Alfa Pages

[gritsop]

Hi,

I was wondering if an electric power switch can be installed on the fuel injected cars. I have seen in a friend's series 2 33 a big switch next to the battery which has purpose to cut out the current from the baterry if a shortcircuit occurs.

He once had a leaking fuel hose which led to fire in the engine and as precaution measures, he had this switch installed.

However his car is a carbed one and most probably there is no problem cutting the current abnormally since it does not have ECU, sensors etc.
What about the injected ones? Will such a switch cause problems in the ECU etc if power is suddenly cut down?

Has anyone seen or installed such a device?

Regards,
_________________
Thanassis Gritsopoulos
1991 Alfa 33 1.4 IE
2001 Alfa 147 1.6 Distinctive

http://www.alfa-restoration.co.uk
Parts Shop: www.alfa-restoration.co.uk/shop

[BILL]

I have a switch for the fuel pump.If you turn it off the pump does not work ,
anti-theft device.It cuts the power from the pump relay at the engine
bay.If you disconect the battery then you have to adjust the clock,the radio ,the cental locking will not work.

[RFlower]

If you disconnect the battery with the engine running you will probably destroy most, if not all of the electronics, and possibly all the lights and other electrics which happen to be on, on either injected or carb. car.

Without a battery connected the alternator voltage will go very high for a short time, maybe 30 volts or more, as even though the regulator will try to maintain a reasonable voltage, it takes a finite time for the field in the alternator rotor to collapse.

There used to be a warning sticker re this when alternators first appeared, but I don't think they bother any more.

As a last resort I suppose it's better than the car burning up, but a better way is to make sure the alternator connection is on the battery side of the switch, so that it's still connected to the battery when the switch is open.

The engine will then just stop; I don't see any reason the electronics should be damaged.
_________________
Dick Flower, Nr. Carcassonne. '94 Trofeo 1.4 ie (F), '93 Imola 1.3/1.4 ie (now for breaking) (F), '91 1.7 ie (GB)(spare car), '86 Sprint 1.5 QV (F). '87 VW Syncro camper (F), '73 NSU Ro80 (F), '99 Fiat Seicento (F)

[gritsop]

Hi,

Dick you are right, the manual indicates with big bold letters "Never disconect battery with ignition on"...

I cannot understand your proposal that the alternator connection should be on the battery side of the switch ...

Can you explain it better?

Regards
_________________
Thanassis Gritsopoulos
1991 Alfa 33 1.4 IE
2001 Alfa 147 1.6 Distinctive

http://www.alfa-restoration.co.uk
Parts Shop: www.alfa-restoration.co.uk/shop

[RFlower]

I know. A battery terminal came off on my m'bike one night. I was going quite fast, and my headlight bulb immediately burnt out (rear lights too), I immediately swiched to dipped beam, and that burnt out also. Quite hairy going around 120 k in pitch dark!

I'm not going to give any step by step instructions, as wiring layouts vary on different cars, maybe from LHD to RHD also.

Find the main cable from the battery to the alternator, and fit the switch so that it cuts off everything from the battery except the alternator. You may have to extend the alternator cable to the battery or move some other wires to do this. Nothing else should be connected to the alternator main cable (some cars have a + wire to other items from the alternator + terminal)
_________________
Dick Flower, Nr. Carcassonne. '94 Trofeo 1.4 ie (F), '93 Imola 1.3/1.4 ie (now for breaking) (F), '91 1.7 ie (GB)(spare car), '86 Sprint 1.5 QV (F). '87 VW Syncro camper (F), '73 NSU Ro80 (F), '99 Fiat Seicento (F)

[Horizontally Opposed]

I guess cutting power to the fuel pump is a nice solution.

I tried to fix the alarm in my 33 once (original alfa romeo unit) so I disconnected the unit from the car to take it home and study it a bit.

When I switched on the ignition I knew something was wrong because even though all the electronics worked I didn't hear the familiar fuel pump sound. I thought: taking off the alarm either fixed the pump or I need to put the damn thing back where it was...

So what I found out was that the fuel pump wires are connected to the alarm circuit. Since they are so close to the dashboard I guess it's easy to make a switch to cut power to the pump.

[gritsop]

Hi,

Well, everything been written sounds to me as good solutions but my main point is how to cut the power from the electrical system of the car when a shortcircuit has occured.

Cutting the power from the fuel pump does not mean that you save your car, as the most common cause of severeshorts which lead to fire come from the + thick cables which, for any reason, touch the chassis.

This will mean severe shortcircuit which will only be stopped if you disconnect the battery...

So, in my opinion it is better to save your car even if you end up burning the electronics, instead of writting it off ...

Anyway thanks for the input.

Regards
_________________
Thanassis Gritsopoulos
1991 Alfa 33 1.4 IE
2001 Alfa 147 1.6 Distinctive

http://www.alfa-restoration.co.uk
Parts Shop: www.alfa-restoration.co.uk/shop

[BILL]

A big fuse,like 40A ?
Like the ones they use for I.C.E. systems .

[RFlower]

A battery cutout switch is fine so long as you never open it when the engine is running.

If you smell burning when you are driving you are going to pull over & stop the engine anyway.

If you are out of the car with the engine running, and open the cutout switch, the engine might not stop.

This is because the alternator will continue to provide power to the ignition/injection system, at least until the electronics are burnt up.

If you had a short circuit, it might draw enough current to drop the voltage down to where the electronics stop working.
_________________
Dick Flower, Nr. Carcassonne. '94 Trofeo 1.4 ie (F), '93 Imola 1.3/1.4 ie (now for breaking) (F), '91 1.7 ie (GB)(spare car), '86 Sprint 1.5 QV (F). '87 VW Syncro camper (F), '73 NSU Ro80 (F), '99 Fiat Seicento (F)

[Matt Stolton]

Being a daft Sound Engineer type chap, I carry an assortment of tools with at all times.

When I had my little meeting the chelsea tractor on the M6, I simply cut the -ve on the battery with my wire cutters, much to the amusement of the fire service who were fafing about with a little spanner, and taking forever. They had lost there wire cutters!!

What Mssr RFlower is trying to describe is Lenz's Law. If you disconnect a steady voltage through an inductor quick enough, you generate a reverse voltage which tries to oppose the change. i.e. if you try to turn off the voltage through a transformer, as the feilds around the transformer start to break down, it generates a measureable voltage in the opposite direction, which tries to maintain the overall voltage. The size of this back EMF is directly proportional to the speed of disconnection.

With this theory, you can get a 9V battery to generate 10 cm long sparks, if you turn off the circuit quick enough! very impressive it is too.

In relation to cars, by simply dissing the +ve on the battery, with the alternator spinning, you a breaking the circuit from the +ve to the inductor in the motor of the alternator. Lenz's law would jump in and things start to fry. Any battery disconnect must be done with the engine OFF. You can then simply get a 100A switch on the -VE side of the battery, and that will isolate the entire cars electrics.
_________________
Regards

Matt

Ex Alfa 33 'GTA' (P4 with Knobs On)
Now cruising in a 166 3.2 Ti!!

[Dick Flower (not loged)]

Well, it's partly Lenz's Law (also called the Law of Natural Cussedness)

(BTW, the abbreviation for "Monsieur" in France is "Mr" - same as in English, and I'm English anyway)

As Mr Stolton says, if the battery cable is disconnected, a voltage is generated, by the inductance of the alternator output windings, which tries to maintain the current which was flowing (in the same direction actually).

As the resistance of the output circuit is now higher, consisting only of whatever vehicle electrical components happen to be switched on, there will be some rise in voltage due simply to this.

However, the voltage regulator for the alternator gets its power supply from the alternator output, (often via an independent rectifier circuit). This supply is internal on most alternators nowadays, as the regulator is inside the alternator.

The regulator supplies the rotating field coils (rotor) of the alternator. Again, the connections are internal. (On old alternators with an external regulator, it would be easily possible to disconnect the either the supply or field wire from the alternator before disconnecting the battery, so there would be no alternator output)

The regulator will try to maintain the alternator output below the maximum, usually around 14 volts, and as it is supplied from the output of the alternator, will continue to do so as long as the engine keeps running, happily supplying all the vehicle's electrical system directly, without the battery. (This is called positive feedback)

Unfortunately, although the regulator tries to reduce the alternator output voltage when it "sees" it go up (when the battery is disconnected), Mr Lenz gets into the picture again.

The windings of the rotor have a very high inductance, and their magnetic field takes a (relatively) long time to decay, even though the regulator tries to reduce it, by no longer supplying current.

This means that the rotor field which was necessary to charge the battery at the moment it was disconnected now produces a much higher voltage, which lasts until the field has decayed below the point where the regulator can control it again.

The voltage peak produced will depend on the alternator's output at the moment the battery was disconnected, i.e. how "flat" the battery was, but in any event will probably be high enough to melt the electronics (unless the makers have cleverly protected them), and a few othe rbits and pieces.

I hope the above is clear.

Dick Flower

[Ben_nz]