I am looking to add a LOW FUEL WARNING LIGHT to an existing fuel
system WITHOUT altering the current configuration except for possibly
adding a connection to the rear post of the fuel gauge. The vehicle is
a 1966 with 12 volts and warning systems were not available then. I
believe the system works by use of resistence as the manufactures
drawing shows the negitive side of the sending unit [float arm] connected to
the neg. side of the fuel gauge. The drawing indicates the positive
side of the sending unit connecting two points with a zigzag line and
connected to the positive side of the gauge.. What I would like to
accomplish is the installation of a wire to some type of
resistor/processor/device or????? to the rear of the gauge and have it
activate a 12 light at a given fuel level. Whatever system being used
must be adjustable as I need to be able to adjust the point at which
the light is activated.. I hope this is enough info to answer my
question..

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Request for Question Clarification by feilong-ga on 15 Mar 2003 07:29 PST

Different cars have different wiring layouts. If you tell us what kind
of 1966 model car you have, someone here may be able to help. Also, it
could help if you already have a preferred wiring layour for this
particular application and post it in the Internet.

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Clarification of Question by bonanza-ga on 15 Mar 2003 08:41 PST

The fuel system is no longer in a car, it has was installed in a speed
boat by the prvious owner who told me it was from a 1966 vintage car.
The drawing he has with the boat is as earlier described..  Sorry I
can't provide a make and model however it appears that it is a fairly
simple system. I am not versed enough to formulate a system by myself
without input as amps and voltage most likely play a part in wheather
I burn out the resistor in the tank if improperly connected..I was
told by a local electronics supply shop that it would be easy and
inexpensive to produce but he didn't have the time now to research the
parts needed..Thanks, Marc

bonanza...

The following circuit diagram, designed by Jeff Anderson,
looks like it will satisfy your needs, inexpensively:
http://users.pandora.be/miata/english/misc/lowfuel.htm

I was an electronics tech in the US Navy, and I can see
no reason why this circuit wouldn't work on any 12V system.

The parts are readily available at any electronics supply,
and the 2K adjustable resistor (2000 ohm potentiometer)
allows you to set the low-fuel level with the tank at 1/8th 
capacity. Please note the warning at the top of the page:

"In case of gross construction error it is possible to blow
 out the fuel tank's sending unit real quick if the tank is
 full, and less so if the tank is at the 1/8 tank indicated
 level that's needed for setting the turn-on threshold. Other
 gross errors could be more likely protected by fuses in the
 car."

The one possible limitation is at the bottom of the page:

"However, it could be improved by adding a timed delay
 circuit so that the warning lamp will not light until
 it receives a signal for about one solid minute. Now,
 upon a hard turn, or fast acceleration, the warning
 lamp tends to flash if the fuel is a little below a
 half tank."

Since you are installing this in a speedboat, it seems 
likely that you will have considerable sloshing in the 
tank, which could cause the light to come on as noted
above, without the addition of a timing circuit. I am
personally unfamiliar with such a timing circuit. Perhaps
your supply shop can assist you with that. However, the
470K hysteresis resistor is specifically included to
address false indications due to splashing, as noted on
the page.

Please do not rate this answer until you are satisfied that
the answer cannot be improved upon by means of a dialog
established through the "Request for Clarification" process.

sublime1-ga


Searches done, via Google:

adding "low fuel warning" circuit diagram
://www.google.com/search?q=adding+%22low+fuel+warning%22+circuit+diagram

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Request for Answer Clarification by bonanza-ga on 16 Mar 2003 07:04 PST

Good morning Sublime1
    Thanks for the response to my question.. I was familiar with this
article from my search on MSN last week and had talked about it with
an individual I know who is familiar with electronics. He indicated he
didn't think this would work until I can determine the ohms at full
and empty then proceed with making a test unit. His statement was the
same as you indicated, that I could burnout the sending unit very
easily by using this diagram if I didn't have the necessary voltage
and ohm inputs..Somehow I thought this was going to be simpler and
less of a chance of destroying my unit. Unfortunantly, the individual
who built this racer produced a tank that is nearly impossible to
remove to replace the sender should it be damaged.. I guess I will
have to be satisfied with the gauge as it is as I'm not interested in
taking a chance of burning it out.. I do appreciate your efforts in
this issue

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Clarification of Answer by sublime1-ga on 16 Mar 2003 10:06 PST

bonanza...

While the inaccessibility of the tank and sending unit is a 
major limitation, I'm not sure you need to give up so easily.
Fuses are cheap, and, during the setup process, you could 
always insert a variety of low-amperage fuses directly in 
line with the sending unit lead. Not knowing how much current
the sending unit can handle without burning out, you could 
start with, say, a 1/4 amp fuse and work up. If you needed
much more than a 1 amp fuse, I'd start to worry.

Note that the warning is talking about *gross* construction
errors - like using the wrong size resistor, or reversing a 
diode, for instance. If you are careful with the construction
or the unit, and make sure the tank is at 1/8th capacity when
you test the indicator and adjust the 2Kohm potentiometer to
turn on the warning light, I don't believe it would be that
great a risk.  Especially if you start with the potentiometer
at mid-range and have a fuse in line with the sending unit.

Nonetheless, I can understand your hesitancy to invest time
and money in building the unit, only to have it fail.

Also, you can contact Jeff directly by way of his guestbook
page. You can opt to make the message private, if you prefer,
and he will email you back. Since he designed the unit, he
may be able to provide you some valuable feedback specific
to your situation, in regards to installing and testing the
unit. His guestbook page is here:
http://users.pandora.be/miata/english/misc/guestbook.htm

Best of luck with your project...

sublime1-ga