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OK, I've got workaround figured out with modern parts.

One of the goals was to design something that could be easily repaired/replaced in the field. I was originally considering, for example, a Bosch DPST head lamp relay, like that found on an old BMW motorcycle. However, those are somewhat rare, and about 10x the price of a $1.79 40A Bosch SPDT relay.

So here's the plan.

The original SWB switch works by grounding the can- power flows from 30 through the thermal flasher to ground. In the ground position, a relay wiper is also pulled into contact, which is a double pole (you must have isolation between both poles of the wiper, otherwise both turn signals will flash when you only apply one). The thermal element heats up and breaks the contact. Mechanically, the thermal wiper moves back into the closed position and it starts all over again.

Now, the modern fix uses a pair of Bosch SPDT relays (terminal 87a omitted for clarity) and a Tridon EL13 Electro-mechanical flasher unit. When the switch is pulled, terminal S is grounded, which allows current to flow through the flasher unit to terminal 86, which energizes both relay coils, allowing current to flow from terminal 30 into the turn signals. When the flasher unit breaks the contact, the coils are de-energized and the current flow stops.

The reason for using two relays is that you must have both left and right signal circuits isolated when off. Rather than use a DPST or a bigger DPDT (like a Hella or the re-badged MSD) I chose the cheap, redundant route-- Bosch ice cube relays are plentiful and can be had at most parts stores.

Also, everything should be small enough to fit in a metal box or can similar to the size of the original unit. I suppose if I really wanted to go crazy, I could fit everything inside the original can, but that would require different components.

Any thoughts?
Last modified: Mon, 20 Jun 2005


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