ammeter

[Chris]

I have a 1931 tourer, the ammeter and switches are early type, located in the centre of the dashboard.  The ammeter has always read in reverse, the ignition light has never worked, the car is positive earth. The dynamo and cutout are both working,  the dynamo has been changed to a two brush set up. I have reversed the terminals at the back of the meter, but the car won't start, it runs when changed back.
 please could you advise a solution. thank you.

[Stuart Giles]

To get the ammeter  to read correctly without a difficult major rework on the switch panel's wiring, you'll need to change the car back to negative earth which is how the car would have left the factory.

Edit: more information on this recent forum thread: [url=<a href=https://www.austinsevenfriends.co.uk/forum/showthread.php?tid=4741 >https://www.austinsevenfriends.co.uk/forum/showthread.php?tid=4741</a>][/url]https://www.austinsevenfriends.co.uk/for...p?tid=4741

[bob46320]

Morning Chris, reversing the wires on the ammeter should work (and the engine should run) I would suspect a break in one of the wires going to the ammeter which makes a circuit when bent in a particular position.

[Chris]

Thanks for the info.  Chris

[Stuart Giles]

If you have a Lucas SM5 switchpanel, reversing the ammeter connections "B" and "A" will not result in correcting the ammeter reading -the reading being the wrong way round on a positive earth car. If you look at this diagram of the SM5 switch panel you can see why. The "A" side has hard wired connections with the brass trace to the two switches and so, the entire vehicle electrical load -lights, ignition etc. By connecting the "B" wire to this "A" terminal you would ensure that the ammeter will not respond to any item switched by the panel, because it now has a direct battery feed. Conversely, connecting the "A" wire to the "B" terminal will mean that the ammeter will show the Dynamo's charge; but not the nett charge after loads like the ignition etc.

[Bob Culver]

Compared with the diagrams for moderns this is a joy to behold. Neverthelss none of the common variants make the workings abundantly clear. I am sure if someone with access to a switch could redraw and expand it would prove useful esp for those with minimal electrical experience. I presume it is possible to select headlamps and summer charge together? The form of the wiper(s) in the ign/charge switch are not obvious

[Stuart Giles]

I presume it is possible to select headlamps and summer charge together? The form of the wiper(s) in the ign/charge switch are not obvious

Hi Bob,
          It shouldn't be possible to have half charge and the headlamps on. I don't have a diagram but this "work in progress" photo of an SM5 I restored recently should help. The "A" terminal is hard wired to the centre posts of both the ignition/charge and lighting switches. When "Full charge" is selected the roller that is visible just below the ammeter on the right is pushed by the brass wiper and shorts the "D+" terminal on the back of the switch panel to the "Shunt" connection on the back, so shorting out the half charge resistor. The roller on the left side will be operated when the lighting switch is set to "Head" -this shorts the "D" to shunt in the same way as on the ignition side. So when the panel is working properly i.e. without 80 something years of tarnish on the roller's Phosphor Bronze spring and on the Brass D+ terminal post, an unthinking owner can't run the battery down in no time at all during night driving.

[David.H]

A bit dangerous me commenting on electrics, but.... Wipers were mostly vacuum on early cars I think, so no need to show them, if fitted as a
modern upgrade then use the A(ccessories) terminal or the fuse box perhaps?