Mi16 Runs On When Battery Disconnected?

[Stu 18]

As above, im not sure if this is in the wrong section, but i couldnt find an 'electrical' room.

 

Soo.. As part of my track day prep ive fitted an FIA 'cut-off' switch to disconnect the battery should the worst happen. Ive cut the +ive battery lug off, took this lead into the back of the switch and then returned to the battery.

 

But.. when the car is running, and the switch turned to the off position nothing happens, the car continues to run. But if the ignition is off, and the switch is off, everything is dead.

 

Ive come to the conclusion that the alternator must be powering the car when the battery is disconnected, does anyone elses car do this?

 

The starting/charging circuit is as per 19/Mi, i have no other wiring gremlins, other than an occasional dim glow from the battery light on the dash. So im wondering if a diode has gone south in the alternator.

 

Anybody got any ideas?

[Tom Fenton 1,526]

You need to wire the alternator output through the auxiallary contacts in the master switch. There is usually a wiring diagram to show you how to do this and a resistor to stop the alternator burning itself out.

 

Dim battery light is usually a bad connection on the spade onto the alternator.

[jackherer 543]

I used to have a dim glowing battery light, then one day the ring connector on the thick wire on the alternator totally snapped off and left me stranded. When I crimped a new ring connector on it all worked again and I didn't have a dim battery light so in my case the ring connector had been breaking down for ages, I posted a thread about it which is worth looking for, I was pretty worried by it as the loose wire could have easily set the car on fire and I had left it in a dark country lane in the middle of the night when it broke down

[pug_ham 242 3 Cars]

Shouldn't the FIA cut off switch have a lead from the battery & then one out to the starter / alternator & shunt box rather than back to the battery? (might just be how you worded it & this is how you have it).

 

Graham.

[M@tt 77]

we had this problem with 309Pug track car when we did the wiring, sounds like exactly the same problem we had.

 

As Tom has said before you also need to break the alternator to battery circuit as well or it won't stop

[XLR8 0]

You could simply wire the alternator/starter motor +ve's direct to the battery (it already has a thick red wire) , then wire the other circuits (the other wires going to the +ve terminal) through the kill switch & to the battery.

That way once the kill switch is turned all power to the ecu & all circuits is removed, this way removes the need for an alternator over-run resistor.

Edited January 9, 2008 by XLR8

[Stu 18]

You could simply wire the alternator/starter motor +ve's direct to the battery (it already has a thick red wire) , then wire the other circuits (the other wires going to the +ve terminal) through the kill switch & to the battery.

That way once the kill switch is turned all power to the ecu & all circuits is removed, this way removes the need for an alternator over-run resistor.

 

Thats a good idea, if i remember correctly there are two leads into the +ive battery terminal, with any luck one will be the alternator/starter feed and the other the feed for the shunt box, so putting the kill switch in this line will still kill the engine and associated circuits with loosing the feed to the alternator and starter..

 

Top man! your Blue Peter badge is in the post...

[XLR8 0]

Thats a good idea, if i remember correctly there are two leads into the +ive battery terminal, with any luck one will be the alternator/starter feed and the other the feed for the shunt box, so putting the kill switch in this line will still kill the engine and associated circuits with loosing the feed to the alternator and starter..

 

Top man! your Blue Peter badge is in the post...

 

 

Your very correct about the 2 leads going into the positive terminal...

No problem, glad to help - Ive built a few rally cars. Currently got a fully prepped 205 gti (albeit on twin 45's) so if you need any advice let me know :-)

[Stu 18]

Nice one buddy, i'll keep that in mind..

[Sandy 189]

If the Alternator and starter leads aren't cut by the master switch, then you can't isolate all the circuits; which is the point of a "Master" switch. If you have a shunt and one of the leads to the Alternator for example becomes shorted, the switch can't isolate it, the battery explodes/catches fire and you suddenly wish you'd done it properly!

It's perfectly simple, all you have to do is take a wire from the positive circuit through the side switch that makes when the main switch is off, through the resistor to earth and have the main power to the coil or ECU through the side that makes when the switch is on. Sometimes the power to the Alternator warning light can back feed into the main circuit and keep the car running (it doepends how the dash is configured), in which case, fit a separate alternator light with a diode in the 12v side to block the back feed.

[Stu 18]

Good point..

 

Id had thoughts to isolate the alt/starter feed with a big amp'age normally open relay, using the main supply also as the supply to the relay coil so that it is permanently energised, and once the battery is disconnected (using the cut-off) the relay falls open and isolates the supply. At least this way ive only got to run one major supply cable inside the car and back, the relay can be done locally quite simply in the engine bay.

 

The only concern is the current drain on the permanently energised relay, id have thought it was milliamps, but ive never tested a large DC relay.

[Sandy 189]

It doesn't need to be that complicated! The FIA switch just needs to be wired as prescribed in the instructions and everything will work as it should.

[Stu 18]

Err.. I may have a little problem..

 

Turns out ive got an FIA 'style' cut off switch, ive just googled for a wiring diagram to see what you mean Sandy, and the FIA ones have extra terminals on the back 2 N/O 2 N/C, mine doesnt.

 

Just two terminals, which obviously isolate when the key is turned.

 

Buggernuts..

[Sandy 189]

Ah. Well, maybe position it so it knocks off an ignition circuit switch as it sweeps round?

[pug_ham 242 3 Cars]

Err.. I may have a little problem..

 

Turns out ive got an FIA 'style' cut off switch, ive just googled for a wiring diagram to see what you mean Sandy, and the FIA ones have extra terminals on the back 2 N/O 2 N/C, mine doesnt.

 

Just two terminals, which obviously isolate when the key is turned.

Its its a two position cut off switch (on or off) you could just run every wire from the battery positive terminal to one terminal & then from the other to the starter / alternator & shunt box.

 

Graham.

[Stu 18]

Can you? I think the only way to do it is to isolate the alt/starter feed from the feed to the shunt box else the alternator will supply the box even with the battery off.

 

I think ill wire the starter direct to the battery in a new lug with a lead that goes to the cut off, then out of the cut off into the shunt box, this way it'll isolate the supply to the loom and hence kill the engine which is what i really want it to do.

 

Its not ideal, as as Sandy says, in the eventuality of a front ender, if the alt/starter supply lead gets cut and earths theres no way to isolate it.