And another thing about my instrument cluster that should be reported
to the Society for Short-Lived Phenomenon is that some months back I
had the turn signal indicator LED working for about a minute.

Jim

On Sat, Feb 21, 2009 at 8:02 PM, Jim Felder <jim.felder@gmail.com> wrote:
> David, it's a 1983 diesel.
>
> Jim
>
> On Sat, Feb 21, 2009 at 7:51 PM, David Beierl <dbeierl@attglobal.net> wrote:
>> 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"
>>
>