using a reed switch to control ecu output

I want to use a magnetic reed switch hidden under the carpet to activate the ECU's Ignition Output signal. While the reed switch is at rest (open), I want to interrupt the ECU's output signal to the ignition coil so the car cranks but does not start. As soon as I activate the reed switch with it's magnet, I want to complete the ECU's ignition output signal which is around 10v.

Using your relay diagrams, I used the following configuration to test the reed switch funcionality, controlling a 12v LED off of the relay's 12v output on 87:

Since my application does not need a 12v output I'm wondering if I could interrupt the ECU ignition output directly by placing the reed switch inline with this output which will see 10 volts when active. Otherwise, how do I wire the relay for my application?

 

 

What do you mean by ECU's ignition signal? If it's a timing signal (to the IgCoil's ignitor), then it will be pulsing - hence maybe looking like a 10V average. And it should be a low current signal suitable for a reed relay. Not that reed relays are necessarily reliable, but as long as it doesn't fail coming into a corner and you can stop and manually bypass the reed... If it is a high-current path, then you you will need a relay.

You are much better off interrupting the starter wire. With your proposed setup if the magnet breaks contact the car will stall. If you interrupt the starter wire the only issue you'll have if the magnet breaks contact is the car won't start up.Kevin Pierson

I don't want to interrupt the starter for 3 reasons:

1) Starter interrupts are usually part of an existing security installation (car alarm) and I want to add an additional layer of security

2) Starter interrupts are common, usually installed near the steering wheel assembly and easily bypassed

3) I can interrupt the ECU's ignition output in a stealth location within the car's wiring harness.

Yesterday I spliced the reed switch directly on the ECU Ignition output wire and it worked really well. I'll have to do a permanent install into my car for long-time testing and also to see if extending this ECU signal wires has any negative affects on the ouput.

Splicing the reed into the output worked but I'm interested in seeing this implemented with a relay - can someone shed some light?

oldspark wrote: If it is a high-current path, then you you will need a relay.

I tested the reed switch directly on the output an it worked really well so I'm assuming it is of the low current type. Can you explain how I would wire in a relay for this application?

As per your original diagram, though with mods to switch the signal rather than +12V. But since it's a signal (under ~0.5-1A for a reed switch) and preumably pulsing, the reed is ok.

As a mechanic, I would have to say this a bad idea. Messing with wires leading to/from the ECU has the potential to cook it. ECU's can cost thousands of dollars. Why would you risk messing with it? As KPierson stated, it would be better to interrupt the starter wire.

Hmmm - I don't disagree... However IMO the ECU can be safer. And not that I know your electrical skills, but I have found many mechanics to be woeful. VIZ the qualified mechanic that could not even test whether the 12V fuel pumps on a mate's car were working - all that was needed was a 12V battery and 2 wires! In fact over a period of 2 weeks, that mechanic managed to get me to spend 2 hours extracting fuel pumps from MY car (for nothing!), waste a lot of my verbal time (2 hours), and destroy the ECU. What was the fault? Bad connections from the +12V battery terminal that I had forewarned the owner about 3 months earlier. Sorry - I get distracted. But even a recent proud mechanic used a 1N4004 diode to keep his second battery charged from his first. The again, he probably still thinks that the torque-horsepower conversion formula is the same for foot-pounds & horse-power as it is for Nm & kW!     All DIY has its risks, and sometimes DIY can be more expensive than going to a professional. (VAG fuel relays anyone?) And this OP seems to be "not incompetent" and somewhat familiar with electrickery and its issues. But that is sometimes why I suggest the grounding of ground-switched inputs (or outputs) as that is reasonably risk free - PROVIDED it is know it is ground switch and not active or hot! However, cooking an ECU is a valid and important warning. I just hope other DIYers would do as I do - get another (matched ECU) from the wreckers for $35, or ~$100 max.

X 3 with KP and Oldie here, if you must run to the ECU try the ignition feed. As Oldspark knows I also ALWAYS feed via ground (0v) it's safer as is breaking the starter wire, that's why it's the recommended method in the US and by the Association of Chief Police Officers here although Germany insists on 2 breaks and Australia 3. As KP and riouxk correctly pointed out ANYTHING that could conceivably break power whilst the vehicle is in a RUN STATE is inherently dangerous and failure prone, corroded ground, bad connection or solder joint, even (about (16 million to one against) relay failure.