From: DANMAS@aol.com [mailto:DANMAS@aol.com]
Sent: Saturday, February 09, 2002 1:27 PM
To: twakeman@cruzers.com; vafred@erols.com; randallyoung@earthlink.net;
triumphs@mcleodusa.net
Subject: Re: gauges
Folks,
I'm not on the Triumphs list right now (too much going on to keep up), so I
haven't been following this thread enough to know what's going on. However,
since you all cc'd me on your responses to the list, I guess I ought to
respond to you all.
First of all, a fuel level sending unit is a pure resistance. A pure
resistance is NOT polarity sensitive. As far as resistance is concerned,
polarity is totally irrelevant. Given that the sender isn't polarity
sensitive, and you can reverse the polarity of the electrical system in our
cars without changing anything in the meter circuits, it is apparent that
the
gauges, regardless of type, are not polarity sensitive either. Therefore,
any
change in the meter reading direction caused by mismatching meter to sender
must be from some other cause than polarity concerns.
OK, what is the reason, then. First, let's talk about the later style
gauges,
the thermal type. These gauges could have been designed to read "full" with
maximum current (minimum sender resistance), or they could have been
designed
to read "full" with minimum current (maximum sender resistance). Had they
been designed to read "full" with minimum current, they would read "full"
anytime you turned the key off. There is nothing technically wrong with
this,
but psychologically, most folks are bothered by seeing gauges read full when
they are off. For this reason, Lucas, Smiths, and all other gauge makers
designed their gauges to read "empty" when off. To make this work, the
sending units had to be designed to give minimum resistance when the fuel
tank is full (or when the water temperature is high, etc).
The "dual coil" type gauges, on the other hand, could be designed to read
"full" with maximum OR minimum current, and still come to rest in the "zero"
position with power off - it was just a matter of choice. Most modern GM
gauges, for example, use 0 ohms empty, and 90 ohms full, while early Fords
used 73 ohms empty, and 8 ohms full, while most aftermarket senders have 240
ohms empty and 33 ohms full (don't ask me how I know this, but it has to do
with my rewiring the Chevy 350 powered TR6 in my garage at the moment. Dang,
that car is fast, and handles like a dream).
(As an aside, placing a 15 ohm resister in series with the sender, and a
1000
ohm resister in parallel with the sender/resister combination, a TR6 sender
matches very closely to the aftermarket sending units - but it creates a
very
nonlinear reading when used with an aftermarket gauge. Full and empty, it
reads spot on, but when the gauge reads 1/2, I only have about 3 gallons
left. When I rewire the car later in the spring, I will replace the sender
with an aftermarket unit to match the aftermarket gauge I'm using.)
Evidently, Lucas/Smith decided to use maximum resistance for "full" on the
gauges designed for the early TRs. No one was any the wiser, nor did it make
any difference, until they decided, for whatever reason, to go to the
thermal
type gauges. Then, they were forced to change the sender to match.
I have a TR3 fuel sender in my shop, so I just now measured it, and sure
enough, it has max resistance when full, and minimum when empty - exactly
the
opposite of the later senders. Mixing the senders/gauges would indeed cause
a
reversal of reading - showing full when the tank is empty, and vice-versa.
The bottom line is, mismatching gauge to sender can cause erroneous
readings,
but it has nothing whatsoever to do with polarity.
TeriAnn - you made one statement that I would take exception to, pedantic of
me though it may be. You compared the Fuel gauge to a voltmeter. Actually,
that is not true; the fuel gauge is not connected as a voltmeter at all, but
is, in reality, connected as an ohmmeter. In truth, however, ohmmeters and
voltmeters are really ammeters. What makes the difference is how they are
wired into the circuit, and the resistance built into the meter itself. An
ideal ammeter has zero resistance, an ideal voltmeter has infinite
resistance, but the resistance of an ohmmeter is determined by the circuit
it
is in, as is the resistance of fuel gauges, water temp gauges, and oil
pressure gauges. In a TR6, the gauge resistance is about 54 0hms, if I
recall
correctly.
Randall - It is my understanding that the changeover to the thermal type
gauges occurred with the TR4. However, it is certainly within the
"character"
of Triumph to have used the early type gauges in the earliest TR4s. More
research is needed on my part.
Regards,
Dan
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