay on when the power is going to go through it, its not the conventional way to do it.

now lets take it another step, you want 12V all the time so you remove the fuel pump ecu line and connect a battery line. now the fuel pump is on whenever the ignition is on and the rocker is on and you just lost the ecu safety function = not good. the answer is to just use the fuel pump power wire for 86 instead of the ignition power, and then no matter what you use to supply the fuel pump power, you have all the safety functions of the fuel ecu (cause fuel ecu controls relay now), and you place the cut off on the ground side.

lets say you don't want 12V all the time, you can just use the fuel pump ecu wire to power the fuel pump, and power the relay on 86. you put the rcker switch on the ground side and this allow you to turn it on and off, and you get normal fuel ecu to fuel pump.

so to summarize:
fuel pump ecu wire on 86
ground with inline kill switch on 85
30 is the power source, you can use the fuel pump ecu wire again or if you want 12V all the time run a proper gauge cable from the battery.
87 goes directly to the fuel pump.

That is how I would wire it anyways. also you dont want a kill switch bypass on the plus side for the same reasons mentioned above (unless you only operte it when power is off and have it mounted in a place the terminals can never touch ground). the kill switch does the exact same function when wired right (turn pump on or off), the bypass switch is redundant and risky as there is no ground side on the power side of the circuit to do the bypassing on. you only want/need the 1 kill switch IMO but that is enough info to get you started. look around on the net lots of diagrams for switch on the ground side of the relay as it is the proper way to hook it up.

CLightning

Ali SC3, Thanks for your input!

So with this configuration quoted above, using 87 (as opposed to using 87A), the relay is energized in order for the fuel pump to get power?

Would connecting the fuel pump to 87A instead (like in your first diagram) be a better alternative? ...where the fuel pump can receive power when the relay is not energized and when energized, it cuts power to the fuel pump? I'm assuming this may increase the life of the relay since it will never be energized unless the thief turns on the ignition with the kill switch engaged (relay ground is not cut by the switch).

I can use a small gauge wire (18awg) for Ground (terminal 85) like I did with the (+) trigger on terminal 86 correct?

Mike

Ali SC3

iTrader: (10)

I didn't notice it was 87a in that diagram I just pulled it off the net I did not make it.

If you use 87a, then the relay would be off when the fuel pump is on, yes you are correct.
if you use 87, then the relay would be on when the fuel pump is on.

either way the rocker will turn the pump on and off (just flips the on and off position), just one way the relay is energizes one way it is not.

there is a disadvantage to using 87a, in that to retain the safety features of the fuel pump ecu the fuel pump ecu output must go to pin 30 then to retain the safety features. if you run 12V battery to pin 30 (12v fuel mod w/relay) in the 87a arrangement, you will loose the fuel pump safety features even if you connect fuel ecu to 86.

Relays last a long time so I wouldn't worry about it going out anytime soon and just do energized with the 12 volt mod, but if you are not doing the 12V mod then you could use 87a and still be safe, but if you are doing the 12v mod, use 87 and run the fuel ecu into pin 86.

To me it makes sense that the relay should be on when the fuel pump is on but it would work the other way.
In your stock cars wiring all the relays are on when their circuits are on. some relays don't even have pin 87a

Only reason I emphasize this is because many members on here tend to do the quick and dirty 12v fuel mod which basically removes all the safety features of their fuel ecu. These tend to be the same people adding alot more power to the car and are arguably at more risk of needing those safety features. I like to retain the fuel pump shutoff features and when using a relay there is no reason not to, just have to hook it up right. There is a good chance in a bad wreck that you may not be able to reach the kill switch or pull the key out of the ignition before it is already too late, its not like abs where you might be better off without it on a track, a fuel pump ecu is always good to have or a standalone that can control it is also fine.