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Avoid the Top 10 Electrical Snafus

How Not to Look Like an Electrical Moron by Avoiding the Top 10 Classic Electrical Snafus

Photography by , Mark Hamilton, Terry McGean


Have you ever had a set of electric gauges change readings when you turnon the headlights? This is caused by a poor ground circuit, a powercircuit that cannot handle the additional load, or both. Most '60s carsprovide power for the wiring harness through a tortuous path leading upto the fuse box under the dash. There are plenty of opportunities forpower loss and voltage drops along the way.

The quick fix is to improve the ground path between the instrumentcluster and the battery by adding those ground straps you threw awaybetween the engine and the firewall. Then add one or two between theinstrument panel and the firewall and see if the performance improves.


If you've ever run a wire from that poor overloaded fuse box to power upan electric fuel pump, or if you've used a large wire from the batteryto a switch and then to a heavy-duty electrical load like an electricfan, there's a better way to go than large clunky switches. Relays are ahandy heavy-duty switching device that can be activated by a very lowvoltage switch. In fact, computers use relays to controlhigh-current-draw items like electric fans and electric fuel pumps.

Here's how relays work. Let's say you're going to bolt on a 20-ampelectric fan. You could run a large 8-gauge wire from the battery allthe way up to a switch under the dash and back to the fan. Instead,mount a relay between the fan and the battery. This creates a shorterpower path from the battery to the fan through the relay. Then you canuse any light-duty switch to control the fan. Relays also work well forreducing the voltage drop on '60s GM car headlight circuit. Mount a pairof relays near the headlights and eliminate the long power path throughthe headlight switch. This shorter path through the relays puts muchmore voltage to the headlights, making them much brighter.


How many wires do you have jammed into that overloaded fuse box underthe dash of your street machine? Most musclecar electrical systems werenever intended to handle great electrical loads. So when you startjamming more wires into those "switched" and "unswitched" fuse-boxpositions, this greater load could eventually cause problems. Not onlythat, but it just looks cluttered.

The solution is to use a couple of MAD's terminal blocks, one forconstant battery power and the other for "switched" power when theignition key is on. The key is to mount two terminal blocks on a commonaluminum mount located under the dash near the stock fuse block. Thebattery live terminal can pull power directly from the junction blockoff the battery under the hood. The second terminal can be switchedusing a relay triggered by a "switched" terminal on the fuse block. Thebattery live terminal should be protected with a fusible link locatedunder the hood. Now you can bundle all those cockpit accessories tosource power from these two terminal blocks and not overload the stockfuse block. If you really want to be safe, design a clear plastic coverfor the terminal blocks so there's no way to accidentally short theblock to ground.


There's confusion about aftermarket one-wire alternators versus typicalthree-wire alternators. A typical production-style three-wire alternatoruses a voltage sensing wire connected to a main power distribution pointin the wiring harness. This "sensing wire" allows the voltage regulatorwithin the alternator to read electrical system voltage resulting inproper voltage delivery to the wire harness. A three-wire alternatoralso has a special switched "turn-on" wire, and this wire can also beused to operate a warning light at the dash. A one-wire alternatorrequires internal voltage created by the spinning alternator to triggeror start the charging process since it does not have a voltage-sensingtrigger connection. When the engine is started, a one-wire alternatormust achieve a certain speed in order to reach that internal voltage.Once that occurs, the one-wire alternator will begin charging and willcharge even at very low engine speeds. This means you must simply revthe engine above a given rpm (usually around 1,500 depending upon thepulley ratio) before the alternator will begin to charge.

According to a rather substantial treatise on one-wire vs. three-wirealternators on MAD's Web site, the voltage drop from the alternator tothe battery with a long wire can compromise the performance of theone-wire alternator compared to a three-wire. This is why MAD recommendsusing the GM 10Si or Cs130 alternators, even in many Ford applications.This is not a condemnation of the one-wire alternator. But if you wantthe most from an alternator, the three-wire version is slightly moreefficient.


What the hell is a fusible link? Think of it as a safety net for yourelectrical system. Most domestic cars from the mid '60s through the '70ssource all the electrical power for the car (except for the starter)from either the main charging harness or, in the case of GM cars, fromthe smaller wire pulled directly off the positive battery terminal. Toprotect the wiring harness, most of the car companies used aninexpensive wire called a fusible link that is designed to melt when thecurrent demand exceeds a given level, like from a direct short. Thisprotects the wiring harness from damage, but also immediately disablesthe car. Often, ignorant wire hackers set the car up for a potentialwiring meltdown and subsequent fire by eliminating this fusible link.

If your GM car is now equipped with a fusible link, you can purchase aninexpensive replacement from MAD in 12-, 14-, 16-, and 18-gauge sizesdepending upon the circuit they are protecting. It's also a good idea toprotect individual circuits with their own fusible links. Fusible linksare sized to protect a circuit four wire sizes larger. Also rememberthat larger gauge numbers indicate a smaller wire. So a 14-gauge fusiblelink would protect a circuit using larger 10-gauge wire.

Voltage-Drop Test

This one simple test will tell you more about any specific electricalcomponent than any other test you can perform. The idea is to measurethe amount of voltage that is lost while the circuit is in operation.What we're really doing is using the voltmeter to learn the amount ofresistance in the circuit without having to remove any components, butcurrent must be flowing at the time of the test.

To begin, set the multimeter on the lowest voltage setting--most oftenthe 20-volt scale. Let's say we're going to test the amount ofresistance in the negative battery cable. A high-quality cable with agood connection should only have about a 0.1-volt drop across itslength. To test it, place one probe on the battery end of the negativecable and the other probe on the end of the cable where it's bolted tothe engine. Disable the ignition system so the engine doesn't start,then have a helper crank the starter while you watch the multimeter.During cranking, current will flow through the circuit, and thevoltmeter will indicate a voltage. If you measure more than 0.1 volt,there is high resistance in the circuit. You can also narrow your focusas much as you wish. For example, to find the resistance in just theconnection between the battery and the cable, place one probe on thebattery post and the other on the cable end, then crank the starter. Ifyou see more than 0.05 volt then the connection is poor.

A higher voltage measured with the voltage-drop test means greaterresistance. Increased resistance means the electrical system isdelivering less power (current) to the load, like the starter motor,headlights, or electric fan. The more power you can deliver to the load,the more efficiently it will operate. It's just that simple.

Volt-Drop Values

Component Acceptable Volt Drop
Battery Cable 0.1 to 0.2 volt
Switch 0.2 to 0.3 volt
Alternator output to Battery* 0.4 to 0.5 volt
Wire to Electric Fuel Pump 0.1 to 0.2 volt

* The voltage-drop spec between the alternator and the battery is thetextbook maximum. However, many factory wiring systems get around thisspec by using a main "buss-bar" for central power distribution. Thisversion routes the alternator output and the voltage-regulator sensingwire directly to the buss-bar. In this layout, electrical systemperformance relies upon voltage level at the buss-bar not at the batteryor alternator. This means a voltage drop greater than allowed by theabove spec may not indicate a problem. Chevy musclecar-era systems wereequipped with this layout with a higher acceptable voltage-drop spec.There's more specific information on this topic

MAD Enterprises
Dept. 5.0
P.O. Box 675
CA  93265
Painless Wiring
9505 Santa Paula Dr.
Fort Worth
TX  76116
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