Finally got around to wiring up an old tach I had from my stash to my 49 3100. I was originally going to wait until I swapped engines and upgrade the electrics to 12v but the old 216 along with it's 6v electrics just won't die and has actually been running pretty good lately. So with an eye towards the future swap, the truck is now running a hybrid electrical system.
I picked up an old fuse box from a fellow Stovebolter, gutted it and modified it to hold a couple 12v Bosch style relays, a couple fuses and a small terminal strip. This is my 12v distribution panel.
To get 12v out of the trucks 6v system I picked up a step up converter from ebay. It'll give a clean and stable 12vdc output from a source of 5vdc to 11vdc. It is also rated at 12 amps. I decided to go with this larger than needed amperage output in case I decide to add anymore 12v accessories before the swap. It was only $5 more than the 4amp rated converter so it was a no brainer to me.
I wired the converters fused 6v input to the key switch and wired it's 12v output to the terminal strip which then throws one of the relays (the other is spare for now) and is also paralleled through the relay contacts and fused to power the tach.
Since the tachs LED lamp is 12v I also wired the tachs light lead in parallel with it's power lead so it lights up whenever the key is turn on. The LED's draw is very low and is a good indicator of the relay's operation.
I didn't want to drill any holes in the perfect condition dash so I used a magnet mount and put it between the gauges just above the shifter rod. I've gone back & forth between mounting higher between the gauges and where I have it now. I've got enough slack just in case. It holds pretty good.
It's great to have a tach and after a 65 mile test drive seems to work good. The charging system is running normally with no ill effects from the additional load.
A few pics.
2 fuse boxes and the converter Lids off Tach Tach 2
