Mobile Radio Wiring and Grounding
There is a
serious flaw with
mobile radios be
wired across battery
negative radio lead
should actually not
have a fuse, and not
be wired to the
History of Vehicle Accessory Equipment Grounds
had both positive
passenger vehicles evolved, standardizing on
“12 volt” negative
ground systems. The resting voltage was around 12.6 volts, with ideal running voltage
in the low 14 volt range.
large commercial vehicles, however,
grounds. Non-standardization of systems meant
two-way radios and other add on equipment was generally designed to
either negative or positive
grounds. This was accomplished by floating the internal negative supply buss,
while grounding all normally accessible external user ports to the case or
These early systems started the trend of negative fuses, with both power
leads directly attached to the battery. This was not harmful, because the
equipment had a completely isolated internal “ground ” that was electrically
isolated from all other leads leaving the device.
Once any other lead common with the negative bus leaves the case, it is no
longer safe to conned the negative to the battery post. It is also not safe to
fuse the negative lead.
In most early CB and
commercial two-way radios,
the negative buss inside the unit fully floated from the cabinet and any
external ports. This included commercial two-way radios
like the Motorola Motrac, Micor, and other expensive, high-quality, radios. The
floating negative was universal across brands including, but not limited to, GE
and RCA land mobile equipment.
The floating negative
allowed use of radios in either
negative or positive
ground vehicles. It also solved ground loop issues, allowing direct connection
to battery posts without fire or equipment damage hazards.
Equipment manufacturers had no way of knowing if the final installation would
be in a negative ground vehicle or a positive ground vehicle. As a result,
systems with a floating negative buss were shipped with both negative
and positive power line fuses. The floating negative buss system inside the radio allowed safe
direct connection to the battery posts, and safe shutdown if either the positive
or negative fuse opened.
The floating negative power buss made it impossible for starter or charging currents to flow through antenna
cables, microphone, or speaker leads. All exiting connections, as well as the
case, were electrically isolated from the negative lead.
Negative Ground Only
Equipment and Radios
Over time, vehicles with positive grounds disappeared. As this
happened, manufacturers stopped using the more-expensive and more-complicated
floating negative power bus system.
Many vehicle manufacturers, and
most aftermarket equipment manufacturers,
never re-thought the systems others were using. Manufacturers carried
over the acceptable negative fuse idea appearing in ground independent power
buss systems, which could also use a negative battery post connection.
Manufacturers misapplied the allowable fused negative battery post connection to
internally grounded negative
Not realizing the safety hazard, they continued to fuse both negative
and positive radio power leads and often advised direct battery post negative
connections. The battery post connection actually created ground loop, fire, and equipment safety
This terrible idea has permeated the aftermarket accessory market,
including amateur radio, audio, and
performance or race car electronics markets. For example, MSD Ignition’s installation instructions advise
a direct battery post negative connection, putting the vehicle’s distributor,
the MSD box, and
other electronics at risk.
Although absolutely incorrect, a popular assumption is fusing the
can protect internal and external equipment wiring, including gauge, computer, speaker, microphone, key leads,
and antenna connections from
ground connection damage.
|The only battery post
negative connection should be to or from another battery negative, the
vehicle chassis, and/or the engine block. There should never be a direct negative post
path to accessory equipment.
Normal vehicle wiring is shown and discussed in detail
on this page link.
current in this
If the ground
or negative wire
from the radio to
the battery opens, radio current
would flow from the
radio out through
the antenna cable,
the speaker jack,
the key jack, or any
other jack or
connects to the
radio circuit board
grounds to the vehicle chassis.
If the battery to
engine block ground
opens, or the engine
block and battery to
open, the battery
will ground back
through the negative
radio connection to
the radio, through
circuit boards or
and to the vehicle
radio negative lead
fuse will protect
circuits, it will
not protect small
traces or components
like those connected
to insulated jacks
For example, when my radio system lost a negative fuse
connection, foil traces for
the keyer paddle
grounds inside my ICOM 751A
burned open. I could no longer send CW.
The traces opened even though the
negative lead was
fused, because the radio’s transmitter current of ~20 amperes flowed through
small circuit board traces to my CW key. The CW key was grounded by touching a
metal bracket, and this melted foil traces on a circuit board inside the radio.
With a grounded negative buss and battery post connection:
- Anytime the battery
negative to chassis
ground opens, vehicle accessory and lighting currents, or a portion of
vehicle accessory and lighting load current,
will flow through the
- Anytime the negative fuse opens, the radio will ground
itself through the coax or other other external connections
- This also sets the radio up for increased alternator
whine, since load currents can superimpose themselves on traces and
connections inside the radio.
Connecting the radio or accessory equipment’s grounded
negative supply does nothing good. Grounding to the battery negative post
whine. It also places the
system at higher
risk, is unsafe for the
radio or accessory, and is unsafe for things connected to the radio.
I’ve had vehicles with higher power radios that have opened a
negative fuse, and then melted the coaxial cable shield on RG58/U cables. This
is because, once the fuse connection was lost, the
equipment grounded itself to the vehicle chassis through the grounded antenna
It is pretty
simple to improve
If we ground the
radio negative lead
to a separate but
good chassis ground near the
battery ground, ground
loops through the
system, through the
radio, and through
to the radio or accessory, are completely avoided.
Ideally, this ground should not share the bolt that grounds the battery to
chassis ground cable. The negative power ground should be
on its own bolt. The fasteners should be good hardware,
with proper star
washers. The ground must provide solid, reliable,
If the radio or accessory device’s chassis
power ground lead opens,
only radio or accessory current
will flow through
the radio circuit
boards. While this
may still open a
trace, it is much
less likely to
happen because we
have eliminated a
needless weak point,
the negative fuse,
and we have
direct connection to
the battery where
battery acid slowly
eats away at
A vehicle chassis power ground completely
current path through
the radio or accessory if the
engine block ground
battery lead opens, and
alternator AC ripple
no longer can drive
the negative lead to
the radio. Any
connection can fail,
and in the very
worse case possible
we have only radio
In most cases, we
will not even have
It also helps to have multiple ground points as additional insurance against
the negative power lead accidentally floating.
Who Else Agrees with Me?
Published standards specifically tell us to not ground to the
battery post, and not to fuse the negative lead!
|4.6.4. Negative Feed Connection
In the case of negative earth return vehicles, the negative power line
connected to the vehicle body as close as practical to the point at
made. Do not connect the negative power line directly to the battery.
For heavy commercial vehicles (>7.5Tonne GVW) only, and those
cab may be isolated from the chassis by rubber mountings, a ground
manufacturer within the cab to provide battery to cab grounding.
fuse box. It is recommended that this point be used for installations
With certain equipment it may be necessary to connect the negative
this case an existing vehicle earth point must be used.
Motorola manuals for radios without an internal floating negative buss
tell us the following:
Motorola warns to not use a negative fuse or battery
negative post connection, but rather directly connect the negative lead to the
Only the positive lead is fused.
Only the positive lead goes to the battery post.