Hi All

I've just finished wiring my dash (26R not 7) and it's all tested OK. I have one minor 'problem' which I think a diode may resolve, but I thought I'd pick the brains of the LCB colelctive before heading off to Maplins!

I've used relays for ignition, start, fuel pump, diff cooler, fan, screen, dip and main beam. Apart from the lighting relays, the others are all controlled by switching the earth (via the FIA Masterswitch) - this is a bit neater and simplifies the wiring as I'm returning all earths to the battery.

I like to have the fuel pump controlled by the ignition switch but also like the ability to prime the carbs, so this is built in. However, with the wiring setup as I originally planned, priming the carbs also switches on the ignition relay as the switch I have used is a combined Off - On - Mom switch (I can't change to separate switches now!).

So, I think a diode (as shown attached ) may solve my problem - any thoughts and suggestions on an appropriate diode?

Ta! Wyn

A picture paints a thousand words!
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The pink and yellow boxes indicate a combined switch.

[Edited on 2/3/12 by lsdweb]

Afaik the diode will prevent the priming function from working at all as it is wired now, I'd put it between the pink and yellow switch ('facing' down)
I don't know how your switches are working though, do they really have 4 poles? If so, the original wiring without diode should work fine. However, if the momentary option also closes the latching circuit I can see your problem, the diode inbetween the switches should do the trick.

Thanks Rost

The switches are double pole (Apem 600 series). The problem is operating the Momentary bit also operates the On position of the other 'half' of the switch.

I put the diode in the original position as I figured if it failed (or more likely my soldering!), I wouldn't lose the fuel pump. I can't see why the prime function wouldn't work like this although I'm probably missing something!

Ta

Wyn

Ah no, I'm the one missing something
Anyway, after a second look I'd say this should work indeed!

Thanks!

Any suggestions on what diode I should use?

One small point... Put a decent Amp Meter in series with the pump and switch on. Measure the current, and then make sure your Diode is at least twice (or more if you can get one) that current drawn by the device and at least 50Volt PIV (reverse voltage), or better still 100Volt. Remember that car electrics is not just straight 12V and has lots of voltage spikes and nasties etc, which dont matter to relays, light bulbs and pumps etc, but Semiconductor devices DONT like them
What you have to bear in mind is that the diode will be passing that current and you dont want to be running it at the max limit and getting hot or overheated, as it could be a potential fire hazard or just even a failure point. (I know its just for priming the carbs, but dodgy electrics can be a real pain)

My other query is that are the devices that you are controlling via a switch in the earth wire, Do they have permanant 12V on the feed side?
Namely are the live all the time, and turned on and off by the switch in the earth wire.
If you are switching the 12V to the devices (via Ignition etc) then this is academic. If you have permanant live to these devices, in an accident the live wires could still short out to the chassis, and be a fire hazard.

The FIA switch should be Isolating the Negative Battery Terminal from the chassis or earth wiring.
Your diagram shows it in the earth wiring to chassis, which will switch off any of the devices, but WILL NOT isolate the Battery, thus leaving the potential for a wire to short out to the chassis in the event of an accident.
I think you may have meant that your Earth Symbol is the Negative battery terminal, and not the chassis.


The point is, your diagram will work. However, a lot of things happen in an accident, and wires get crushed to the chassis, and thats a fire hazard, if the BATTERY TERMINAL to CHASSIS connection is NOT isolated by the FIA switch.

Sorry if I sound a bit overbearing, but what you are doing will work, but may not leave you with a safe situation in the event of an accident. This is the whole point of the FIA switch is to Isolate the Battery from the chassis / earth wiring, to completely sever the connection so no current can flow in the event of an accident.

I might has misread your post. However, getting the electrics wrong on Kit cars is not uncommon. Whilst it may work, is it actually safe and covers every eventuality, say when in a critical situation like an accident?

Bottom Line, is make sure the Battery Earth terminal is Isolated from ALL the wiring AND the chassis, by your FIA switch.

Have fun, and Be Safe.

Thanks Geoff

I am switching the earth of the relays supplying these devices, so the current through the switches and proposed diode is merely that to switch the relay. All the relays are fed directly from fuses.

And the wiring to the FIA switch is to the auxiliary contacts - sorry I didn't make it clear in my original post! The main terminals of the FIA will be switching the battery earth.

Regards

Wyn

If it were me I would want to switch the power side of the fuel pump rather than the earth

Dash


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This will not work with a single diode, you will have exactly the same problem as you did before. You need two diodes, one in each circuit that you need to separate.

EDIT: Now I understand the switch arrangement you can use a single diode, but it has to be in a different place:


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[Edited on 2/3/12 by MikeRJ]

Thanks Mike!