Re: Fuel Senders

[EldonMcf@domain.elided]

>John Landry wrote:
>The closest is the Chrysler type.  My bench testing of the IH gauges would
>suggest 10 Ohms will get you a full or near full reading and 78 Ohms will
>net a reading of dead empty.

   <Allan wrote>
I believe John is correct, but would like to see if we can get this cleared
up so I can dump something into the FAQ.  My IH service manual 
(CTS-2303, for T-all/Pickups) identifies the sender as a
0 to 90 Ohm unit, with a resistance of 88-92 ohms in the 'FULL'
position, and having less than 1 ohm in the 'EMPTY' position.
Now the kicker:  While a GM sender is 0-90 ohm, I _THINK_
that they are backwards from the IH sender.  I don't have
a GM service manual (since I don't, and won't, own a GM
vehicle) but I will try to verify this tomorrow at a bookstore.
If someone else has a GM product service manual, please
post the resistance of the sender in the full and empty
positions.

   <John wrote>
Boy... I don't know what's going on in your reference material Allan, but I
can swear to you that my in every single IH gauge I've gotten my hands on...
both out of my 1976 Traveler and various Scouts or pickups from wrecking
yards... my bench testing shows that near zero (0) Ohms of "sending unit"
resistance results in full deflection of the gauge.  Full deflection means
FULL, HOT, or HIGH PRESSURE, depending on which gauge and how it's marked.
All of the gauges operate the same, only the index markings are different.

Maybe that's a misprint in your shop manual???>>


     Who is right? 

   The answer is both. There are two types of guages "constant voltage", and
"air core magnetic", some confusion, and a misprint in the shop manual.
    
    I'll try to explain a little about each type, and add my $.02

   CONSTANT VOLTAGE GUAGES  (CV)
   
    This type of guage operates by voltage applied to a heating coil that is
wound around a bi-metal strip, attached to a needle in the guage reciever.
When full voltage is applied maximum heat is produced, causing full
deflection of the needle. As resistance is increased by the sending unit
between the voltage source and the heating coil, less deflection of the
needle occurs. At the sending units maximum resistance little or no heat is
produced, and the position of the needle remains at its lowest point. This
type of guage requires a "Constant voltage regulator". The function of the
"CVR" is to supply a preset voltage to the guage, allowing for variable input
from the vehice's electrical system. This maintains the accuracy of the guage
throughout varying operating conditions.

     AIR CORE MAGNETIC GUAGES

   This type of guage operates by voltage applied to a coil of wire on a
spindle, attached to a needle in the guage receiver. When full voltage is
applied a magnetic field is produced, moving the needle to its lowest
position.  As resistance is increased by the sending unit, less voltage is
applied to the coil moving the needle towords the full position. At minimum
resistance the needle reads full scale. This type of guage does not require a
voltage regulator. Note that the resistance that varies to operate this type
of guage is opposite of that used with "CV" type guage.

   MY $0.02

   "Air core magnetic guages" were used by IH beginning in 1977 on new
models. The first truck to use them was the "S" series, then 9670, 9300 HD
models beginning in the early 80's. Since there were no new light line models
after '77 no Scouts, pickups, etc.. use this type of guage. No doubt the
Scout III would have had 'em....
   I checked back as far as the '50's models till the end of the Scout, and
Loadstar models, and my guess is everything from the early days of IH trucks
till the end of these models were all the same. The "CV" guages were the
industry standard untill the mid '70's or early '80's. 

    The spec's I found apply to all "CV" guages used on IH truck models,
Scouts, Pickups, and Travelalls (fuel, temp, oil press.). They are: LOW SCALE
= 73 OHMs,   FULL SCALE = 10 OHMs. These agree with John Landry's bench
testing, except for the low scale reading. The 5 ohm difference could be from
the slight resistance that  the wiring would have, or that the test called
for the pointer to be "bottom line on dial plus or minus the pointers width"
so it would also depend on the angle of John's eyeball. Or maybe John's "dead
empty" is a little below the line. John, what reading do you get at 73 ohms?
Also I have a guage tester similar to the one used in the IH (and others)
shop manual. If anyone (thinking of the electrical engineer guys) would like
to build one, I could describe it for ya. Actually a rheostat 0-100 ohms, a
couple wires with alligator clips would do for this application.

    If you are using the IH service manual section CTS-2118 INSTRUMENTS
beware it is confusing. Apparently the "air core" information was added to
the "CV" info to save paper or something. It doesn't list the test spec's
seperately, or tell how to identify which type of guage is used where, or
when, or how to identify them. Since the discussion on this list pertains
mostly to light line vehicles, the only type guages that apply here would be
the "CV" type. For information, testing, or specifications for "CV" guages
refer to pages 3 thru 10. For information that applys to "air core" guages
refer to pages 14 thru 18. To further complicate and confuse things, a
mis-print exists in the test information on page 15. In step 4 the guage
tester is set to "90" (ohms), the guage should read slightly above "full"
(this is correct). In step 9 the same test is being performed except the
wires between the guage and sender have been eliminated to determine if a
wiring problem exists, therefore the guage should read the same as in step 4,
or slightly above "full". Step 9 states that the guage should read slightly
below "empty" this is the incorrect part.

   Allan,  The spec for the GM sender is:  LOW SCALE = 1 OHM,  FULL SCALE =
90 OHMs. You are correct it is backwards because its an "air core" type. I
don't have the spec's on early GM stuff, and the page with Chrysler spec's
was missing from my book.

   Another note: Someone was going to redo their whole dash, starting from
scratch. If you want to do this you can use the "air core" guages. they are
available from AC Delco, Stewart Warner, Isspro, etc.. in standard 2" size
guages. The only tough part would be the fuel sender, which would have a
different style mounting, but no problem for the master fabricators out
there.

   I hope this helps clear things up

   Eldon Mcfarling
   Ceresco, NE
   80 Scout II Turbo Diesel
   Other stuff in the weeds 
   http://members.aol.com/EldonMcf/scoutd.html