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Old 11-25-2012, 11:02 PM
wilburtual's Avatar
240D + 240D = :)
Join Date: Oct 2010
Location: Los Angeles,CA
Posts: 192
installing volt / amp meters gauges..

Hey everybody, I run a single tank system on one of my 240Ds in los angeles utilizing both a FPHE and 12v heaters. it has worked great for 3 years and 45k miles for electrical, I use injection line heaters, a fuel line heater and a 12v filter heater. i upgraded the alternator to a 95 amp model to get more juice.. I am wanting to monitor my systems power/charging a little closer just to make sure things are good and mostly to detect any issues before they are a problem - i recently had a loose alt belt that was only partially charging the battery, so it left me hanging, but i didnt get any pre-warning such as the battery light coming on. it was sorta charging, just not enough to keep up with my electrical demands.

has anyone installed a volt/amp meter on their dash? if so can you recommend which one you used? I see a few cheap little ones on ebay from china that look like they would do the trick, although I'm a little confused on where to wire them in- directly across the battery, through the fuse box.. ?? will the volt/amp of the whole system read differently at different places or in an electrical circuit will it always read the same no matter where i tap the meter in?? i know you can plug a volt meter into the cig lighter, but i'd love to have an amp readout as well, so i can see how much current my 12v heaters use. i have them on switches and use them before start up and until the FPHE gets to temp, then i just leave the injection line heaters on...

here is the meter i found on ebay:
1PCS DC 4.5-30V 0-100A Dual LED Digital Volt meter Ammeter Voltage AMP Power | eBay

it says it needs a "shunt" to work, which i also found on ebay for like $6. although i dont really understand what that is or where to put it either!

thanks for any insight you guys may have!!

1981 240D ... 265k
1977 240D ... 265k
1977 240D ... 250k
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Old 11-26-2012, 12:24 AM
fashion victim immunizer
Join Date: Feb 2012
Location: oakland
Posts: 930
i have a small, cheap backlight analog voltmeter mounted under the dash on the right side. it's wired behind the console and spliced into the power that goes to the radio. you could wire it into the cigar lighter as well.anything else might be needlessly complicated (in terms of going through the firewall, etc)

from my reading of the description for that piece, you'd only need the shunt if the input amps were over 100?
1981 NA 300D 310k miles
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Old 11-26-2012, 12:40 PM
Registered User
Join Date: Sep 2006
Location: MA
Posts: 524
A little electricity 101...

A voltmeter is a parallel device. Touch the negative to ground, and the positive to any 12v line in the vehicle...the battery, the lighter plug, you will get the same reading. With the engine off, you will be reading battery voltage...12v or so. With the engine running and alternator working, the reading will be 13-14v.

An ammeter is a different animal. It is a series device. It measures current flow. In order to measure current flow, the electrical connection must be broken (i.e. the supply wire from the battery), and the meter placed in series. (one side of the meter to the battery, the other side to the wire you just removed).

When they specify a shunt resistor, you must still break the connection, insert the shunt resistor in series, and then use a voltmeter to measure the voltage drop across the shunt resistor. The voltage drop across the resistor will be a function of the current going through it.

DO NOT put an ammeter directly from positive of the battery to ground. It will short out the battery, and fry the meter.

In short, voltmeters are a great measuring device for the health of your battery and charging system. They are easy to use, and you're not in danger of doing much harm. Ammeters, on the other hand, you need to know what you're doing.
Mark in MA
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Old 03-01-2013, 07:35 PM
wilburtual's Avatar
240D + 240D = :)
Join Date: Oct 2010
Location: Los Angeles,CA
Posts: 192
Thanks for the info & tips, I appreciate it. i'm gonna just wire the volt meter to ground and then to the cig lighter 12v. I'll probably just pass on the amp meter so i dont fry anything (including myself!)
1981 240D ... 265k
1977 240D ... 265k
1977 240D ... 250k
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Old 03-02-2013, 11:27 AM
Registered User
Join Date: Apr 2011
Location: SF, CA, USA
Posts: 931
i'm also considering running a length of 4 or 2 gauge wire through the firewall so i would have an ammeter in the glovebox. i suppose putting it under the hood would be easier, but much less convenient.

i have some ammeters that don't use a shunt but work in the clamp style - you have to thread the insulated wire through a metal channel. so one loop of ground wire through the FW, through the ammeter, and back is not too heinous a mod to make, especially since my car has only one negative strap from battery to chassis.

PS, it will be safer to measure amperage on the negative side of the battery; in case there is a catastrophic wire failure, no short to the chassis.
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Old 03-02-2013, 01:40 PM
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Join Date: May 2009
Location: Sacramento, CA
Posts: 631
The one I found on ebay just plugs right into the cigarette lighter.
Much easier to install

1984 300SD 300,000 miles--two tank WVO setup
2.88 diff & 500SEL anti-squat rear end
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Old 03-03-2013, 08:35 PM
Silber Adler's Avatar
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Join Date: Oct 2012
Location: Living on a gravel road in a Red State
Posts: 593
If you battery voltage is good You should not need anything else to tell you if your alternator is keeping up.

85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.
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