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Switchboard Identifying help

Can somone recognise this switchboard? From the front it looks like an SM5 with the two switches, the charge light and ammeter but it looks like it has an extra terminal on the back marked F1 to the right of the D+ terminal. I am trying to sort out how it should be wired before I try and start the car

(and also trying to work out how to post photos)


Location: wilts

Re: Switchboard Identifying help

Looks like you worked out how to post a photo !

Is the F1 terminal connected by a link to the +B terminal above as the terminal on the opposite side is connected to the A terminal above? If so I would leave it unconnected.

Cheers, Tony.

Location: Melbourne, Victoria, Australia.

Re: Switchboard Identifying help

Tony, having spent some more time taking the switch apart to see how it all works it looks to me as though the switch does some odd things (by design!).

in the switchpanel:
When switched to Full charge F1 is connected to D+ (Otherwise F1 not connected to anything)
When switched to Headlights F2 is connected to D+ (Otherwise F2 not connected to anything)
When both Full and Headlights are switched F1 & F2 connected to D+

This seems to give the possibility of 3 different charge rates.

1/2 Charge where the field is fed via the ign warning light/D+ (via 2 resistors in my cut out which is some electronic one with a diode, 2 resistors and a fuse)

Full Charge where the field is fed by D+ (via one resistor in my cut out)

Headlights where the field is fed direct from D+

Does this sound "normal"

Location: Wilts

Re: Switchboard Identifying help

A bit more reading and it looks like it's a bad idea to put 12V direct on the field windings of the dynamo so I have joined F1 & F2 on the switchboard and wired them into F1 on my electronic cut out so I only have two charging modes (and effectively only 5 terminals like the other panels I have seen)

Full charge (when turned on or when the headlights are on) the field is fed via a 1 Ohm resistor and half charge the field goes via and additional 3.3 Ohm resistor (so 4.3 Ohm altogether).

Battery flat now so find out tomorrow if the dynamo works wired like this

Location: Wilts

Re: Switchboard Identifying help

I am unfamiliar with early switches but some makes in the 1930s had 3 stage charging as you have fathomed, and that was the basis for the two F terminals (or vestiges of) on some switches and cutout units, whether used or not.

With conversions anything is likely and the workings of electronic regulators uncertain. The field may have been rewound for 12v. Compare resistance. The original field was subject to only 4-5 volts normally; on 12v directly the cooking effect increased 5 or more times.

Location: Auckland, NZ

Re: Switchboard Identifying help

Er Chris, I assumed from "12 v direct" that working as 2 brush; if 3 brush setting the 3rd brush back reduces current.Is just the cutout electronic or is a regulator somehow incorporated??

Location: Auckland, NZ

Re: Switchboard Identifying help

There doesn't seem to be any form of regulation on the car only an electronic cut out. I started the car today and tried measuring things with my electronic multimeter. The voltage on D seems to be all over the place and the field voltage was tiny! on 1/2 charge the current was not enough to swap from discharge to charge but on full charge there was a charging current. It does seem odd not to have a regulator I might sort something on that or else I will fry my battery I reckon.

Location: Wilts

Re: Switchboard Identifying help

This article does describe the 3 settings I have found on my switch being used on the A7 (last section). (but not the switch itself) I don't feel confident that my dynamo has been re-wound so I will stick with the two settings for now so I don't put the full 12V on the field windings.


Location: Wiltshire

Re: Switchboard Identifying help

Hi Chris

As mentioned, until someone publishes a circuit, I am not familiar with the pre RP arrangement, and it is a pain to fathom from wiring diagrams.

At the risk of prolonging this, the direct links shown on the switch in Fig 1 of the article introduce confusion.
Whilst the article may have come out of A7CA mag, as far as I know it does not apply exactly to any Seven.

Digital meters do not work well in a car ignition environment, and they can get confused by erratic and noisy voltages.

Much depends on whether your system is operating as 3 or 2 brush.
Allowing for meter lead resistance, the resistance field terminal to 3rd brush (or earth if none) should indicate if wound for 12v or not. I measure at about 1.5 ohm for 2 pole models originally but others quote somewhat more.

From playing on the bench the field seems to tolerate considerable current without getting very hot; this may explain why many somewhat random 12v conversions survive. However in the car has a lot of other heat to contend with.

Location: Auckland, NZ