alarm wiring: engine running detection

Technical Area Electrics & ICE
2

After trying the trial and error way, and poring over the wiring diagrams for a while, I’m close to giving up, but perhaps someone here can help.

I’d like to find a wire in the car that is live when the engine is running, but not when it’s cranking/being started.  Does anyone know where I can get a signal for this?  It’s for a remote start alarm, so it can know when it’s successfully started the engine.  At the moment, I’ve just tied the line to a 12v point, so it tries to start the car for just a brief moment and then assumes that it’s worked.  This is fine for now, but in the winter when starting may take a longer turn of the key, it’s just not going to work.

Any advice?

3

Which version of the car do you have. Most radio power connections have such a feed.

4

ah, yes, I forgot to mention the car, that may help a little!

It’s a Mk1 / /NA 1993 Unos Roadster 1.6l

Generally, the connections to a car radio will be a permanent live, a switched live from the ignition, and a wire that’s live when the radio is in use, to trigger the power antenna, none of those will do it.

5

On mine I thought the radio turned off for cranking its common practice. Sorry good luck.

6

Oh, it does.  Things like the radio are powered in both the On and Accessory position of the key, when you move the key past On to start the engine, the radio power is dropped.  But you know when to let go of the key when you hear the engine fire, and so you let go of the key and power goes back to the radio, if you were to keep the key turned past when the engine had started, the radio would not turn on.

7

Ah do you mean you are trying to design a remote start system that remotley operates the starter and then drops the starter solenoid out when the engine starts?. I would put a circuit on the tacho signal that detected the rpm rising above a given level, (I am guessing 500 to 800 rpm would do). the best way would be a small micro but I guess you could make an analogue filter circuit. One lazy way would be to monitor the alternator/battery light as it is on for non-engine running and drops out when the engine rotates fast enough.  I would worry that you could get a failure to start then, so you may need to also set a minimum crank time. Oil pressure could also be used.

Check the ECU wiring to see if it has an engine running output.

 

8

Yeah, that’s the kind of thing, only I’m not designing it, it’s a retail unit.

There’s a wire on the alarm that the instructions say should be connected to a wire that’s at 12v when the engine is running.  It works like this:

When the alarm is told to start the car, it closes three relays, one of which is connected to the accessory position of the ignition switch, to power the car, the second is on the On, or Run position of the switch to power the engine management unit, fuel pump etc, and the third is connected to the non-latching position of the ignition switch, which powers the starter motor.

It will send power to the starter motor for three seconds and then stop.  If it senses 12v on it’s “enging running” wire, then it will stop sooner than the 3 seconds, and wont try again (because it worked, the engine is running) If it does not detect the 12v, then it will pause for a moment and then try again. 

So if you leave this wire disconnected, it’s a whole mess, the alarm will start the car, then keep trying to start the car to the sounds of a starter motor grinding - ask me how I know :slight_smile:

The safe way is to connect it to a permanant live feed so it thinks the engine is always runing, so it wont cause any damage.

And of course the right way, is to fit it properly!

I did consider the connection that drives the rev counter, because that shows nothing when the engine is stopped or cranking, but moves to 600-800 rpm when the engine ir running, but I assumed that this is an analogue device, and is recieving parhaps just a few volts for a low speed display, and closer to the 12v when at redline; but that’s just a guess.  

Anything above ground may be enought for the alarm, I’d test it but I couldnt find a good place to get at that signal but I may try removing the instrument cluster and poking around with a multimeter.

I think I have a printout of the ECU connections somewhere, I’ll go get a coffee and have a good look at it.

9

I think the rev counter receives pulses so I do not think you can use it as you suggest. Was the remote start designed for your car? Also the MX5 runs very rich when cold (well my MKI does in its attempts to light the cat) and I would worry about excess petrol washing the bore if you leave it idling to warm up. and in the very cold ice and snow, its handling is pants, believe me I tried it Big Smile

 So to be honest, I would just start it when I am ready to go.

10

I expect you’re right about the pulsed signal for the rev counter.

I’m not too worried about damaging the car with the remote start because it wont get too much use anyway; it’s very much a ‘because I can’ type thing.  I really only needed the keyless entry feature of the alarm, and buying an alarm over a keyless entry system cost just a few pounds more, and the remote start is just an added feature.  It’s nice to show off with every now and then - plus I got the elevtronic boot release setup with it too!