Date: Thu, 2 Jul 1998 17:42:46 -0400
Reply-To: averill@LDL.NET
Sender: Vanagon Mailing List <Vanagon@vanagon.com>
From: George Averill <averill@LDL.NET>
Subject: More light on the 84 "red light" mystery
Content-Type: text/plain; charset="us-ascii"
Here are the results of my efforts to solve the mystery of the red low
level coolant light that never stops blinking. Some of you may be
interested in trying to figure out how the electronics work.
So far I have replaced everything. The light stoped blinking for several
days after I installed a used temp gauge then started blinking constantly
again. My next step will be to purchase a new gauge assembly from the
dealer, but I really don't want to spend that amount of money and have the
same problem occur again after several days.
There are four components in the system.
1. (volt stab, part No. 171-919-803, around $10 from a VW dealer.)
This is really a three prong 10v regulator which supplies a constant 10v
to the temp gauge and the fuel gauge. One pin goes to 12v. One pin goes
to ground, and the other pin supplies a regulated 10v. The tab also
connects to ground and uses a large copper portion of the flexible wiring
harness as a heat sink.
2. ( Cool low lvl wrng sw, part No. 251-919-372, around $20 from a
VW dealer.) This isn't really a switch, but consists of two probes with
about 1cm of exposed wire on the ends which extend into the
water/antifreeze solution in the coolant expansion tank. A single wire
connects one of the probes to pin S of the cool low lvl wrng contro unit
(No. 2 below). The other probe is connected to ground
3. (Cool Low Lvl Wrng Cntro Unit, part No. 191-919-376A, around
$30 from a VW deaer.) This is a plug in module mounted on the left side of
the body beneath the dash. There are several other modules/relays attached
at this same location. This one can be recognized by the markings on the
bottom of the pins, G, 15, S, and 31. It also has two yellow/red wires
attached to pin G. Pin 31 connects to ground. Pin 15 connects to +12v.
Pin S goes to one probe of the cool low lvl wrng sw. Pin G connects to the
negative terminal of the temp. gauge. (Note: There are other
modules/relays mounted directly behind the fuses. These modules/relays are
not the group of modules where the cool low lvl wrng cntro unit is
located.)
4. (A gauge module consisting of temp. gauge, gas gauge, clock,
part No. 251-919-511A, around $117 from the dealer. You can't buy just a
temp gauge.) The temp gauge contains an integrated circuit consisting of
one eight pin integrated circuit marked SEC 2741D0, two diodes, four
resistors, one red LED, one 10mf capacitor. This circuit has three
connections. One connection goes to the side of the temp gauge which is
connected to the 10v voltage stab. One connection goes to the other side
of the temp gauge which then connects to the cool temp sndng unit mounted
on the engine. The third connection goes to a pin which connects to ground
(the negative side of the 10v voltage stab.
Anyway, that's what I have learned for what it's worth. I still have
several questions.
1. What type of signal does the cool low lvl wrng cntrl unit output?
2. How does the circuit built into the temp guage work to detect the
signal and cause the red diode to start blinking.
3. How does the circuit built into the temp guage test the diode upon
turning on the ignition and then stop it from blinking.
4. A schematic diagram of the circuits. (not the wiring diagram from the
Bentley)
Hope I didn't take up too much space with this message. I'm looking
forward to hearing from someone who can shed more light on this subject.
