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.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

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.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

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.

Your on to the right idea interrupting either of the coil feeds, except they are both probably in bus bars in the fusebox. Is there a fuse marked Engine Control anywhere? That's the one to go for, except in practical terms cutting any ignition/run (as against starter) is DANGEROUS.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

madmanuser - thanks for asking for more. Being new to this is no problem, but I have troubles assessing poster's knowledge, and others get cheesed off else run off the road after falling asleep with my long replies...


Forget cutting - at least in terms of power etc.
Placing a switch in a solid link is an extra point of failure. Wire falls off, connectors or switch corrode or fail... Bingo - no go!
Guaranteed that happens as on a level crossing in front of that freight train, or driving the loved one to hospital... in fact - anywhere!
Even if a mere relay-ground switch (#85 to ground), compare - with simple consideration only - a #85 connector to wire to ground connector to ground to a #85 connector to wire to switch connector thru switch contacts out switch connector to wire to ground connector to ground.
Which has the greater risk of failure, and by how much?

My suggestion to switch (interrupt) a relay-coil's ground was merely in preference to interrupting the relay's much higher power output or source. (Removing its fuse is ok though, but that could wear the fuse contacts...)
Besides, isn't stopping a relay from energising the same as interrupting its power? Carry a spare (or the original) #85 to ground cable and easily restore a failed switch circuit.
(Yeah, ok, smart stealer merely jumpers the #30 - #87 contacts, but they have to know what is wrong AND which relay - and it won't be clicking to assist them!)


But to disable ignition...
Typical ignitions involve grounding a hot ignition coil and breaking that ground for the spark. (+12V from IGN +12V to coil+. The coil- is grounded to charge the coil, then broken/opened to get the spark across its secondary winding.)
Maybe see https://en.wikipedia.org/wiki/Ignition_system but... why is it I can never find the diagrams I am looking for???!!!... I think these diagrams are better...

From
www.oldmarineengine.com/discus/messages/2/95648.html but resized:

....which shows the grounded IgCoil and its +12V supply, and the points/contacts that open and close to ground.

And from www.allaboutcircuits.com/worksheets/specialq.html:

... is the equivalent but shows how the +12V coil supply is IGN switched, and the points are buffered by a transistor.
That transistor could be ant transistorised ignition - eh, HEI etc (but not CDI = Capacitive Discharge Ignition!).

In BOTH cases, the coil- is interrupted to produce spark.
If it is NOT interrupted, then no spark.
So if the points or transistor is short-circuited to ground, no spark.   
How? Simple! A hard wire from coil- to ground. Short or fat enough to handle coil current (typically 5-10A).
Add a switch to break or open that short, and the system functions normally.
That is merely an addition to existing wiring, and can be easier to hide. There is no cutting of wires.
And if the wire to the switch shorts, or the switch fails, just cut or disconnect that wire.

The catch? The coil has continual current through it. Eventually it may overheat and be damaged or fail.
But old coils usually had enough thermal inertia to be okay for 10 minutes or more.
Newer coils (E-types etc) might have reasonable thermal inertia.
(Author's note to self: Suggest to readers to carry a spare coil - just in case...?)

FYI - electronic ignitions usually have a means of cutting power after a certain time to prevent coils melting.

But that's only a problem if the thief leaves the IGN +12V on...
But the if the coil melts, they can't drive the vehicle can they?
(Where did I put that reminder note?)


BUT if it's a CDI system, that's totally different. You can probably short the pickup (points; else reluctor, optical or Hall-effect sensors) but not the coil-.
In a CDI, the CDI switches high-voltage DC to the coil (typically 300V - 450V) and shorting that can damage the CDI capacitors or circuit. (CDIs use the IgCoil as an "impulse" transformer; the coil does not charge as with points and transistorised ignitions. Hence CDIs can be used to 30,000 RPM etc!)


That shorting principle (to ground) can apply to many vehicle sensors. That's because many are "open collector" outputs. That's just like points - either they are grounded, else they are open and floating. It's a common signaling arrangement for various reasons - in particular between systems at different supply voltages. Computer chips use it inside PCs.
But that for on-off sensors only - like grounding oil pressure switches, handbrakes, most door switches etc.
Find the right sensor(s) which - if disabled - prevent the vehicle form starting or running, and they may be other or additional options. Diode-OR those sensors to a single grounding switch to   disable all those inputs. (Poor thief - jumper new ignitor or points/pickup wires - still no go. Jumper the airflap - still no go. Hey - there's another car....

Sorry for the Ignitions-101 ramble.

Luckily Stargate Universe is on the TV so I'll save you from more....

And they've shut down Stargate Universe halfway through the current season load of #@".
Will this car have a separate coil?
I've not really done any coil - (31) grounding since the early 70s.
You might also simply ground the sensor side of the camshaft position sensor, if the vehicle has an engine management system. i.e. 12V+ ignition, ground and trigger; ground the trigger.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Howie - when you say it is dangerous to cut ignition, is this because of the possibility of accidentally cutting it while driving and the possibility of losing power while on the road? Or a connection failing and then cutting power while on the road?

Also, sorry, but I'm not sure what you mean by separate coil. The car has only one ignition coil at the moment. It is stock ignition.

This is what the coil looks like, but it's a stock one not msd

https://www.americanmuscle.com/msd-tficoil-8695.html

Oldspark - WOW! good info haha. definitely learned something new. I don't know how I could hard wire my coil, it has a stock plastic connector that the wires are in and that connector snaps onto the coil I am not sure what kind of ignition i have <---nooob! I know I have a tfi coil and a tfi module on the distributor.

I found this schematic for a similar year mustang. it is correct for my car so I have been using it. take a look

https://brembs.net/cars/maf_conversion/mustang_wiring.jpg

Either way I found this remote controlled relay and I wanted to use it to control the EEC power relay. Also I have my dash out so I can hide the relay somewhere good!

I don't know how comfortable I am locating sensors and shorting them.

Hey! do you think I could short the power wire to the coil for the EEC power relay? If I understood you correctly the current or power would go towards the ground and the relay would not receive any power. Or would it just make sense to use a relay to control the power for the coil to the EEC power relay?

Thanks again. I feel I am getting closer to figuring this out and another step closer to taking my baby out on the road!

YES

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Two wires on that coil. When you disconnect that plug and turn on the ignition, one reads 12 volts+, the other nothing. If you ground the "nothing" wire with a secret switch, (safer than the R/Controlled relay), the car simply won't start.
But to be honest, someone like me would find it in 2 seconds.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Howie - so if I'm willing to take the risk of the car shutting down on the road by wiring a relay to the EEC power relay wouldn't this be more difficult for someone to find as compared to grounding the ignition like you mentioned above?

The EEC power relay is tucked deep in the kick panel and by disabling the eec power relay wouldn't it be hard for a thief to figure out why nothing is getting power when they turn attempt to turn the vehicle on? Also without ripping the dash out again I doubt anyone would ever find the relay. I see the entire wiring harness and I can find a way to make sure that the wires and relay are hiding extremely deep behind the dash

We are running around in circles. If you must immobilise it's safer to ground rather than cut. Second, the first thing I look for with a "mystery" breakdown apart from the obvious fuel supply is engine bay power.
A single grounding wire if properly loomed in will be MUCH harder to find.
Third from your questions and lack of theoretical knowledge, don't do this.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

x2. Think about it - vibration, heat, faifique, installation and meterial quality.... Your car stops anywhere anytime anyspeed anyrain anybend anyrailwaycrossing any freeway entrance etc etc etc.

Usually "cut" or "active supply" systems are set up with redundancy - ie, two switches, two relays, 2 parallel sets if wiring. But that is useless unless monitoring is available to alert the failure of one of the supplies (useless - other than perhaps maybe halving the chance of failure).
Hence the short/bypass system is used in preference....


But yes - circles x2 too.


PS - I did points-shorts in the 00s (that's 2000s - not 1900s!) but that was on ignitor circuits and some transistorised (HEI) which is the same thing - a connection else not to ground to the coil-.

Ok sorry for the circles. I re-read the entire thread and have a better understanding of what you recommend. I see why you recommend it and have decided even though I don't have as great a knowledge of electrical circuits that I will continue. Hopefully you guys can help me do this right. Again I want to thank you for your patience and help.

Two quick (maybe not so quick :-D) question sorry!

Now I have checked the coil and located the wire which does NOT receive +12v when the ignition is on.(I will call this wire "ign coil ground wire")

I don't want the thief to find my grounding wire. Does it matter where on this ignition coil ground wire I wire in my switch? Does it have to be very close to the coil?

I noticed that the wire goes to my EEC and my tfi module as well. My research on my vehicles ignition system indicates that the tfi module is the device which switches ground for the coil allowing spark to be sent to the distributor. It may make a huge difference where I wire in my switch.

Old spark you said my switch should be "hardwired" into the ign coil ground wire. Does this mean it has to be attached at the connector that connects to my ign coil? Or can I tap into the ign coil ground wire and splice/solder in my wire at any point on the ign coil ground wire?

My preference would be to tap into the ign coil ground wire near my EEC inside the car.

My other question is if someone happens to know I have grounded/shorted out my ign wire, would they have to find the switch wired into it to start the car or can they just wire my ign coil straight to my tfi module?

Thanks again

The answer to your first question is yes, you can make the connection anywhere on that wire.
No they wouldn't have to find the switch, simply disconnect the grounding wire.
Make sure you SOLDER, TAPE over and STEALTH the connection.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Sorry just to make sure I follow you, as long as my wired switch and the wire for the switch are hidden they cannot start the car correct?

Or are you stating that all they need to do is disconnect the ign coil ground wire?

PS The wire runs from the EEC to a resistor to my tfi module and also to my coil. Does the resistor affect anything?

Opps, I can't edit my post so I must add a reply.

I just want to make sure it's not easy to bypass my switched grounding wire. If all they have to do is easily ghetto rig a wire and run the car then this may not be a good option for me.

Howie-you mention that you could find the wire easily or get around this type of anti theft device?

The whole point is that you're an amateur and a newbie, I've been doing this kind of work since 1972 and NO after market immobiliser has held me off more than the 3-5 mins. to strip and find, the 2 mins. to rewire and another 2 mins. to put the car together.
An alarm with remote control from a reputable manufacturer is a much better bet

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

But I don't understand how you could find it if its tucked deep into the dash and only actuated using a remote control switch connected to a relay.   

Unless you are bypassing it. In that case what is needed to bypass this type of anti theft device if you cannot located the switch and grounding wire?

And I understand I'm new to this but in the time you spent telling me that I shouldn't do this we could have been more productive. The front page of 12volt.com says the site is here for "do it yourself" technical assistance and tips and tricks of the trade.

Apologies, I didn't consider the remote control relay, since that eliminates the switch.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Howard beats me on both this sort of expertise (like, he blows me away!). And he has 10% extra years too.

ITO the coil's -ve circuit, yes - all that needs doing is disconnect that; disconnect the other end (points or ignitor); and join those 2 with a new wire.
There are some provisos though. The ignitor isn't always in the distributor - you the may have to disconnect & bridge the dizzy to ignitor (which could also be ground shorted) AS WELL AS the IgCoil to ignitor as before.
Or the dizzy timing may be fixed and the EMS provides the timing (advance/retard etc) which then feeds the ignitor. Usually the dizzy is timed so that it will be good enough for starting and low RPM, but it could be way off that so that dizzy to ignitor (bypassing the EMS) is useless.

Whether switch or relay doesn't change the method of defeat, but a switch could be easier to find.
Then again, a relay with remote requires the remote, its batteries etc. And it is more complex hence - in simple terms - more prone to failure.


But I recall a simple yet effective thief deterrent a (555) timer that - unless disabled - would turn on every (say) 20 seconds for (say) 10 seconds and energise a points-shorting relay.
Any thief would probably think it's an engine fault rather than an anti-theft device. IE - start and run for 10-20 secs, then cutout and not start nor run for 10 secs, then run again until it cut out.
Bad luck if the got it as far as the freeway or railway crossing though.

And there is always the 125dB internal sirens etc. You enter the vehicle (or house) and get a 5 or 10 sec warning peep, then 10 seconds later if not turned off - a deafening LOUD scream....