The active shunt in http://slantsix.org/forum/viewtopic.php?t=18493 is cool, but I only need to bypass the ammeter when I give someone a jump (or if my battery's dead and I get a jump).

What if I ran an extra 8 gauge wire from the alternator output to a battery disconnect switch mounted next to the battery? The 4 ga positive battery cable has an extra 8 gauge wire that I'm not currently using - that would connect to the other terminal of the switch.

This would bypass the ammeter, but also bypass the crowded starter relay terminal.

Here's the switch I'm considering:
http://www.boatersworld.com/product/196978241.htm

Will this work OK, or am I overlooking something? I have a 63 Valiant with an 80 amp alternator

That looks like a very good switch for the money; thanks for letting us know about it. However, 275A is far beyond what you need unless you've got a killer alternator. Harbor Freight sells a $8? 100A one I've been thinking of getting.

When jumping somebody else, the switch sounds like a great idea if you've upgraded your alternator, but not the rest of the system. The stock alternator shouldn't overload the stock system, but I'll admit it the design is marginal and a bypass wouldn't hurt.

When they're jumping you, you won't need it if they're jumping to your +battery terminal and to the starter since little current is flowing through your ammeter.

Once your car starts with a flat battery, then current will flow and you could stress the system just as if you're giving a jump.

If you forget to turn it off your ammeter won't work well and you'll have no protection should something critcal shorts since it bypasses the fusible link. If you've got a >100A alt, I'd also consider using #6 wire to the alt.

Please let us know how it goes.

What gauge fusible links do you recommend for my passive shunt?

http://www.harborfreight.com/cpi/ctaf/d ... mber=92688

If this is the switch you referred to, it is on sale for $3.99 !!

The 19.99 doesn't have exposed terminals like this one, though.

The Harbor Freight one could be mounted on the firewall with the terminals facing the engine compartment.

Or I could mount it on a cheap plastic box and place it near the battery, where I was going to put the other one.

If I use it I will glue a permanent knob to the switch.

Here's the product manual for Harbor Freight switch:

http://www.harborfreight.com/manuals/92 ... /92688.pdf

If these links expire, it's U.S. General model 92688

Yup, that's the switch I had in mind; 100A continuous/1000A surge should be enough unless you've got something exotic in mind.

The rule of thumb is to make your fusible link 2 sizes smaller than the cable and to put it as close to the battery as possible, but it's exact size isn't really critical.
For example, if you use #8 wire, use a #10 link. It's just a last ditch effort to prevent a fire from a massive short; you'd like to know and control where the weakest link is that'll melt first.

If you short out the alternator, it is pretty much self-limiting.

The quality if the connections are important; I used an uninsulated crimp (done with a proper crimping tool) w/ 2 or 3 layers of heatshrink on it for protection & to prevent flexing at the joint. Make sure it isn't too close to anythig grounded.

You can see how I did it in Fig.4 of http://www.tidewatermoparclub.com/TECH/ ... shunt5.pdf - it's the blue thing at the end of the red#6 wire.

It is only in case of disaster; such as when your $4 switch breaks in two and the battery end falls down and wedges against the body. [BYW, I'd mounted the switch securely to prevent any stress on it.]