beg3yrs

I've incorporated a couple of common relay kits for headlights and marker/dash lights in my 260Z. These take much of the load off the combination switch. They source the high load power via a connection directly to the battery and only use the combination switch to energize a low power relay. All in all, a good idea.

But ...

The ammeter itself is just a voltmeter connected across a shunt. This shunt is a very low resistance, high current device that develops a voltage that is proportional to the amount of current flowing through it. The voltage developed will change polarity depending on if current is flowing. When everything is stable (fully charged battery and charging system supplying all the car's electrical needs), the ammeter reads zero (centered).

Here's my concern: If high draw things like lights and amplifiers are connected directly to the battery, their current doesn't go through the shunt and isn't accounted for in the ammeter's reading. Thus when the car is running and lights are on, the ammeter will show charging because the alternator/voltage regulator circuit is "amping it up" to power these devices and current flows back toward (but not necessarily into) the battery, causing an erroneous reading on the ammeter.

Looking at the wiring diagrams, it seems WHITE (W) wire typically means +12V before the shunt and WHITE with RED stripe (WR) means +12V after the shunt.

Is it then a good rule of thumb to connect these accessories to WR rather than direct to the battery?

I can see a trade off as some devices might then overpower the fusible links but I'm not sure. I'd like the ammeter to read correctly but realize it might not be practical.

Looking for some direction from those with more experience than I. What have y'all done about this?

theramz

The amp gauge should only read zero with the engine off. The WR wire is the hot from the alt. If you had the gauge out and hooked it up backwards it will not charge. If you run enough accesories the alternator might not be powerful enough to keep a + reading at idle.
Also any add ons should be fused in case of accidental short or overload. Each new circuit needs its own hot and not added on to an existing load.

beg3yrs

Tom, thanks for the quick reply but I'm not sure I understand what you're saying here.

The ammeter will read zero only if there's no current flowing through the shunt. You can have the engine off and turn on the headlights (factory circuit configuration) and you should see a discharge on the ammeter.

If the alternator is supplying the same amount of current as the devices on the car is consuming and the battery is fully charged (not consuming current), then you should have a zero reading. Essentially the ammeter is supposed to tell you if the battery is consuming power (charging) or supplying power (discharging). If accessories are connected directly to the battery, the ammeter gets biased as all the current "drains" are supposed to be on the WR side of the shunt.

Don't understand why you say if the gauge is hooked up backwards nothing charges. It's just a voltmeter across a resistor (the shunt) and all that should happen is the meter reads backwards.

I completely agree about the accessory load and the need to fuse them.

Anyway, my original question still stands, although I'm becoming more convinced that if you want the ammeter to read correctly, you should wire your additional accessories off the WR rather than the battery directly.

I suspect this kind of complication is why manufacturers just went to voltmeters on later cars.

theramz

I was thinking back to when I rewired my '70 dash fuse block. I should clarify that the zero reading with the engine off and everything else too. The alt should be putting out more than the demand even at idle if the alt is sized correctly. There is no provision in the early cars to boost the idle for ac.
I took the fuse block from a 180sx and spliced the 240 plugs onto it. It wasn't that hard because the color codes are the same. It also gave me extra ouputs and the blade style fuses.

beg3yrs

That's a great idea Tom. Thanks, I'll look around for something like that.

theramz

Zcarparts sells one plug and play for $200. Mine was $5 and a couple of hours.