inline fuse and measuring vpwr wire amps?

Hey guys, I am new to this site and hope to gain a lot from the great knowledge out there. Quick couple questions. 1)Right now I am looking to interrupt a vehicle power(VPWR) wire with a relay to add security to my car from theft. I am unable to find information online about how many amps the wire carries and thus I am unable to figure out how many amps the relay needs to be. Is anyone familiar with the VPWR wire on cars and how many amps are likely drawn from it? My understanding is the the VPWR wire carries battery voltage to different solenoids and relays and thus the amps should be quite low. I was hoping a 15 amp relay would do the trick. One idea I had was to just buy a PCM power relay for the vehicle and use that. The reason is because I was looking at a wiring schematic and it appears the PCM power relay sends power to the VPWR wire once the ignition is turned and if it is used to send power to the VPWR wire then buying the same relay should suffice correct? 2)Also I am just cutting the VPWR wire in one place and placing a relay in between. Since I am not running a power wire from the battery should I still use an inline fuse? I know when you wire something new and make a new connection directly from the battery you should use a fuse as close to the battery as possible, but since this wire is coming from the vehicles computer it maybe protected already. What's your suggestion on using an inline fuse and what amp rating should I use? Same rating as the relay I will use above or? Thanks in advance for any info, I really appreciate it.

Vehicle is a 1992 ford mustang

You are looking for a starter cut/vehicle immobiliser, in nearly 40 years I've never heard of a VPWR wire, are we talking about the main feed from the battery to the fuse/relay box/ If so in excess of 100 amps rating.

Hey thanks for the response. I am not looking for a starter cut, the vehicle can still be push started without a starter, so a starter cut really isn't protecting much with a manual car imo. I mean they will tow it if they want but I'd like to have some protection from the not so professional tow truck thief. I am looking at a schematic straight from ford and it shows VPWR wire from the computer. If you google it all that is mentioned is that it is vehicle power and carries the battery 12volts and it actuates injectors and the fuel pump and other solenoids/relays. If I am able to cut power from this wire then the car will not have power to the injectors or fuel pump and other critical components. From my research someone stated that it doesn't carry much load, but I am looking for a little more specific info on it. And to make matters worse it appears there are 2 VPWR wires in the car and from the schematic the wires are spliced together somewhere so I need to find out where the best place is to cut the wire. Oh as I mentioned in the first post the wire is receiving power from a relay when the ignition is turned on so I figured if I can find out the amp rating on the relay I should have an idea of how many amps the wire carries etc.

Another thought I have is just tapping my relay into the power wire to the coil for the computer's power relay. Do you guys think this would work? Then I would have control over the the computer's power relay. I could just purchase a new ECU power relay and use it to control the power to the ECU power relay, this way it's rated at the same amperage. It should be more safe right? I mean how amps can a wire carry when it is used to actuate a relay? Just putting my thoughts out there for some opinions. Thanks again

Just add a switch that cuts the relay's signal or ground.

Hey old spark, That's what I was talking about in my post above yours. Looking at the schematic it seems the signal wire for the ECU power relay is also my ignition wire. Is there a way to test amperage or should it be safe since it's a signal wire? I thought ignitions have really high voltage but some aftermarket ignition coils claim only 1 amp per 1000rpm. So maybe I will call MSD today and see if they have tech support with that information. Thanks again guys.

The relay signal wire just like the rest of the electronics on that vehicle with two or three exceptions* is ALL 12volts. It's the current used by the objects being powered up that varies. The trigger wire from the ignition is the one to cut. Since it's triggering a relay, current draw is about 1.5 amps. max. Except you shouldn't cut it because if any of your joints fail at speed.... I'm surprised Oldspark hasn't tried to put you right on your marked misuse of the terminology. On a car of that age, coil HT leads if a single coil will be about 12-15000 volts. Certain sensors and injector wires will pulse (showing as AC) at between 1.5 and 7 VAC.

MSD CDI? Irrelevant. The best anti-theft is shorting, not breaking or cutting. EG - for ignition systems, shorting the points or pickup or ignitor output to ground. Though in some cases - like if via a long or thin cable - it may not be enough to kill a high-revving engine, it virtually guarantees no engine start or low RPM etc.    It is also safer - a broken connection merely means the vehicle operates normally. Open or break systems are simple to defeat. Some idiots in the old days were dumb enough to mount the "braking relay" next to the thing they broke (eg, ignition coil). (Until then I thought those with VISIBLE break points were the stupid ones!). They are so easy to defeat. And for older vehicles, most ignition-break systems were defeated by the "standard" kit that thieves would have.   (Same as many starter inhibit systems today.)

Howie thanks for the info i will keep that in mind. Old spark - I'm a little new to this so please bare with me. When I mentioned msd I was going to call them to ask how much current is pulled from their single ignition coil and the tfi module. You mention shorting, braking and cutting. By shorting do you mean that I would interrupt the circuit on the ground side rather than power side? this way I eliminate the ground, instead of the power? and by braking or cutting do these terms mean that you would interrupt the power side rather than a ground side using a relay or switch? Basically at this point I believe my best action is to cut the power or ground to the coil for the ECU power relay. This way nothing receives power because the computer will not turn on. I think once I find out what shorting vs breaking/cutting is, I will have a better idea of whether or not stopping the ECU power relay will work well and whether to interrupt the ground or power side of the relay's actuating coil...conventionally pins 85 and 86 Thanks again guys I really appreciate your help.