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All,
I fitted an overdrive, but the deal didn't include a gearstick or steering column switch, so I've used an ordinary one, but one that is illuminated, so that I'm in no doubt that the O/d is on or off. The switch is in series with the gearbox 3rd/4th switch and is wired to trigger a relay that switches the actual circuit to the O/d.

O/d works fine, but the switch light doesn't! I've used these switches elsewhere on the car - lights, fans, winscreen wipers etc and they work fine. The switch light itself works when tested on the bench, there is useful amount of volts at the switch supply and the earth is good. Whats more, if the wire from the switch to the relay is disconnected, the light comes on appropriately - the O/d doesn't of course in this situation.

Why won't the light work normally?

Thanks,
John

John, I think you are running the two circuits in parrallel, the o/d actuation relay (for the solenoid) and the indicator light.

might it be that the resistance of the indicator light is so much greater than the relay that the current flow to the indicator light isn't great enough to light it?

hence if you disconnect the solenoid the current has no where else to go 'cept through the indicator light.

can you wire the indicator light into the feed cable to the solenoid, in series, or even to the relay in series, rather than parrallel?

jonath

Hiya John. I think the switch needs to be in parallel. I did the same in my car using a home made switch in the gearknob wired in parallel with the 'fasten seat seat belts' light.......sorry!! It seemed funny at the time......Ahemm.....

John,

Looking at the wiring diagram for overdrive in a Spitfire with a relay fitted, it gets interesting.
I'll have to assume that yours is the same setup.

one side of the solenoid is connected to earth, and the other side goes to the C2 terminal of the relay
the C1 terminal on the relay goes to 12V
the W1 terminal on the relay goes to 12V
the W2 terminal on the relay goes to one side of the gearbox switch
the other side of the gearbox switch goes to the "common" terminal of the illuminated switch.
the other switch terminal on the illuminated switch goes to earth
the other lamp terminal on the illuminated switch goes to 12V.

Therefore the switch will illuminate when the relay, and hence the OD is activeated.

*** "Common" terminal on the switch is the one that is common to both the switch and the lamp. The switch should have three terminals.

Kevin

Thanks, guys,

Picture=1K words so here's a diagram of how I've wired the system.
The driver's switch and indictaor light are one unit, with three terminals, Supply, Load and Earth, the last for the light.

And having to redraw it and look at it again, I think I may know the answer.
As I wrote before, I've used these same switches for the lights and the very powerful fans, and the indicator lights come on as required. BUT I think that before I have inserted the switch before the load. As it stands, the 'Load' beyong the swirtch is just a short circuit to earth - no wonder the light doesn't come on!

What do you think? If I put the switch between the +12V and the relay (terminal 65) with a side connection to earth, then there should be lots of volts to split between the relay etc and the light.

I'll try that - and if it works it's down to you guys. Wonderful what 'takling' to other people does to the brain!

Thanks for the input!

John

John,

Your original circuit is very nearly correct to what I described.
The Drivers OD lamp needs to be wired to the other side of the Drivers OD Switch, and the earth side of the lamp needs to go to +12V (not earth)

See the modified picture

Your modified version will also work

Kevin

Thank you Kevin!

The driver's switch and lamp are one unit. I would guess that turning it around from its normal orientation - so that the supply was connected to the common earth, the load towards the gearbox switch and the 'earth' taken back to +12V - would achieve that arrangement! And I might not have to rewire it very much, except at the relay end, near where the lamp is currently (!) earthed anyway.

I hadn't though out yet how to rewire my modified arrangement, and I think it would be more complex, so that's a great help. I'll let you know!

Thanks again,
John

John,

It is as simple as swapping the wires around on the back of the switch and moving the lamp earth to 12V.

This second modified image might help you visualise it a bit better.

This will tell you when the OD switch is active, and (but) will remain illuminated when the car is in 1st or 2nd gear, so you know that when you change up to 3rd it is OD 3rd.

Kevin

Kevin,
Thanks for the alternative.

But it looks to me as if, when the gearbox switch is closed, the relay is then exposed to +12V on both sides, less that due to the resistance of the indicator lamp. I'm afraid I'm very simple minded about electrics - is this likely to make the relay less reliable? (The volts, not my simple mindedness)

Would that have the same effect as putting the drivers switch upstream of the relay, when the light will 'bleed off' some volts back to earth? (see what I mean about simple minded?)

JOhn

John,

For current to flow through a resistance, you need a potential (voltage) difference between the two ends.

If both ends are connected to 12V (or earth or 1million volts) then there is no potential difference, and therefore no current will flow.

Therefore in the case of this circuit, with the gearbox switch closed, and the OD switch open, both sides of the relay will be at 12V (see note below), so no current will flow through the relay, therefore no problem. With both switches closed, one end of the relay is at 12V and the other at 0V, therefore there is a potential difference of 12V and the relay activates because current is flowing. The same goes for the lamp.

Note: being really pedantic and picky, with the gearbox switch closed, and the OD switch open, it is possible to get a very very small current flow through the series combination of the lamp and relay IF AND ONLY IF there is a small difference in the 12V voltage that each end is connected to. For example, if the relay 12V was wired direct to the battery, and the lamp 12V was wired to the your electric waterpump 12V at the waterpump - due to the current that your waterpump draws, there is a small (say 0.25V) voltage drop in the wiring between the battery and the waterpump (the heavier gauge wire the less the drop), therefore you would have a 0.25V potential difference across the series combination of the bulb and the relay. The "trick" is to ensure that the same 12V point is used for the lamp and the relay - an ignition switched 12V, otherwise the OD may remain engaged when the ignition is off. I believe that the 12V supply for the solenoid side of the relay goes direct to the main unswitched supply - therefore if the relay contacts fail short circuit, the OD is permenantly engaged.

Kevin

Section : General

Overdrive wiring problem

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