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Voltmeter hookup

New 04/20/2006

The volts measured at most locations within the passenger compartment
will read lower than the volts at the "core" of the electrical system.
So those of you that have voltmeters hooked up in the SHO, what are
you using for the switched 12V+ that turns the gauge on and supplies
the voltage that is read by the gauge? My thought was to use a relay
triggered by the ignition switch, with the power supply coming
straight from the MegaFuse so it reads the volts at the core of the

Also, the wire I tapped for the dimmer function is not working as
intended. I'm pretty sure I got advice on which one to tap somewhere
around here, but it isn't working. I need ~12V+ when the lights come
on to lower the brightness of the LEDs (I'm using digital gauges). I
could always tap one of the three relays I have on the headlight
circuit, but if there's a better source within the cabin I'd rather go
that route first, as I already have a dozen or so extra wires under
the hood.
Dan Carman
'97 PG
Philadelphia, PA
In my experience almost any dissimilar material connection can act as a resistor which is aggravated by temperature fluctuations. Pretty much the principal of the thermocouples I've used over the years. In fact, applying a voltage to these is the principle used from those electric cooler/heater boxes you can plug into your cigarette lighter. We used those (thermal-electric-coolers) all the time in molecular gas measurement for maintaining a constant temperature on the cell and optics measuring the gas concentration when exposing it to ultraviolet light or ozone under a vacuum. 
I put an ammeter on a voltmeter one time and found that it draws less current than an LED (as any properly designed voltmeter should) so I just hooked mine up hot. Its proven useful that way and Iím glad I have it setup that way. My car sits for long periods of time sometimes, and I can always look in at the A-Pillar to see if the batter is getting low. I try to keep it on a charger, but not always. Even though itís garaged I keep the alarm on all the time. I have a couple of asshat roommates and I came home from class one day to find a beer can sitting on the car. I still have to fix the resulting hole in the wall from that incident. Iím glad it wasnít my roommates head though, that could have been bad.
I suspect it's a ground loop issue between the engine block and the
ignition switch. So I need to tap the + and - in the same area of the
car. No biggie, although I was hoping to get away with using the
wiring I had already done before with the last gauge install.

Thinking back, I had the same ground loop issue with the trans temp
gauge sender. Even though the engine is grounded securely to the
firewall, I'm thinking the stuff in the dash area isn't... I had to
run an auxiliary ground from the gauge right to the sender housing to
get accurate readings.

On 4/19/06, Paul L Fisher <sho@paul-fisher.com> wrote:
Why would volts be inconsistent on a voltmeter? The only time you will get a
voltage drop is if you have a load. Just putting a voltmeter, even if you
put 500 yards of copper 24 gauge wire between the meter and the battery will
lead the same as if you direct connect it with 2 inches of double 0 cable.
Paul L Fisher
Any reason you are not coming off the MEGA fuse?

Just so that it doesn't go unsaid, an inline fuse would be a good idea, and shouldn't cost too much.

Best Regards,
Because then the gauge would be on all the time. It's a DIGITAL gauge
(LED). Probably should have capitalized that in the first place. ;-)
So not only will it draw a little more current than an analog gauge...
it would light up the cabin pretty well 24/7 if it was getting a
constant 12V. I'm either going to run a small lead from the Mega Fuse
to a relay and then the gauge ("keyed" 12V+ as the trigger input) or
I'm going to run the gauge ground very close to the 12V+ supply to the
ignition source.

As far as the dimmer goes, I'm going to tap my foglight control relay.
The gauge does not have a variable/progressive dimmer, just a
high/low switch essentially, so it needs a full 12V+ not the
reduced/dimmed instrument panel voltage. Wow... lots of slashes in
that sentence. :-S

Any switched setup will have a voltage drop.

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