Date: Sat, 21 Feb 2009 20:51:50 -0500
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Where does this relay go?
In-Reply-To: <4d1b79350902211713u1a5098f0o2953a5271f06e1e0@mail.gmail.co m>
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At 08:13 PM 2/21/2009, Jim Felder wrote:
>the coolant light blinked slowly at first, but as the engine heated
>and the temp needle rose, the blinking became more and more of a
>constant red light until, at needle center, it was on full time. When
>I reached my destination, I turned it off and restarted and it was
>fine ever since.
Fascinating. I've never heard of like behavior. What year is this beast?
>I don't know what's in the control module, but I doubt that it is
>really a relay...
It's not. It's a rather fancy circuit that sends a very
high-impedance (weak, if you will) square wave out on the active
level sender terminal. If that square wave is tied to any constant
level between ground and +12 through a resistance of about 180K or
less it will be quenched, and the gauge driver remains inactive and
floats with the temp sender output. However if the square wave
continues for several seconds without being quenched, the gauge
driver goes active and pulls the sender line down to a level
corresponding to an overheat condition (not all the way to ground,
which would burn up the gauge). That triggers the flashing light in
the gauge, which flashes for several seconds whenever it's triggered
(and it's this behavior that gets out of hand inside the gauge and
leads to constant flashing for no cause).
At this point the old (large) and new (small) controllers part
company. The old one applies a steady overheat signal until the
square wave can reestablish itself, which drives the needle into the
hot zone. The new one applies an intermittent signal which keeps
triggering the blinker without greatly affecting the gauge
reading. The new one also latches on, so the blinking will continue
until key-off regardless of what the level sender sees.
I had a lot of enjoyment tracing out and analyzing the interior
circuit of the old-type controller. It was cleverly done. I haven't
seen inside the newer one, but I suspect they put things into a
single no-doubt-proprietary chip that were wired out on the old one,
since the new one is half the size and does more.
--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'89 Whitestar "Scamp"
