Date: Tue, 6 Oct 1998 17:47:02 -0400
Reply-To: Bulley-Hewlett & Associates <gmbulley@BULLEY-HEWLETT.COM>
Sender: Vanagon Mailing List <Vanagon@vanagon.com>
From: Bulley-Hewlett & Associates <gmbulley@BULLEY-HEWLETT.COM>
Subject: All you want toknow about Auto electrical,
was: tracing electrical problem
Can someone walk me through the process for tracing a failed electrical
part such as power mirrors? Maybe clue me in on what I would need to do
such a thing. I've checked the fuses and they seem fine, but I am unsure
of how to tell why something isn't working and where to start the
deductive process---
T'would be a pleasure Tony-
First off, congrat's on checking the fuse FIRST. In a recent USA TODAY
poll, 94% of the mechanics surveyed said they would replace FOUR expensive
electrical parts before remembering to check the fuse.
Here is a primer on VW electric, just in case.
All electrical items in your vehicle require two forms or "directions" of
electricity: Positive, (supplying electrons), and Negative, (sucking in
electrons). You can think of these like the water pipes in your home (if
you are lucky enough to have indoor plumbing). The positive is like the
pressurized water coming in, and the negative is like the water (sewage)
going out.
The important difference in an electrical circuit, you must have BOTH
positive and negative simultaneously to each device... otherwise the device
doesn't work. That is VERY important.
In a simplified view of your car(forget about the ignition system, and
charging the battery), if you started the car, and the alternator was
charging, but you had no electrical devices on, it would be like turning on
all of the faucets in your house(positive electron flow), and letting ALL
of the water go back down the drain (negative electron flow),. What goes in
comes right back out.
If you turned on a few "Loads", like the headlights, and the fan, it would
be like turning on all the faucets in your home but using some of the water
for watering your lawn, and filling the swimming pool. Some water still
goes down the drain, but some other just "goes away" from the circuit. And
so it is with the electrons in your wires, wherever you turn on a load, the
electrons change state, becoming light, or heat, or motion. I'm simplify
ing, but you get the idea.
To save money, (since cars are made out of metal, which conducts
electricity), car companies (like VW) use the metal body of the car to
conduct one of the "directions" of electricity. Almost universally, the
body of the vehicle carries the negative charge. This saves money because
the car company only has to run ONE wire to each electrical gizmo, the
positive wire. They use the body as to other wire. No matter where you go
in the car, the body is always nearby, so a short little screw or short
wire usually suffices to "ground" the electrical device. This idea of
"grounding" is also VERY important. Remember, the ground provides the
negative electron flow.
Now, most car companies (like VW) do something that is very clever. They
color the wires that lead to all of the electrical devices. Unless the
wiring is in a specialty harness (like the fuel management system) the
wires conform to a special wire color scheme. VW conforms to certain "tr
ends" in wiring coloration, certain types of devices having (historically)
a certain color wire.
Many a VW home mechanic will tell you right away what a turd-brown wire is,
a GROUND. Turd-brown with a white stripe is a switched ground, and so on.
Getting back to you mirror problem, it boils down to a couple of simple
tests. First, you have to make sure that positive and negative are flowing
all the way to/from the power mirror motor. It is highly unlikely that the
motor is not "grounded" as (I think) it is bolted to the door, which is
grounded. So that leaves the positive, flowing from the fuse, to the
switch, and on to the power mirror motor.
You need two things to proceed without undue strain. First, a
current-sensing device. This could be an expensive
double-jeweled-multi-phase-chromium-plated multi-meter (list price, $200)
or it could be a COTS (Commercial, Off the Shelf) test lamb with probe
(list price, $8.00 @ FLAPS [Friendly Local Auto Parts Shyster]) The other
thing you want to have on hand is a Bentley or Haynes manual. These will
display a schematic (electrical map) of your vehicle, with all of the wire
colors listed.
The test lamp will provide instructions for how to use the probe to gently
pierce the wire and test for current. Obviously, you will do this with the
vehicle on, and will start in the least obtrusive manner, testing the wires
as they lead INTO the door. If you don't have current there, then the
problem lies "upstream", that is, between where you are and where the
battery/alternator are in the circuit. In between you and the
battery/alternator is the fuse box, the ignition switch and a long harness.
If there IS current flowing into the door, OFF comes the precious door
panel (use a pick-ax and chisel to hack it away from the door frame).
Now test at the point where the current flows into the power mirror motor
(don't forget, the ignition will need to be on, and the mirror switch will
need to be in one of it's "active" up/down/left/right positions) Got
current there? Well then the circuit is good, and the motor is a bum. Call
Ron or Kyle or VW or God. No current there? It's the switch or the wiring
to the switch.
Still no current? Oh yeah, Check the fuse. I always forget that. :)
Hope that helps.
G. Matthew Bulley
Principal
Bulley-Hewlett & Associates
Communications for Organizational Development
www.bulley-hewlett.com
(888) 468-4880 toll free
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