From: Ian Williams
Sent: Friday, 27 August 2021 4:30 p.m.
To: '[email protected]' <[email protected]>
Cc: '[email protected]' <[email protected]>
Subject: RE: Dimona stuff
A slight addition .. Of course the other end of the pressure switch is
connected to ground.
So no hour meters work until you are in the air. When engine is stopped in
the air, the Airframe one keeps going and the engine hourmeter stops.
From: Ian Williams
Sent: Friday, 27 August 2021 4:25 p.m.
To: '[email protected]' <[email protected]
<mailto:[email protected]> >
Cc: '[email protected]' <[email protected] <mailto:[email protected]> >
Subject: Dimona stuff
Hi Kurt and Michael,
I must have bad breath or something because some of my posts to the DOG
group seem to vanish into free space. However I hope I can help
Because our Dimona was ex Thai, it had a lot of wiring which was not needed,
so I completely re wired it. Rather than use the original cable driven
taco, I installed a VDO electric taco Part No 333-953 driven by a little
Hall effect sensor from Aircraft spruce which screws into the bottom vent
hole of the Slick magneto. A real plus is being able to calibrate it using
a 50Hz mains sample . ie 50Hz equals 3000 RPM.
Refer to the attached picture . the 2 hour meters are also from VDO.
The way I connected them is the common (-VE) of both hour counters connected
to a small pressure switch (HERGA Manuf part No 6743-0003) .. operated by
pitot pressure at around 30Kts. The positive end of one switch (airframe)
connected to the plus bus, the other (engine) connected to the relay next to
the voltage regulator on the right hand side of the firewall looking
forward.
If you refer to the wiring diagram on page 25 of the service and maintenance
manual, notice the oil switch (the oil pressure sender) has 2 connections
(actually 3 if you consider the earth being the case) It is 12 and 17 ..
One output drives the VDO oil pressure gauge (15) and the other is a
switched output driving the relay (9)
That relay when active enables the voltage regulator to work only when the
engine is running. Presumably a cunning trick to keep the current draw as
low as possible when the engine is off soaring.
So the plus supply for the engine hourmeter can be taken from the output of
that relay.
It is a real simple system and has worked faultlessly for 500 plus hours.
Of interest, I have since replaced the Cyl Hd temp gauge with a MGL 4 cyl
colour barograph unit
I would be interested to know if you actually have received this email.
Best regards
Ian Williams
ZK-GCB
