[Elecraft] Connecting Transceivers in Vehicles - Change ofRecommended Practice

[Lew Phelps K6LMP]

Major auto makers offer conflicting guidance on this subject.

In sum, Chrysler says negative lead to the body near the battery, not fused. 

Ford says both leads to the battery, but doesn't give clear guidance on fusing the negative lead.

GM says it's best to connect the negative lead to the chassis, but IF you connect it to the battery terminal directly, then you should fuse it, in case the ground lead from the negative side of the battery to the chassis becomes disconnected (in which case your rig will be the car's only path to ground.)

No wonder there's confusion.  

What makes the most sense to me is to attach the negative lead directly to the chassis, near the battery, and with no fuse. That's not what I have in my vehicle right now (currently have both leads to battery, both fused, but I will change it based on this thoughtful dialogue.

Here are direct quotes from the documents available online provided by the "Big Three" US car makers:

Daimler-Chrysler says:

Special design considerations are incorporated into this vehicle's
electronic system to provide immunity to radio frequency signals.
Mobile two-way radio and telephone equipment must be installed
properly by trained personnel. The following must be observed during
installation.

The positive power connection should be made directly to the battery
and fused as close to the battery as possible. The negative power
connection should be made to body sheet metal adjacent to the
negative battery connection. This connection should not be fused.

(and later in the same document)

>DO NOT FUSE THE GROUND LEAD. If the ground-side fuse were to open,
the entire supply current would be conducted by an alternate current
return path, which could cause the feedline to overheat, with
possible resulting damage.


Ford Motor Company says:

Transceiver power connections should be made directly to the battery and appropriately fused as close to the battery as possible. A weatherproof fuse holder is recommended. Twist the positive and negative power leads together to enhance noise immunity.

(Ford is silent on the issue of whether to fuse the negative power lead or not, but implicit in their instruction (fuse holder is singular, not plural), is the notion that only the positive lead should be fused.


GM says:

3.) Connect the positive lead to the auxiliary power terminal and connect the negative lead to the battery body connection point (identified by a short #10 AWG or larger wire running from the negative battery terminal to the body of the vehicle).

If connections are made directly to the battery terminals, the GM approved methods of connecting auxiliary wiring include the adapter package illustrated in Figure 2, NAPA-Belden replacement battery bolts (part # 728198), or drilling and tapping the hex end of the original battery bolts 10-32 X 3/8" deep. NOTE: It is recommended that a fuse be placed in the transceiver negative lead to prevent possible transceiver damage in the event the battery to engine-block ground lead is inadvertently disconnected.