The dreaded electrical Gremlins strike again!!

The breaker for the landing light is tripping. In a Mooney it is built
in to the switch and the switch will not stay on. I'm thinking that
there is a short somewhere but I don't know why that would cause the
whole avionics bus to go flakey...

Jon Kraus
'79 Mooney 201
4443H @ TYQ

Dan Luke wrote:

wrote:


From what you describe, the multisystem failure would tend to indicate
something in common. Possibly a shared breaker (avionics avionics
master), but most
likely a single-point, shared ground that's flakey.

Most GA aircraft are a multi-layered hodge-podge of crappy electrical
wiring.
In particular, grounds tend to be wherever the last boob to install it
had a pre-cut
wire long enough to reach. Having a single-point where everything
grounds is a good
thing to eliminate ground loops. Gotta make sure that one point is
solid, though.


Concur. Most likely it's not a short, which would pop a breaker. An
intermittent open or high resistance connection would fit the symptoms,
and the ground connections are the places to start looking.

"Jon Kraus" wrote:

The breaker for the landing light is tripping. In a Mooney it is built in
to the switch and the switch will not stay on. I'm thinking that there is a
short somewhere but I don't know why that would cause the whole avionics
bus to go flakey...

Ah, that's different. That might account for some crazy symptoms if it's an
intermittent, high resistence short to ground. It could be pulling the
voltage down enough to make the avionics act up if *they* have a power or
ground connection problem. Perhaps a wire is being pinched somewhere, so
that changing the rpm causes the airframe to flex enough to affect the short.

At least the LL breaker gives you a clue where to start looking. I'd go back
to where the bus feeds the LL circuit and disconnect it, then see what
happens. I'd also still check all the ground and power bus connections,
master switch, etc. for high resistance, as you might have one of those
delightful two-headed gremlins.

Good luck and keep us posted.

--
Dan
C-172RG at BFM

The landing light had a wire shorting to ground causing the breaker to
trip. The avionics issue looks like it was a loose wire to the avionics
bus. At least we'll see anyway. Nothing glaringly obvious like the
landing light.

Jon Kraus
'79 Mooney 201
4443H @ TYQ

Dan Luke wrote:

"Jon Kraus" wrote:


The breaker for the landing light is tripping. In a Mooney it is built in
to the switch and the switch will not stay on. I'm thinking that there is a
short somewhere but I don't know why that would cause the whole avionics
bus to go flakey...


Ah, that's different. That might account for some crazy symptoms if it's an
intermittent, high resistence short to ground. It could be pulling the
voltage down enough to make the avionics act up if *they* have a power or
ground connection problem. Perhaps a wire is being pinched somewhere, so
that changing the rpm causes the airframe to flex enough to affect the short.

At least the LL breaker gives you a clue where to start looking. I'd go back
to where the bus feeds the LL circuit and disconnect it, then see what
happens. I'd also still check all the ground and power bus connections,
master switch, etc. for high resistance, as you might have one of those
delightful two-headed gremlins.

Good luck and keep us posted.