Consequences of installing relays in various parts of 97 Civic
02-20-2006, 09:21 PM
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Consequences of installing relays in various parts of 97 Civic
I'm looking at building a hopefully more sophisticated/challenging immobilizer/security system for my 97 Civic.
The plan is that the ignition key will no longer act as a main switch connecting critical ignition wires. Rather, it will simply act as an input to a microprocessor. This microprocessor will then control relays on critical ignition electrical lines in the vehicle. Once the steering lock is disabled, this also raises the possibility of designing a number entry pad for the ignition as well, thus eliminating the need for any key at all. (Different pin numbers could mean different things - one might make the car run normally, another might make it normally but then mysteriously "die" - e.g. useful in a carjacking). Monitoring vehicle speed and waiting for it to hit zero would provide a good place for killing the engine. There's a number of different ideas I am toying with, including RFID and biometric identification (e.g. fingerprint scanning). (I am a CS major; implementing this stuff doesn't really phase me).
Well my question then is where do you *safely* install the relays so as not to cause engine damage, a dangerous situation, or a major accident while killing the engine (well you'd lose power steering I suppose but other than that I would expect the car to still be drivable). Obviously if you electrically emulate the behavior of the ignition key *exactly* and have it switch *everything* on or *everything off*, there should be no problem. But what happens when things get turned on and off individually? This would certainly happen if (1) relay fails, (2) thief starts screwing with electrical system and manages to turn on a relay or two.
After looking at the official electrical schematics, I was thinking of the following possible locations, and had some concerns if the relays should turn off while the car is running:
* Relay on distributor: What happens if the distributor loses power while the fuel pump / fuel injectors are powered? Is it just going to keep pumping and squirting fuel even though it's not burning? (thus probably causing damage?) This would normally be the issue only if your IGN COIL fuse blew while driving.
* There's a little box called the Main Relay, which based on the diagram seems to keep either the fuel pump / injectors both on, or both off (with a brief exception when cranking). Ideally I would like to put a relay on the fuel pump line after the main relay - this way you can hide it under the back seat. But then there arises another situation that normally wouldn't come up - what happens if the fuel pump is off while the injectors are on? Engine damage? Or does it just die from lack of fuel?
* Closely related to the previous point, if I installed a relay going to the fuel injectors, what happens if they are turned off while the car is on and the fuel pump is pumping? Car dies from lack of fuel, or engine damage? Again, not a chance this would normally happen unless your injectors outright broke while driving (shorts would blow the fuse and then shut off the fuel pump) or the main relay itself malfunctioned.
Certainly stuff could be placed before the main relay, thus electrically causing your relay to look like a blown fuse. But that would really limit my hiding spots for relays.
The plan is that the ignition key will no longer act as a main switch connecting critical ignition wires. Rather, it will simply act as an input to a microprocessor. This microprocessor will then control relays on critical ignition electrical lines in the vehicle. Once the steering lock is disabled, this also raises the possibility of designing a number entry pad for the ignition as well, thus eliminating the need for any key at all. (Different pin numbers could mean different things - one might make the car run normally, another might make it normally but then mysteriously "die" - e.g. useful in a carjacking). Monitoring vehicle speed and waiting for it to hit zero would provide a good place for killing the engine. There's a number of different ideas I am toying with, including RFID and biometric identification (e.g. fingerprint scanning). (I am a CS major; implementing this stuff doesn't really phase me).
Well my question then is where do you *safely* install the relays so as not to cause engine damage, a dangerous situation, or a major accident while killing the engine (well you'd lose power steering I suppose but other than that I would expect the car to still be drivable). Obviously if you electrically emulate the behavior of the ignition key *exactly* and have it switch *everything* on or *everything off*, there should be no problem. But what happens when things get turned on and off individually? This would certainly happen if (1) relay fails, (2) thief starts screwing with electrical system and manages to turn on a relay or two.
After looking at the official electrical schematics, I was thinking of the following possible locations, and had some concerns if the relays should turn off while the car is running:
* Relay on distributor: What happens if the distributor loses power while the fuel pump / fuel injectors are powered? Is it just going to keep pumping and squirting fuel even though it's not burning? (thus probably causing damage?) This would normally be the issue only if your IGN COIL fuse blew while driving.
* There's a little box called the Main Relay, which based on the diagram seems to keep either the fuel pump / injectors both on, or both off (with a brief exception when cranking). Ideally I would like to put a relay on the fuel pump line after the main relay - this way you can hide it under the back seat. But then there arises another situation that normally wouldn't come up - what happens if the fuel pump is off while the injectors are on? Engine damage? Or does it just die from lack of fuel?
* Closely related to the previous point, if I installed a relay going to the fuel injectors, what happens if they are turned off while the car is on and the fuel pump is pumping? Car dies from lack of fuel, or engine damage? Again, not a chance this would normally happen unless your injectors outright broke while driving (shorts would blow the fuse and then shut off the fuel pump) or the main relay itself malfunctioned.
Certainly stuff could be placed before the main relay, thus electrically causing your relay to look like a blown fuse. But that would really limit my hiding spots for relays.
02-20-2006, 10:46 PM
#2
Honda-Tech Member
Re: Consequences of installing relays in various parts of 97 Civic (PredatorCoder)
Can't immediately answer all your questions, but I'll try to give you some thoughts on the matter. If you cut off fuel suddenly to your engine, yes it will die, but you also run the risk of having it run too lean and cause damage.
Depending on how the ECU actually reads it, if you have no spark and continue to pump fuel into the engine, it may pull a reading that it's running super rich and lean the mixture out, or shut the pump off entirely at the main relay. Not sure, but you may want to check to see if that would actually happen. Too much gas will flood the engine, which isn't a huge deal, but if you keep on pumping it in, you'll run into troubles.
As far as I can tell, at least in an EG chassis, the main relay gives power to the ECU, then the fuel pump. From the research I did, apparently, if one of the signals is interrupted, it won't let the other work. I haven't verified this.
I suggest that if you do go through with the project that you rig a fail-safe into the system. If you can get it set up right, I'd suggest doing something that would make sure that if either gas or spark were absent, the other would be shut off immediately. This would probably be the best way to make sure you don't damage anything.
Depending on how the ECU actually reads it, if you have no spark and continue to pump fuel into the engine, it may pull a reading that it's running super rich and lean the mixture out, or shut the pump off entirely at the main relay. Not sure, but you may want to check to see if that would actually happen. Too much gas will flood the engine, which isn't a huge deal, but if you keep on pumping it in, you'll run into troubles.
As far as I can tell, at least in an EG chassis, the main relay gives power to the ECU, then the fuel pump. From the research I did, apparently, if one of the signals is interrupted, it won't let the other work. I haven't verified this.
I suggest that if you do go through with the project that you rig a fail-safe into the system. If you can get it set up right, I'd suggest doing something that would make sure that if either gas or spark were absent, the other would be shut off immediately. This would probably be the best way to make sure you don't damage anything.
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