Years ago, the 2 way shop I was working in ran into the same issue with
a truck in a fleet that we took over. The previous shop had just detuned
the receiver to the point that it was nearly deaf, but since it was a
city utility truck that ran around under a repeater, the symptoms were
so masked that the customer thought it was fixed. Anyway, we ended up
calling GMC (it was a Chevy truck) and they had a factory bulletin on
how to fix this. The result as I recall was that it was a master
reference oscillator in the computer. The fix was to obtain a free xtal
from GMC to change the clock speed, and therefore move the birdie to
another, non interfering frequency. Crude, but it worked.. Mike Pugh KA4MKG
Tim S. wrote:
> This is a little off topic but I could use some help.
>
> I am working on a fire truck that is getting interferrence on the VHF
> (156.075mhz) from the trucks wiring.
>
> If I put an HT near I get the interferrence on the HT. When I disconnect the
> battery it goes away.
>
> We removed one fuse at a time and it went away when we pulled the fuse to the
> trucks computer ECM module.
>
> We tried an ECM module from another truck w/o the interference problem and we
> still had it in the bad truck.
>
> Next I tried sniffing around with a spectrum analyzer and found the highest
> noise near a large bundle of vehicle wiring. We are talking a tightly
> bunched 2-3 inch bundle.
>
> Tried adjusting the squelch to open at .5uV but that did not help.
>
> Need some idea on how to proceed.
>
> Thanks
>
> Tim
>
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> Yahoo! Groups Links
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