Download Basic Electrical Diagnosis

Basic Electrical Diagnosis
By Dave Brisco
http://www.eastcoastroadster.com
I seem to spend a lot of time with people going over the basics when diagnosing an electrical issue, so I thought it may
be helpful to get it in one place. I repair fire alarms and other DC circuits for a living, some with miles of wire, but the
basics are the same, whether it’s a flashlight, a circuit on a car or a 30 story building. What you need to start with are
a wiring diagram, a volt meter, a 12 volt test light and a fully charged battery. No fancy tools needed, total investment
as low as $40.00. An auto-ranging digital multimeter can be found at Sears, The Craftsman Multimeter is Good,
the Amprobe Compact Digital Multimeter, AM-240 is Better, and yet the Fluke 115 Multimeter is more
Professional. I use a Fluke 77III, and the Good as a backup. They work equally as well.
A 12 volt Test Light is under $10.00; get one with a long cord and a strong clip. A fully charged battery is 13.2 volts
or above. Tape or heat shrink the long metal probe of the test light up to ¼ inch of the tip. The test light is good for
fast checking, the meter for more exact readings. Wire Diagrams (found on www.311s.org Tech WikiWiring
Diagrams) for almost every Roadster are on 311s.org with many thanks to those who drew them and posted them for
us; they’ve saved me many hours. Print it up nice and large; a few people have put it under a sheet of Plexiglas.
Without these tools and information, you can waste a lot of time on simple things.
I’ll use 12 volts in the discussion, but it would be whatever the battery reads, + to – terminal, usually about 13-13.5
volts. (Test light in parenthesis) All this is for negative ground cars.
The basic of any circuit starts at the battery and returns to the battery via the ground cable. Using a simple switch and
light bulb as an example, you start at the battery + terminal, through a fuse, then to the switch in, switch out, to the
bulb center, through the filament, through the bulb base to the socket, and to body ground, then to the battery -. Other
circuits on the car have more connections, and components, but the basics are the same.
Set your meter to DC voltage, or DC20V if it’s manual. The concept is that the problem is between your meter leads
(test light clip and probe). To find it, one lead stays on - and you move the + lead along the circuit to the point at
which you stop reading voltage (light out). The problem is between this point and the last point where you had 12 volts
(light on). It’s sometimes helpful to practice on a working circuit first, so you can see what normal readings are. In
the chart below, if the meter has one lead on black, the other on red, you read 12 volts. (With a test light, if the clip is
on one color, probe on the other, the test light will be on.)
Normal
Switch on Bat+__FUSE__SWITCH___SOCKET+__BULB__SOCKET-__BODY___BatSwitch off Bat+__FUSE__SWITCH___SOCKET+__BULB__SOCKET-__BODY___BatFaults..(switch on)
Blown Fuse Bat+__FUSE__SWITCH___SOCKET+__BULB__SOCKET-__BODY___BatBad bulb Bat+__FUSE__SWITCH___SOCKET+__BULB__SOCKET-__BODY___BatBad ground Bat+__FUSE__SWITCH___SOCKET+__BULB__SOCKET-_ _BODY___BatBroken wire Bat+__FUSE__SWITCH_ _SOCKET+__BULB__SOCKET-__BODY___BatBad switch is the same diagram as with the switch off.
With the switch on, and the black meter lead (or test light clip) to ground, follow the voltage with a meter or test light
from end to end. First, to verify you have a good ground, you should read 12 volts to + battery (light on). On a good
circuit, you’ll see 12 volts (light on) all along the circuit wherever the text is red. The filament of the bulb “consumes”
the voltage, so from that point of the circuit back to the battery - terminal, you would read 0 volts. If you put the red
meter lead on battery +, you should see -12 volts all the way from the battery- terminal back to the bulb socket. In a
good circuit, all readings should be equal to or within half a volt of battery voltage. That’s the basics; the real world
varies in interesting ways. Any reading less than 12 volts and more than 0 indicates a bad connection or other
problem, like in the next example.
We’ve all seen it; the car in front of you in traffic has taillights acting oddly. When they step on the brake, the brake
lights come on, the other lights go out or dim. In some cases, if you step on the brakes the front parking lights come on
dim. The battery + voltage seeks ground, and if that connection is broken, it finds a way through any available path.
Most commonly for our cars this is a bad or disconnected ground wire, a corroded connector or a single filament bulb
in a dual filament socket, a common Previous Owner issue.
It’s a tough concept until you actually do it for real. A volt meter measures the difference in voltage between + and -.
This is where the voltage difference comes into play. In the example above, the rear stop/tail lights have a bad ground,
but the front parking light ground is good. The voltage travels through the stoplight, to the disconnected ground shared
with the rear parking lamp, through the rear parking light bulb + wire, to the front park lamp bulb and to ground. Since
the voltage is now going through 3 bulbs, they each consume 1/3 of the voltage, 4 volts, so they light dimly. You’ll
get readings of 4 volts, 8 volts etc. instead of 12 volts and if you remove a bulb, other things light up or the whole
problem changes.
For general diagnostics, it’s best to isolate the circuit with the problem and break it down to its simplest components.
If the readings get strange, stop, look at the diagram, clean any relevant connections you can get to and try again. Even
though it looks like a nightmare, if you take a minute and follow the wire diagram on paper, things become apparent
you wouldn’t see looking at the car. Things like a wire shared by 2 different things, and they both don’t work, so you
now know it’s a problem from the point where the circuits join back to the battery. It takes time, but one day it all
becomes clear and you can almost figure out the problem from looking at the diagram before even touching the car.