Message-ID: <000b01c16a2c$460013c0$5c30b43f@f3g6s4>
Reply-To: "Dan Schaefer" <dfs155@earthlink.net>
Reply-To: lancair.list@olsusa.com
From: "Dan Schaefer" <dfs155@earthlink.net>
To: "Lancair list" <lancair.list@olsusa.com>
Subject: Relay Problem
Date: Sat, 10 Nov 2001 13:11:29 -0800
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Rick, you have a classic case of a bad, or intermittent connection somewhere
in the circuit that energizes the relay. Found one of these on a friend's
Lancair down in LA where the crimp on a wire terminal just wasn't making a
reliable contact. Tracing down an offending connection is difficult
particularly when the thing works some of the time. It's often the result of
using the wrong size terminal barrel (too large) for the wire size.

Another likely culprit is the master switch contacts. If you don't have a
diode across the relay coil, each time you turn the switch off, there's a
substantial arc generated by the coil inductance across the switch contacts.
This will cause the contacts to burn and become unreliable in a relatively
short (but indeterminate) length of time.

For the bad connection case, what usually is happening is that when a crimp
is first made, the wire has just been stripped and is nice and clean and
corrosion-free. Under these circumstances, a poorly crimped barrel still
allows good enough contact for the problem to be masked. As time passes, the
barrel and the wire begin to accumulate some surface corrosion because they
are not mashed together tight enough to create a gas-tight pressure bond (a
good crimp creates close to a pressure weld between the wire and barrel). At
best, this causes an unreliable, and intermittent connection - at worst, an
open circuit.

The only way I know to find a bad crimp is to go through your entire relay
switching circuit from the relay terminal, through the master switch and
eventually, to ground (most master relays are energized by the switch taking
the active terminal to ground with the other terminal taken permanently to
the battery positive) and giving each crimp an enthusiastic "pull test".

A way to get an idea if your problem is in the wiring or master switch
contacts is to activate the relay with a clip-lead right at the relay. If
the intermittent action goes away doing this, you can be sure your problem
is in the wiring or the master switch. If it still occurs, there's not much
left causing the problem but the relay.

If the clip-lead test gets rid of the problem, you're left with determining
whether the problem is a crimp or a bad switch. Use the clip-leads to jumper
across the switch contacts - if the problem persists it's not the switch and
a bad connection in the wiring is the cause, if the problem again goes away,
the switch is the bad guy.

Incidentally, the probable reason that the thing works after some chattering
is that arc I mentioned. Each time a bad connection allows the relay coil to
de-energize, the inductive arc is generated. Sooner or later, the arc
creates a weld (usually microscopic, but enough to hold the relay energized)
at the offending connection and the relay stays on. Due to the (usually)
very small weld produced the last time this all happened, the next time you
try to energize the relay, the inrush causes the tiny weld to act like a
fuse and it burns open, starting the process all over again.

Hope this helps.

Dan Schaefer

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