Top 12 Wiring Mistakes
Jim Smart
November 11, 2015
After walking car shows for more than 40 years, we have seen tons of
interesting rides. Open hoods say a lot about the person who owns the
Mustang and how much they know about automotive electrical systems. We
see wiring bundles, splices, and connections that leave us awake at night
wondering if the guy got home without help from the fire department. We’re
talking horrendous electrical wiring mistakes that not only lead to darkness,
but disaster. Un-protected circuits void of fuses or circuit breakers, vulnerable
wiring subject to chaffing and short circuits, way too many accessories on one
circuit, HID headlights without relays, big subwoofer sound systems with the
original low-amp Autolite alternator and a single battery, engines not properly
grounded, and the list goes on. A safe electrical system is your wealth in a
classic Mustang. All wiring, connections, switches, and accessories should be
adequately protected to the point where they can be considered failsafe and
foolproof. We asked the pros the most common mistakes they’ve seen over
the years, how to avoid them, and how to correct them.
1. Improper Grounding
This is easily one of the most common mistakes enthusiasts make with their
electrical systems. Unlike your household electrical system where you have
three wires—neutral, hot, and ground—automotive electrical systems have
one wire (positive) and the vehicle body/chassis (negative/ground). Electricity
flows through the positive lead through to the accessory or light to negative
ground at the body/chassis and back to negative at the battery. There are two
theories on current flow. Some say positive to negative while others say
negative to positive. Nonetheless, you must have a solid connection between
the two.
“The majority of our wiring questions from customers can be traced to
improper grounding,” Andrew Erichson of Painless Performance told Mustang
Monthly. “You need to be sure you have completely rid a grounding surface of
any paint, rust, or other contaminants.” Andrew added this also applies to
painted surfaces where light sockets fit and make contact to ground. All
grounding surfaces must be metal to metal.
When we spoke with Bill Tumas, CJ Pony Parts’ video restoration expert, he
echoed Andrew’s comment on grounding. Improper grounding causes more
electrical gremlins than any other issue. It is best to have several grounds
where possible to ensure a complete circuit. You must always have a ground
strap between the engine and firewall because rubber engine mounts tend to
insulate the engine from the chassis (ground).
2. High Amp Devices Without Relays
We see this one all the time—too high a load for the switch or circuit-breaker
capacity. If you’re lucky, this pops the circuit breaker or blows a fuse. If you’re
unlucky, your Mustang will burn to the rims. Wiring, relay, and circuit-breaker
capacity are crucial to proper and safe electrical-system function.
When you look at your Mustang’s electrical system like a fuel or plumbing
system, it makes sense. You wouldn’t expect a 600hp engine to be able to run
and make power with a 5/16-inch fuel line would you? Your engine would
sputter and fall flat on its face. Not enough electrical capacity is like trying to
drink a Slurpee with a cocktail straw. You can draw on that straw with
everything you have until you have busted eardrums, but the icy stuff just isn’t
coming. This is called resistance. Do this with a greater diameter straw and
flow is plentiful and resistance low.
Wiring is the same way. If you tried to turn your Mustang’s starter with
speaker wire it would immediately heat up and melt. Your high-amp starter
requires very heavy-gauge cable and a starter solenoid to make the
connection. Your ignition switch sends power to the solenoid, which is a relay
that makes the connection between battery and starter. The starter winding
energizes, thereby cranking the engine. The starter relay also gets full power
to your Mustang’s ignition system for start-up.
The same can be said for high-amp devices like sound systems, electric
radiator cooling fans, and high-intensity headlights. You can’t just switch them.
You must have a switched relay to handle the amperage, along with the wiring
capacity to handle the load. Andrew Erichson of Painless not only suggests
relays, but a complete wiring package engineered to handle greater electrical
loads.
Accessories like electric radiator cooling fans, subwoofer sound systems, and
high-intensity headlights pull down a lot of amps. You need a heavy-duty
switching device to cycle power on and off with high-amp electrical devices. A
light duty switch energizes the relay, which completes the high-amp electrical
connection. Never be fooled into thinking you can run high-amp accessories
through a light-duty switch.
Halogen headlights can generally be powered via your classic Mustang’s
headlight switch. However, if to do it right, opt for a headlight relay and take
the load off your Mustang’s headlight switch.
3. Insufficient Charging System
Classic Mustang owners tend to get lost in the 1960s when it comes to
electrical system demand. We perceive a 65-amp Autolite/Motorcraft
alternator will carry a high-output sound system, electronic engine control, or
high-intensity headlights.
If you’ve upgraded your classic Mustang with a fuel-injected 5.0L High Output
V-8 engine, as one example, you will find quickly a 65-amp alternator won’t
keep the battery charged. It will slowly run down until you’re unable to start the
engine.
If your classic Mustang is little more than an AM/FM stereo and halogen
headlamps the 65-amp Autolite/Motorcraft alternator with a solid-state voltage
regulator will get you through, along with a 700 to 800 cold-cranking amp
battery. If you have added power accessories and electronic engine control,
you’re going to need a 130- to 200-amp alternator to keep your battery happy.
While you’re at it, disconnect and bypass your Mustang’s factory ammeter.
Most burned up during the first year of operation; leaving them connected
makes for a fire hazard.
Autolite/Motorcraft 65-amp 1G alternators are adequate for concours
restorations and dead-stock daily/weekend drivers—and only with solid-state
voltage regulators. However, if you’ve fitted your classic Mustang with the
latest modern conveniences like a hot sound system, high-intensity
headlights, and electric cooling fans it will not have the capacity to keep the
battery charged.
The Mr. AMP 3G single-wire internally regulated alternator from Performance
Distributors is an easy drop-in replacement for your 1G charger. This guy is
prewired and Performance Distributors gives you detailed instructions.
Installation is easy and you’re not likely to screw it up. The Mr. AMP 3G
comes with cables and circuit protection for your safety. While you’re at it,
bypass your Mustang’s factory ammeter.
4. Wiring in Harm’s Way
This one ranks right up there with poor grounding. We see some of the
darnedest wiring jobs in our travels, and quite frankly, it makes us nervous.
Willy-Nilly unprotected wiring strung all over the place is dangerous, especially
if unprotected by a fuse, fusible link, or circuit breaker. We see connections
directly to the positive post or starter solenoid, with no obvious circuit
protection. It spells disaster if it shorts to ground. Wiring should be wrapped
and protected and follow a logical path from origin to destination. What’s
more, it should be protected with a fuse, circuit breaker, or fusible link. Fusible
links are generally located between the battery and main wiring loom, though,
early Fords didn’t use them. A fusible link, when overloaded with a short
circuit or extraordinarily high amp draw, will melt and open the circuit like a
fuse does.
Your wiring should never look like this. Tangled and twisted up is hazardous
to your Mustang’s health and safety. Unprotected, they become vulnerable to
chaffing, heat, and the elements. And what’s with this upside-down ignition
coil?
Wiring should never be this vulnerable. An engine harness, which includes
ignition, senders, and charging system, should always be wrapped and
protected.
If your existing wiring harness is loaded with splices and open butt
connectors, it is oftentimes smarter to replace the entire wiring harness with a
new Alloy Metal harness from CJ Pony Parts. A new harness is simply safer
and can be installed in a matter of hours. What makes the Alloy Metal harness
superior to other reproductions is close attention to detail. Wire colors are
extruded, not painted, for permanent identification. Plugs are easier to
connect. Instrument lamp sockets will accommodate all LED bulbs without
popping out. Too many of us try to save old wiring harnesses when it’s wiser
to start over again with new.
5. Careless Splices
Careless splices are another careless mistake enthusiast’s make time and
time again. If you’re in the habit of using household wire nuts in your
Mustang’s electrical system Andrew Erichson of Painless stressed to stop it
immediately. He has seen it enough times to comment and add it to our
electrical do’s and don’ts. Household electrical wire nuts work fine in home
use because houses don’t move, as a rule. Automobiles move and vibrate,
calling for a failsafe electrical connection. When you need to splice electrical
connections, use butt connectors and crimping pliers at a minimum. Do not
use diagonal cutting pliers. For best results, solder all connections and
insulate them properly.
Be careful with your splices and wrapping. This is how you do it. Mustangs &
Fast Fords OC (MFFOC) is installing an Alloy Metal Products stock electrical
system in its 1965 Mustang convertible showroom project. Paul Gammerino of
MFFOC has chosen Powerbraid from Painless Performance to protect his
Alloy Metal electrical system. This is how you splice and protect a premium
electrical system.
This is not how you splice and connect. These are vulnerable connections that
can leave you stranded. Ideally, solder insulated boots to your ignition leads
and protect them from the elements.
6. Dimmer Switch Protection
There’s a reason Ford installed this plastic insulator between the dimmer
switch and floor pan. If you don’t have one, please get one immediately. The
dimmer switch is made of cast aluminum and your floorpan is steel, and
touching dissimilar metal can cause corrosion. This is a region where moisture
can get in, so the problem becomes even more critical. You don’t want a short
to ground at the dimmer switch. This piece also properly routes switch wiring.
We see more dimmer switch installations void of this important insulator,
which goes in between the dimmer switch and floorpan. Don’t forget to install
yours.
7. Improper Use of Tools
“It is always surprising to me how many people get into their Mustangs to do
electrical work without a set of wire strippers and crimpers,” Andrew Erichson
told us from his desk at Painless Performance, “Many connectors and
terminals require the use of a jaw style or rollover crimpers to be safely
crimped.”
Pertronix has long been known for innovative ignition products that have
made our lives as hobbyists easier. Check this out. We like this professionalgrade wire crimper from Pertronix. Put your diagonal cutting pliers away and
invest in this timesaving tool. It is designed to crimp virtually any size wire and
yields professional results.
8. Battery Unsecured
We’re astonished how many batteries we see in Mustang battery trays that
are unsecured. We’ve seen them tied down with bungee cord and coathanger wire. Fact is, your battery must be secured with a stud and nut battery
hold-down in the interest of safety. We’ve seen batteries fall into radiator
cooling fans. We’ve also seen them explode because they moved about the
battery tray and shorted to ground, causing the spark that ultimately ignited
hydrogen gas fumes at the battery vents. Boom!
9. Resistor Wire With Pertronix Ignition
Here’s a mistake we’ve made ourselves. Pertronix Ignition instructions are
extremely specific, and they tell you to bypass Ford’s pink factory resistor wire
when installing the Ignitor electronic ignition module. The Ignitor must have 12
to 14 volts to perform as designed. When you use your Mustang’s factory
resistor wire, which was designed for your factory ignition coil and pointtriggered ignition, the Pertronix Ignitor won’t get enough current to function
properly. Another mistake we see, and have made, is proper grounding of the
Ignitor module. Your Autolite/Motorcraft distributor must have the ground strap
between breaker plate and baseplate or your engine won’t start. And if it starts
without the ground strap, the engine will stall sooner or later.
Classic Mustangs were factory fitted with this pink resistor wire, which was
engineered for the original Autolite/Motorcraft ignition coil and point-triggered
distributor. This wire is between your Mustang’s ignition switch and the factory
ignition coil. If you’re running a Pertronix Ignitor and Flamethrower ignition
coil, you must bypass the pink ignition wire. Aftermarket ignition systems must
have the full complement of 12 to 14 volts to function properly. Another option
is to use the Pertronix ignition relay kit and trigger the relay with the stock
resistor wire, too.
10. Overloaded Circuits
Jim Thayer at Alloy Metal Products, which makes outstanding reproduction
wiring products for CJ Pony Parts, National Parts Depot, and other major
Mustang parts retailers, told us one of the biggest mistakes people make is to
second-guess electrical engineers. We splice and look to factory fuse boxes
when adding circuits, Jim tells us. Jim stressed to use the additional factory
plugs Ford designed into your Mustang’s main wiring harness if you’re adding
light-duty accessories. All Alloy Metal Products wiring harnesses have the
auxiliary factory switched and constant live plugs found in original wiring
harnesses. What’s more, all Alloy Metal harnesses have extruded color-code
wiring to keep wire identification from being lost via chaffing or cleaning. You
cannot remove the stripe identification.
Proper wire sizing is what building a safe electrical system is all about. Too
many of us improperly size wiring for the load and wind up in trouble. You’re
always safer by oversizing wiring to the load. American Autowire’s website
stresses proper wire sizing, switching, and circuit protection for the load. You
can buy wiring systems ready made for your application or consult with
American Autowire’s tech team to get the right answers.
When we spoke with Jim Thayer at Alloy Metal Products about how to add
circuits to the factory wiring harness he stressed using the connections Ford
and Alloy Metal provide, and knowing when and how to add protected
auxiliary circuits with relays for high-amp draw accessories. Never overtax the
stock wiring harness with a dangerous splice.
Absolutely never operate a Mustang’s electrical system without circuit
protection. And never run a higher amp fuse than a circuit is designed for.
Fuses are there for your protection. Bypass them or install an accessory
without them and you risk fire.
Circuit breakers do the same thing as fuses, except they automatically reset
themselves when the short expires and contacts cool. Never use more circuit
breaker capacity than a circuit is designed for. Your Mustang’s headlamp
switch is equipped with an automatic circuit breaker. It cycles the headlights
off and on when there’s a short circuit or contacts in the switch are corroded.
From 1965-1967 circuit breakers for accessories like air conditioning are
mounted and connected on this switched post. This wasn’t one of Ford’s
better ideas. It can be unsafe if it shorts out before the breaker.
11. Watch For This Post
We will always wonder what Ford electrical engineers were thinking when
they came up with this one—the power post from 1965-1967 Mustang ignition
switches. It is live when the ignition is on or in accessory mode. It is also
vulnerable to short circuits and fire. We often add a long rubber vacuum cap
or electrical tape to cover and protect the stud from accidental contact.
From 1965-1967 Ford ignition switches had this auxiliary power switched post.
It provides power for accessories and is normally fitted with a circuit breaker
but not all of them were. Always cap this post with a rubber or plastic insulator
to prevent accidental short to ground experiences.
12. Not All Bulbs Are Safe
We learned this one the hard way. Periodically inspect your Mustang’s light
bulbs for safety issues. Closely inspect the filament and filament support posts
for distortion that can cause an internal short circuit and potentially a fire. We
have experienced at least one taillight bulb internal short that melted a main
wiring harness. When we traced the cause it was learned one of our taillight
bulbs suffered an internal short when one post leaned over and touched the
other. We’ve only seen this once, but it could happen to you.
Periodically perform a light bulb inspection on your classic Mustang. Some
parking and taillight bulbs have been known to short internally.