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Posted by: lmcchesney Aug 6 2004, 08:53 AM

What is the wiring for the addition of a volt meter?
The VDO gauge has a + and -. Do I run a wire from the + pole of the battery to the gauge and ground the - side of the gauge? Since Ohms law applies, what gauge wire should be ran from the battery to the gauge?
Thanks,
L. McC

Posted by: ArtechnikA Aug 6 2004, 09:11 AM

ideally, you run the biggest, fattest wires you can possibly fit directly to the terminals (posts) of the battery.

anything short of that is a compromise. you can "calibrate" whatever wiring you use by measuring the battery voltage directly at the posts of the battery and comparing it to whatever you see at the ends of whatever wires you use.

Posted by: Joe.D Aug 6 2004, 09:32 AM

I'd put a fuse in that big fat wire right after it leaves the battery post :lightningbolts:

Actually, I'm about to install one myself. I was planning to conenct it to the hot feed at the fuse panel. Which is a "big wire" coming directly from the battery. But I think I'm not sure I like the idea that the volt meter is powered at all times. It could kill the battery, if the car sits for a while, so I might look for a "switched" source. And yes, its a compromise regarding accuracy.

Joe

Posted by: Demick Aug 6 2004, 09:34 AM

If you run it to the battery, then it will always be 'on'. In other words, it will run down the battery when the car is off.

So you need to connected it to a switched source. The factory decided to do that by wiring it into the circuit with the rear lights, which is not a very good place as anyone with a factory volt gauge will tell you (changes by 2 volts everytime you step on the brakes or turn on your headlights). But depending on what year your car is, the factory wiring for the center console gauges (which included a volt gauge) is all there under the carpet next to the shifter. That's the easiest place to wire it in.

Demick

Posted by: Demick Aug 6 2004, 09:37 AM

Also, you don't need a big fat wire. The voltmeter draws very little current so your voltage drop over even a small gauge wire will be small. 18awg wire is more than big enough - especially for the accuracy you are looking for.

Demick

Posted by: porschefool Aug 6 2004, 10:15 AM

You only need a big fat wire when wiring up an ammeter (measures current), not a voltmeter. A voltmeter is pretty harmless, I recommend it's switched, though.
BTW, anyone got references on adding all the center gauges to my '72?
I haven't looked yet, but I'm pretty sure it's too old to have the wiring just laying under the carpet...

Posted by: type47 Aug 6 2004, 11:06 AM

lmc, what is the year of your car? i'd look for the console wiring over the center tunnel. the factory console had a voltmeter and i think most cars came prewired for the console and if not equipped with a console, the wiring was left under the carpet just behind the shift lever. haynes elec schematic could help with which wire colors to connect.

Posted by: lmcchesney Aug 6 2004, 11:20 AM

Jim,
Mine is a typical 914. The VIN frame is a 72. The front suspension is a 76-911. The engine was a 1.7L and now a 2.---. the D-jet FI is a 74 2.0L.
L. McC

Posted by: Joe.D Aug 6 2004, 11:22 AM

The only Ammeter I would consider for permanent installation would use a shunt and a meter calibrated to indicate current flow based in the millivolt difference across the shut. I would not want the entire electrical flow of the car (even if you leave out the starter current) running directly through a meter.

Larger gauge wire has a smaller voltage drop over distance, so "larger is better" is still a good guideline, but its certainly true that the wire length in a 914 won't amount to much drop at the current load of a volt meter.

Joe

Posted by: SirAndy Aug 6 2004, 11:46 AM

you *might* get lucky tonight. they added the basic wiring for the center console somewhere mid '72.
soooo, check under the carpet just in front of the shifter. if you see a bunch of square plastic connectors that are unused and just dangle there, you got yourself a factory hookup for a volt-meter ...

Andy