[Milsurplus] Accessory Bus in vehicles

[Sheldon Daitch]

Tom,

My comment about the differing accessory bus setups was prompted
with what I have found with some newer GM vehicles.

I rented a 2004 Blazer and found that the radio would stay on,
after I had turned the vehicle off and the ignition switch in the 
OFF position, until I opened the door.  I never tried it, but I 
assumed the radio would have stayed on if the ignition switch was 
in the accessory mode, no matter what the position of the door, 
open or closed.  I guess the beauty of this was that you could
switch off the vehicle and remove the key, yet still listen to 
the radio until you got out of the vehicle.

I also did not figure out if the ACC position actually powered
anything else other than the radio, such as the cigarette lighters,
power windows and ventilation fan, or if the cigarette lighter 
also switched off with the radio upon opening the door, was hot
at all times, or only hot when the key switch was in the
ACC or run position. I also never checked to see if the radio 
mode in ignition switch OFF would time out, or the radio would 
play until the battery died.  Another question for inquiring minds.
Maybe someone here have a late model GM that keeps the radio on, 
until the door is opened, and could check. 

In the case of this Blazer, there is the possibility of 
3 ACC modes:

 1 - operates only when the switch is in the RUN position

 2 - operates when the switch is in the RUN or ACC positions

 3 - operates when the switch is in the RUN or ACC positions 
     or until the door is opened.

I whole heartedly agree with you on using the ACC bus to drive
a relay, for both safety and the life of the ignition switch.
And for those of us who run/ran Progress Line or other vintage
radios of that nature, the relay was the only way to go!

73
sheldon
WA4MZZ





Tom Norris wrote:
> 
> I always *made* a standardized hookup when I was doing installs - which
> I did for over 20 years, both in private shops and for various govt agencies.
> Simply used the ACC bus to drive the relay coils that actually switched power
> to the power accessories used. Almost always tapped it at the fuse block, since
> it was easier - for the standardization reasons already cited. Since I kept
> my own switched power bus the same from vehicle to vehicle, I only had to
> dig for the accy power once per vehicle.  And with each relay fused, I never
> had a problem with the equipment damaging something expensive like an
> ignition switch. :-)
> 
> Tom
> 
> >It makes it a lot of fun when you are trying to install alike
> >radios, equipment or
> >lightbars in dissimilar govt, mil or public safety vehicles.  No
> >standardization...
> >Jay
> >
> >----------
> >Just a little comment on the ACC mode in automobiles, it is
> >interesting to note how the different manufacturers model
> >their ACC circuits in today's
> >vehicles.
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