Date: Thu, 14 Oct 1999 04:53:46 -0400
Reply-To: David Beierl <dbeierl@IBM.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@IBM.NET>
Subject: Re: [vanagon] need tech info for tach cluster circuit board
In-Reply-To: <0.3e1aa158.2536dc82@aol.com>
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At 03:13 10/14/99 , you wrote:
>David, thanks for responding so quickly.
>
>I just went and rechecked some things. First, I am using a pretty basic
>analog meter. Second, all the ohm values I received were the same when I
>reversed the leads from one side to the other.
Ok, good.
>Whatever these are, two of them that are banded exaclty the same, only
>installed opposite of each other yielded 300 ohms. These are the bottom
>two. The next one up yielded 80 ohms. The one that I am thinking I smoked
>shows no movement of the needle (infinite resistance?).
Ok, I'm guessing that you have an '85 or later. What I'd expect to see
(there's some guesswork here, the stuff in Bentley about 80-84s doesn't
agree with the '84 panel I have in my hand, and they don't describe the
'85-up as well) is two resistors on the bottom coded
yellow-purple-brown-(gold) which is 470 ohms. These are current-limiting
resistors for panel LEDs. The next one up is much larger physically and
probably has a value of 150 ohms. Mine is coded
brown-green-black-black-red -- the first two would indicate 1-5 but I don't
understand the rest. That I believe is to deliver a trickle current to the
alternator field windings so it will start to generate.
The next one up might very well be coded red-red-black and have a value of
22 ohms; and connect from ground to the middle lead of the voltage
regulator (three-pin device screwed to the foil next to the tach). If that
is open, it might be because the voltage regulator fried and then cooked
the resistor. You should see 9.5-10.5v between the left and middle pins of
that regulator; if not it's probably bad. Follow the foils around and see
if the connections are as I suggest.
>Do you know these circuit 'foils'? Can this one part be replaced?
The regulator is a plug-in part. Dealers have them but maybe a bit
pricey. The part should be worth about a buck or so.
The resistors are trickier, they're crimped onto the foil and you don't
want to mess with the crimp. You can clip leads and solder a new resistor
to the leads of the old, but since you asked the question I can say that
you don't have skill to do that. But someone with good electronic
soldering skills could do it for you.
>What is the difference between a resistor and a diode?
A resistor is just a lump -- or film, or coiled up wire -- of something
that impedes electrical flow. They are rated primarily in ohms of
resistance and watts of permissible power dissipation. The little guys are
quarter-watt, the larger ones probably half-watt, and both are probably a
film of carbon on a little ceramic tube, covered with epoxy paint. The
more voltage you put across them the more current flows and the more power
is dissipated as heat (Watts = Volts^2 / Ohms ). Resistors don't care
which way they're installed.
A diode is a semiconductor device, usually made of silicon (not silicone),
which absolutely resists current flow in one direction, and below a certain
voltage in the other direction. Above that voltage (roughly half a volt
for silicon diodes) they tend to act like a dead short. LEDs are special
diodes which have a "forward voltage" that is much higher, from about 1.8
volts (red) to 2.2-2.5v (yellow, orange, green) to almost 5 volts
(blue). If you hook any diode directly to a power source it will destroy
itself; that's why the 470 ohm resistors are there for the indicator
LEDs. My '84 panel has a 1N4004 diode in the second place from the bottom,
molded black epoxy case with a white band and numbers -- but from your
description that's not what you have. NB: This is a barely adequate
description of diodes, I glossed over a lot...
>Thanks for your input.
Yr welcome. Good luck.
david
David Beierl - Providence, RI
http://pws.prserv.net/synergy/Vanagon.htm
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
