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Ammeter Wiring Help

Discussion in 'Technical Q&A' started by Mike328, May 5, 2004.

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  1. Mike328

    Mike328 F1 Rookie

    Oct 19, 2002
    2,655
    Boulder, CO
    Full Name:
    Mike
    #1 Mike328, May 5, 2004
    From time to time, I investigate (if purely for academic reasons) the installation of an ammeter into my 308/328. I like to know what's going on with the electrical system aside. Let's put aside for the moment arguments about the effectiveness in automotive applications of voltmeters of ammeters, as well as the risk of electrical fire if large enough wires are not used, etc.

    One gauge I'm looking at is a +/- 60A VDO Vision series ammeter. Basically, their installation instructions say, "Put the ammeter in series between the alternator (+) terminal and the starter solenoid (+) terminal." (According to the 328 wiring diagram, normally there is already a 16 gauge wire between the alternator and starter. It appears we're putting the ammeter in series with that connection.)

    Here is VDO's installation instructions for ammeters:
    http://www.egauges.com/pdf/vdo/0-515-012-154.pdf

    The part that I *don't* understand is, WHY do they have the ignition switch wired into the "Load" terminal of the ammeter?

    According to the 328 wiring diagram, the "hot" or swtiched ignition +12 voltage is already connected to the alternator, making this connection redundant. (I.e., if coneptually it's the same "wire", it doesn't matter "where" on the wire you connect it to.)

    It seems then that, in the 328 application then, all that needs to be done to wire an ammeter is to remove the wire leading from the alternator (+) to the starter (+), and put the ammeter is series with this connection (now requires two wires: one leading TO the ammeter, one leading FROM the ammeter). Using, say, 10 gauge wire. Part of the challenge is getting two thick wires from the cabin to the engine area, of course.

    Do I have it right?
     
  2. CraigFL

    CraigFL Formula Junior

    Jan 17, 2001
    954
    Panama City, FL
    Full Name:
    Craig
    #2 CraigFL, May 5, 2004
    According to the VDO wiring diagram, the sole purpose of the ammeter is to measure the net current going into or coming out of the battery(as a plus or minus reading). The regular electrical load is attached to the alternator side of the ammeter so the alternator can be used to supply the necessary current without indicating the load current- you only want to show the "charging" current in and out of the battery to see if your alternator is working properly. The VDO information is only shown that way to be sure that you put the load current on the correct side of the ammeter. If you would break into the alternator-starter wire in the wrong place, you would be showing Load current and not Charge current.
     
  3. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    9,632
    #3 Mitch Alsup, May 5, 2004
    By locating the ignition switch after the ammeter the meter sees the current that the ignition switch allows to go to the rest of the car (e.g. the switched 12V source). What the ammeter reads is the amount of battery current the goes to the switched load (alternator off) in the negative direction, and how much current the alternator can supply to recharge the battery. Note: ammeter is NOT measureing how much current the alternator supplies to the switched load, but only how much is used to recharge the battery. Now if you wanted to know the current going to the load, you put another ammeter in the load circuit.

    Note 2: the ammeter itself requires very little currerent (microamps typically) to move from null (zero) to full scale. So there is a very low Ohmic resistor in serives with the ammeter itself to allow current to pass with very little voltage loss (around 10 mV). The resistor should be in 0.001 Ohm range.
     
  4. Mike328

    Mike328 F1 Rookie

    Oct 19, 2002
    2,655
    Boulder, CO
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    Mike
    #4 Mike328, May 5, 2004
    Mitch, Craig: Thanks so much for your posts.

    1. I'm still a little bit confused about the actual WIRING, because the VDO instructions DO NOT SAY to "break" the wire that otherwise pre-installation went from the alternator to the starter. Should I just infer this as "intuitively obvious to the casual observer"? :)

    2. OK, I think I got the idea that the ammeter is NOT measuring TOTAL alternator output, that is, NOT measuring the total current draw required by all the devices in the car. If this were the case, then as you turned on more accessories, the ammeter would read more. This is NOT that case, as has been pointed out.

    Rather, the ammeter is measuring NET FLOW to the battery, in other words, "what charge or discharge is the battery seeing?" If this is the case, we'd like to be a somewhat on the "+" side after the car starts, and then during normal operation, once the battery has been recharged, we'd expect it to be basically "dead on" except for sudden loads (cooling fans coming on) for which the voltage regulator in the alternator has to "catch up" to... This might result in a brief "dip" of the needle but if the alternator is working properly, it should beef up power output to meet the demand, and get the battery back up to "just barely charging if at all."

    Do I have that right?

    3. I've read about two types of ammeters. One measures big time, thick, heavy current in the system... The other just does some trick and measures "voltage drop" and converts that to a current reading. Also, this gauge uses an internal "shunt" I believe. Which type is this particular ammeter?

    4. VDO recommends 10 gauge wire to and from the ammeter. With this type of setup, and I really carrying that much current (up to 60A) up to the cabin and back?

    5. VDO offers these gauges in +/- 30, 60, 100, and 150A. Which one is best for this particular applicaiton, in your opinion?

    6. I'm still unclear on what, if anything, I should do to get the switched +12V voltage to the ammeter. The wiring diagram shows a wire ALREADY running from ignition switched +12V TO the alternator (+), right there with the wire leading from the starter (+) to the alternator (+). Does this need to be connected to the ammeter "L" terminal INSTEAD of the alternator, IN ADDITION TO the connection at the alternator, or NOT AT ALL given that it's already running to the alternator (+) to which the ammter ("L") terminal will already be connected to?

    7. Ideas on how to run wires from the starter/alternator area up to the cabin?


    As you can see, we're well on our way to writing a "Technical Installation Guide" here, and as I did with the brake lines with your help I would be glad to do so! It'll be the best guide ever.

    Thanks all for the help...


    Take care,

    --Mike
     
  5. atlantaman

    atlantaman Formula 3

    Mar 31, 2002
    1,726
    Roswell, Georgia
    Full Name:
    Charles
    #5 atlantaman, May 5, 2004
    well the ammeter should have 3 wires from it--1 to the battery, 1 to the alternator, and the warning/alarm wire--it runs up under the drivers seat and attaches to the drivers left testicle.
     
  6. jselevan

    jselevan Formula 3

    Nov 2, 2003
    1,873
    #6 jselevan, May 5, 2004
    Mike - the switched 12 volts is merely the source of power for all items controlled by the ignition switch. Thus, you need a 10-12 gauge wire from the L+ of the ampere to the ignition switch "hot" contact. (Yes, you will draw 60 amps easily during normal evening driving).

    Current leaving the battery (forget the starter for a moment) will flow through the ampere meter, through the on (closed, i.e., conducting) ignition switch, to the various loads.

    Current leaving the alternator will flow to loads (through the ignition switch) AND to the battery. When the battery voltage has dropped below the regulator threshold, the alternator generates more current. "Excess" current produced by the alternator, not going to loads, will flow through the ampere meter into the battery.

    Thus, when the battery is supplying the load, current flows through the ampere meter (from battery to load) and the ampere meter reveals negative amps (discharge). When the alternator is supplying the load current, AND the battery is below regulator threshold, some current will flow through the ampere meter to the battery (charge).

    Hope this helps.

    Jim S.
     
  7. BigTex

    BigTex Seven Time F1 World Champ
    Owner Rossa Subscribed

    Dec 6, 2002
    79,321
    Houston, Texas
    Full Name:
    Bubba
    #7 BigTex, May 5, 2004
    Someone along the way installed a VDO Voltmeter in my 308GTB.

    It was recently disconnected by an enthusiastic stereo installer, who did not know how to reconnect it!

    Has any one seen my clock??? I want it back!!!

    The voltmeter did show the barely strong enough alternator output, but was redundant to the stock indicator light in the dash.
     

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