Electrical Ramblings                        05/93 R. Kwas

At the risk of explaining the obvious, an amp-meter, or ammeter, is a measuring device that indicates amps or electrical current (GEE!). It may be installed in any circuit to show the magnitude of current flowing to the load (load being the general descriptive for the device which does the work--light bulb, electric motor, radio--you get the point. Isn't electricity a splendid thing?) The good folks at Robert Bosch even use a neat little trick that balances bulb currents against each other to determine a difference between sides, i.e., right and left brake light current, which enables them to inform the driver that a bulb is out, although it may be out for a number of reasons.

When referring to the ammeter in a car, however, the electrical location, and therefore its information to us, is very specific, but sometimes the source of some confusion which I would like to clarify. To aid in this, I present my (almost) copyrighted "H" diagram which I have been using forever:

If you follow the four different current paths around, you essentially have all possible paths that occur in a vehicle. Path (1) Starting--straightforward--the actual magnitude of this current is not really of interest (unless we are in the business of repairing and checking starters) and quite high, so high in fact, that we would certainly risk damaging a +60 amp meter. Let's not measure it, since we will know when it flows by other means. (2) Load--again we will lump a bunch of things into this category and again we are not really interested in seeing a precise indication of the current--we will know in short order that the radio or the blower or seat heater is or is not working.

(3)(4) Charge/Discharge--now we're getting to the important stuff. In this path we will have the currents going from the battery to our loads. We will call this discharge current. This current becomes of concern to us because it serves to relieve the battery of its ability to get us started (if we're parked, motor off) or tells us our alternator is not making it! (motor running). Current in the other direction, charging current, is an indication of energy going back into the battery from our source, the generator/alternator. Sometimes, such as right after using current from the battery to start the motor, we would expect to see all of this current returned to the battery (the voltage of the battery drops because of starting, so the regulator orders up a recharge from the alternator, lead-acid batteries like being fully charged--always!) Our ammeter is effectively connected to show us which of these currents--charge or discharge is greater at any one time. I present this definition: the ammeter shows the mean battery current. It is therefore an excellent indicator of the state of health of the charging system of the vehicle. It's too bad but many cars just aren't outfitted with amp-meters anymore. Wendy and I have just become parents so we replaced our Merkur XR4Ti with a Mommy Wagon 745 Turbo (once you go turbo, you never go back). On Volvo turbos such as this, a voltmeter is standard equipment. Quote from the manual: "The voltmeter indicates the voltage in the electrical system and thereby also the state of the battery." Did you notice the difference? Ammeter monitors charge system. Voltmeter monitors battery.

The information supplied by the voltmeter, however, is only marginally useful since (see chart, special thanks to Mr. Yoder of Interstate Battery) the 100 to 0 percent charge condition happens within a mere 8 tenths of a volt span! Even with an expanded scale voltage meter (many automotive voltmeters are) by the time we notice a low voltage condition we are already in trouble. This, as opposed to a current meter which as soon as it shows discharge while the motor is running signals a charging system problem--thankfully while we still have an almost fully charged battery to get home or to safety on. Furthermore, the only absolute voltage reading that is truly indicative of the "state of the battery" is the "open circuit voltage," that is, before the alternator is charging or the loads are discharging the battery. When the motor is running, the voltmeter indicates the vehicle's system voltage which will be pulled up or down as a function of mean battery current.

I believe the reasoning behind the trend of installing voltmeters instead of current meters is (without getting too technical, I guess it's too late already?) because voltmeters are (slightly) simpler to incorporate into the automotive electrical system. I suggest, therefore, for those drivers cursed with a voltmeter (It does beat an idiot light only*) to make this simple modification which will make the instrument more useful: with a fully charged battery and the motor not running, observe and mark the "low normal" voltage with a small dot of white paint on the meter glass. Start the motor and after a minute or so, after the voltage settles out, mark the "high normal" voltage in the same way. Later, when the indication deviates from these levels it will stand out, clueing you into a possible abnormal condition. By making this simple modification, you have essentially turned the voltmeter into a 3 zone instrument (Under, Normal, Over), and like the 2 zone ammeter (Discharge, Charge) have made it, I believe, somewhat more informative.

For my money, I always install an ammeter when doing my alternator retrofit/upgrade (I guess that's a plug for my kit), but I'm including a basic troubleshooting chart which covers all metering situations and hopefully helps you interpret better the information your meters are giving you. In conclusion, don't let your ever-increasingly complicated car electrical system drive you crazy. Remember, take troubleshooting one logical, informed step at a time and eventually you will get to the root of the problems, and please...remember not to call it "juice."

*122S Owners read on: Speaking of idiot lights, did you ever wonder why the amp light glows faintly when you switch on your blower? First of all its not indicative of a major problem, although the brighter it glows, the less your battery is charging, and with the original 30 amp generator let's not waste any charging ability if we can avoid it. The effect is due to an excessive voltage drop across the fuse block and it usually gets worse with age since it's due to corrosion between the stacked brass/bronze contacts. The low-contact-area, usually also corroded fuse ends, don't help either. Even the gold and /or stainless end fuses that are available now aren't a long-term fix since they still result in a condition of dissimilar metals in contact. The long-term fix I consider a must-do for this condition is a dab of Penetrox (the anti-corrosive grease required by electrical code for aluminum to copper connections) for all euro fused Volvos which makes a "Gas Tight Joint" and prevents this corrosion by excluding the electrolyte-moisture. My recommended fix for the corrosion between the bronze contacts involves a Restored Fuse Block. I have these available also.

Link to good additional battery info:  http://www.uuhome.de/william.darden/

Link to AmpMeter discussion:  http://www.brickboard.com/RWD/index.htm?id=1213119

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