On Wed, 28 Oct 2009 16:28:04 -0400, wrote:

On Wed, 28 Oct 2009 15:30:30 -0500, rich
wrote:

My homebuilt's master contactor is going bad. Sometimes when I turn it
on it doesn't make connection. It's got 1700 hours on it, so I'd just
as soon replace it. But the way the builder wired it, he's has
positive power from the battery going through the master switch to the
small terminal on the contactor. (cole-Hersey type) But the master
contactors, such as Spruce sells, are set up to actuate with ground
power going to the small terminal. They also have plastic around their
mounting feet so their case doesn't make ground. A starter contactor
would work perfectly with the way the plane is wired. I just wonder,
are starter contactors made to withstand continous use, like a master
contactor does? And how can one tell the difference in the two, they
look identical? And if not, can the master/continuous duty type be
made to work with postitive power to the small terminal?


Check the coil resistance. Constant duty contactors are higher
resistance, so they draw less wower and heat up less than starter
solenoids.

Does the coil resistance change much as the contactor wears out?
Somehow, the contactor coil seems to get weak over time, or the
contacts inside are dirty or both. My master contactor would click,
but not turn everyhing on until I cycled it a couple times.
One thing I've discovered is, on new contactors, the coil resistance
on continuous duty vs. intermittent duty isn't much different. Just
barely enough to make a determination of what type they are.
I've got one new intermittent one reading 14.7 ohms, and a continuous
one brand new from ACS reading 15 ohms, then another continuous one
that reads 17.3 ohms. I wish there was a more difinitive reading
between the two types. They are so close, it's hard to make the call
using that method.