View single post by Mark Rosenbaum
 Posted: 03-15-2005 06:02 pm
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Mark Rosenbaum

Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532

Gauge questions seem to come up fairly often.  Hoping to answer some of these, I've gathered data from various sources on the internet and elsewhere, and presented a condensed and simplified version below.

A Note on Voltages.  In automotive usage, the term "12 volts" means the output of a charged six-cell lead-acid storage battery in good condition.  As it happens, the actual output of such a battery is around 13.6 volts.  But the terminology is unlikely ever to change, so one must get used to the discrepancy. 

General Operation.  The volts, fuel, and temp gauges all operate on the same basic principle: an electric current flows through resistance wire inside the gauge.  This current causes heating of the resistance wire, which is wrapped around a bimetallic strip.  This strip bends in response to the heating.  The gauge's pointer is attached to the strip and the more the strip is heated, the more the pointer deflects.  The response time of these gauges to a change in the current is typically about 3 to 10 seconds.

The fuel and temp gauges work in conjunction with external sensors (senders) that vary the current flowing through the gauge.  The volts gauge is its own sensor and is connected directly to the switched 12 volt supply.

Voltage Stabilizer.  There are two types: mechanical and solid-state.  The mechanical stabilizer works something like a turn signal flasher, producing 12 volts for a while, then 0 volts for a while, so that the average output over time is about 10.0 volts.  This switching effect confuses many digital and some analog voltmeters.

The solid-state stabilizers are externally the same as the mechanical ones but rely on solid-state devices for their function, and produce a constant output of about 10 volts.  You'd expect them to be far more reliable than the mechanical ones.  However, these stabilizers do not seem to have any protection against a short circuit on their output, and thus can be expected to fail instantly upon the occurrence of a short.

If the output of the voltage stabilizer increases, the readings of those gauges connected to it will normally increase.  Conversely, if the output decreases, gauge readings will normally decrease.

Oil gauge.  Smiths PL 2312/04.  Unlike the other gauges used in a JH, this gauge is purely mechanical and uses a Bourdon tube to deflect the pointer.  Some non-JHs use electrical oil pressure gauges, however, and their operation follows the same principles discussed below.

Voltage gauge. Smiths BV 2204/05.  This should be connected to the switched 12 volt supply.  A good gauge will measure about 120 ohms between its two terminals when disconnected from the wiring.

Fuel gauge.  Smiths BF2201/27.  This should be connected to the output of the voltage stabilizer.  A good gauge will measure about 60 ohms between its terminals. The fuel tank sender varies from circa 30 ohms (full) to circa 370 ohms (empty).

Temp Gauge.  Smiths BT 2204/24. Like the fuel gauge, this should be connected to the output of the voltage stabilizer.  A good temp gauge will measure about 60 ohms between its terminals.  In a properly operating system, the mid-range of the gauge is about 80-85°C or 175-185°F -- the temperature at which the thermostat opens.

The correct temp sensor is the Smiths TT6811-01, which has an 1/8" x 27 pipe thread.  The resistance of the sensor varies with temperature as indicated below.  

 °C         25   50    70    80    90   100  110  

 °F         77  122  158  176  194  212  230

 Ohms  820  306  170  142  106   79    53   All plus or minus perhaps 10%

Interchangeability. Most electrically operated Smiths gauges are substantially identical internally. This means that you can very often repair a burned-out temp or fuel gauge by installing the mechanism from a Smiths gauge scrounged from the junkyard.  This is just as true for MGs, Jags, and Lotus as it is for JHs.

Calibration.  A close look at the face of a Smiths gauge will usually disclose three pairs of dots that align with the low, middle, and high ends of the gauge's range.  These are calibration markers.  When the gauge is correctly calibrated, the pointer will lie between the two dots of a pair at three specific inputs.

Gauge calibration can be checked by applying several accurately known DC voltages across the gauge terminals.  For 2.0 volts, the gauge pointer should lie within the low pair of dots.  For 4.8 volts, the pointer should lie within the middle pair of dots.  And for 7.6 volts, the pointer should lie within the high pair of dots.  Unfortunately, there does not seem to be any publicly available information on how one adjusts the internal parts of a gauge so that everything will line up correctly.