Date: Tue, 8 Jul 2014 21:39:55 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Westy water tank level indicator problem
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>1) You can test the input with a potentiometer and voltmeter. Input
>lead is biased to B+ with one megohm, electrodes ground out
>successive resistors in a voltage divider inside the little black
>module by the tank. Result compared against similar network at the
>left-side LM324 chip.
First ground the green wire. If all LEDs do not go out, skip
immediately to the second section, LM324s die a lot. Read this
section for info later. Otherwise, the test for the thresholds is to
put a digital voltmeter from the green wire to ground and either:
a) operate the operate the sender float on newer ones (be *very*
cautious, if you bend the rod the tiniest bit in the wrong direction
it will break one or both reed switches inside because no slack was
left when soldering them). The meter will pull the voltages down
somewhat, see NOTE 1 below and method/formula at c).
b) ground all the sender terminals on the older tanks to the bottom
terminal and then remove them beginning from the top, same comments
about meter as above.
c) disconnect the green lead and connect to ground via one end and
middle of a 10-20 megohm pot, preferably linear taper, and adjust the
pot. With this method you can find the actual thresholds. A ten
megohm pot and a ten megohm meter will give you about 10.5V maximum
which is plenty. If you want to know what the actual applied
resistance is at each threshold, unhook the green lead, switch the
meter to ohms, write down the value and calculate Reffective = Rmeter
x Rpot / (Rmeter + Rpot). Same method if you want to know how the
meter is altering sender resistances in previous methods.
d) disconnect green lead and connect positive side of a variable
power supply to it, negative side to ground.
CAUTION: do not apply power with the panel turned off (if the panel
power switch needs is pulling out of the solder fix that first to
avoid possibly losing panel power while your supply is driving the line).
CAUTION: Do not exceed the measured voltage present on the green wire
with panel on and nothing connected, and do not allow the voltage to
go negative more than a few millivolts. Some variable supplies may
go slightly negative when set to minimum, so check and beware. The
chip can handle a very slight negative input; that's how the flame
detector LED operates, the fridge thermocouple is the only voltage in
the van that's officially negative to chassis ground. But the
emphasis is on very slight.
e) connect various combinations of dry cells likewise. See CAUTION above.
f) connect individual resistors likewise ranging down from ten
megohms. This is the hard way for sure, but if you have a bunch of
resistors it works.
NOTE 1: with either method where you're actually supplying voltage,
adding about 5,000 ohms in series will prevent any excitement from
shorting a wire to ground. The green wire itself can be grounded safely.
NOTE 2: If you're not actively supplying voltage you will get most
accurate results with a ten megohm (or higher) voltmeter. Most
digital meters have ten megohm sensitivity on voltage ranges, but
some cheap ones may be one megohm. A ten megohm meter will drag the
green wire down to about 11.5V all by itself, and a one megohm meter
will drag it to about 6.3V, severely limiting your testing.
Assuming the LED panel is being fed by 12.6V, the following voltages
at at the end of the green wire should have the following results
with a few per cent variation for differing resistor errors on the
LED board (if all resistors were x% off in the same direction the
thresholds would not change):
12.6-9.45 --> No lights (no bolt jumper/not reachable with float
sender. Signals </= one liter in the tank)
9.45-7.35 --> Red (bottom sender bolt jumpered to next up/float
sender at bottom)
7.35-5.25 --> Yellow (add jumper to next bolt/float sender at middle)
5.25-0.00 --> Green (add jumper to top bolt/float sender at top)
Note that varying battery voltage will change the voltage thresholds
proportionately; but will not cause them to shift relative to the
operation of a particular sender. The circuit employs three of the
four amplifiers contained in the left-hand chip on the panel(the
fourth one is a spare). Each amplifier drives one LED, and
independently drives that LED any time the output of the voltage
divider formed by the various discrete resistances of the sender in
series with one million ohms becomes even microscopically less than
the output of a particular tap on a reference voltage divider (made
up of five resistors on the panel).*** Since both the sender-side
and reference-side networks are powered by the same battery voltage,
the ratio between them does not change.
***Ordinarily that would mean that the lights would show red,
yellow+red, green+yellow+red; but the LEDs are wired in trick fashion
so the red one loses its ground when the yellow comes on, and the
yellow loses its ground when the green comes on.
Incidentally a different arrangement is used for the battery meter,
because there you *want* the result to change when battery voltage
does. There a very similar ladder of resistors becomes the sensing
part of the arrangement; and the resistor+changing resistor pair used
for the tank sender becomes instead a resistor+zener diode pair which
changes output voltage very little with any reasonable change in
battery voltage. As with the water side, lighting one LED removes
the ground from the previous one.
If you were to add the 500k resistor I refer to in my schematic, the
thresholds would change thus:
12.6-9.72 --> No Lights
9.72-7.56 --> Red
7.56-5.4 --> Yellow
5.4-0.00 --> Green
I made this change on my '84 to prevent the panel indicating red when
it should have been showing yellow. Adding that single resistor
changed all the thresholds because it altered the overall value of
the string; to manipulate one threshold without altering the others
could be done in various ways. The simplest would be to leave the
original resistor string alone and simply re-route the input to that
amplifier to a separate pair of resistors having the correct
ratio. If the designers had envisioned a need to adjust the
thresholds they would have used a separate divider for each
amplifier, probably with a tweak adjustment included. It all costs
money though, and board real estate.
>2) The LM324 chips die a lot, at least on this board.
>
>Seriously.
If the battery lights work ok but the water tank lights don't all go
out when you ground the green wire (or if the battery lights start to
misbehave), your first action is to replace the left-hand LM324N with
another LM324; because it's highly likely that the chip If Radio
Shack still carries them that's fine, otherwise get one or a few from
wherever. Chip price used to be in the dollar range; I haven't
looked for a few years. If you aren't up to soldering (and this is
about as easy a board to work on as exists, see below for method) get
whoever does the job for you to install a socket on both sides
instead, so you can easily change or swap them yourself later. The
three troubles that afflict this setup (no special order):
1) Learning the hard way that the float-type sender as originally
built (and I hope and believe that Trevor has arranged for some slack
in the assembly of the new ones) is ABSOLUTELY INTOLERANT of being
bent the slightest amount in the direction away from the foil side
of the circuit board inside, where the reed switches are soldered
on. One or both of the glass reed switches will burst because their
leads will separate and glass doesn't stretch much. Repairing is
often practical and once you're inside you can put some slack in the
soldered leads so it won't happen again. Best way I found to open
them was with a plumbing-type tubing cutter used very carefully and
rejoined with a rigid oversleeve.**
1a) Getting a drop or two of water inside said float sender, either
from a leaky joint or more likely through the top where the wires
come out. This causes troubles where the tank reads too high, but
the reading is likely to shift as the panel stays on for
awhile. This can be repaired by drilling a small hole next to the
bottom, breaching the seal around the wires if need be, and flushing
through several changes of 190 Everclear or Denatured Alcohol or
high-strength isopropyl alcohol; then re-cementing assembly seams as
well as the hole you made and the top seal.** But don't be surprised
if you break a switch in the process.
1c) Having to scrub off the older-type tank electrodes with green
Scotch-Brite, not the wussy blue stuff. Maybe adding a few pinches
of salt to a tank if the water's really pure.
** I'm sorry, you'll have to experiment or do some research for a
cement that will bond properly. Cheri or Trevor can no doubt supply
that info now; they weren't involved with the senders when I was fixing them.
2) Power switch working loose from the solder. I used to routinely
re-solder mine any time I had the panel out.
3) LM324 gone bad for no obvious reason.
That's it. I have never heard of any other difficulty with them ever.
>3) Some info and schematics here
>http://pws.prserv.net/synergy/Vanagon/LEDpanel.htm
d
--
David Beierl -- dbeierl@attglobal.net