In most cases when updating an older instrument panel the range of the oil pressure and temperature gauges will be increased and a voltmeter will be substituted for an ammeter. In our case the original amp gauge on the ’41 simply read “Dis” for discharge on one side and “Chg” for charge on the other. To maintain the original appearance our new voltmeter uses the same markings—the middle of the 13-gauge now represents 14 V.

We couldn’t be happier with our updated gauges. Ironically, in both cases the end results look so right that the improvements go unnoticed, but they’re certainly not unappreciated.

Gauge Basics

Here’s some general information on gauges from Classic Instruments:

Fuel Gauge

The most common fuel level sender that Classic Instruments supplies operates in a resistance range of 240 ohms empty and 33 ohms on full. Other senders available operate in resistance ranges of:

~0 to ~30 ohms (typically used with original GM fuel gauges before 1964)

~0 to ~90 ohms (typically used with original GM fuel gauges after 1966)

~78 to ~10 ohms (typically used with original Ford fuel gauges before 1986)

Classic Instruments carries fuel gauges that work with multiple types of fuel level senders

240-33, 33-240, 0-30, 0-90, 78-10, and 16-158 ohm range gauges available in 2-1/8 inches

240-33, 0-30, 0-90, and 78-10 ohm range gauges available in 3-3/8-inch quad

240-33, 0-30, and 0-90 ohm range gauges available in 4-5/8-inch quad

It is very important to run a dedicated ground wire to the fuel level sender to ensure it has a good ground. Don’t rely on the tank to provide the ground, since a lot of tanks are isolated from the ground by things such as rubber grommets. Without a proper ground on the fuel sender, the gauge circuit is not complete and the gauge will not work. The gauge will register infinite resistance for the sender and this will cause 240-33 and 75-10 gauges to peg below empty while 0-30, 0-90, and 16-158 fuel gauges will peg above full.

Oil Pressure Gauge

Classic Instruments’ 100-psi oil pressure gauges register 0 psi at 240 ohms resistance, 50 psi at 105 ohms resistance, and 100 psi at ~33 ohms resistance (the lower the resistance, the higher the psi reading).

Classic Instruments’ SN52 (100 psi) oil pressure sender works in a resistance range of 240 ohms at 0 psi to 33 ohms at 100 psi.

SN54 (80 psi) oil pressure sender works in a resistance range of 240 ohms at 0 psi to ~33 ohms at 80 psi.

The Classic Instruments’ oil pressure sender gets a ground connection through the mounting threads. Without a proper ground to the sender, the gauges will read low or peg below 0 psi. Pipe sealer should be used sparingly on the threads of the sender and Teflon tape should not be used.

Temperature Gauge

Temperature senders should be mounted in the intake manifold of an engine when possible. Installing the sender in the head or in close proximity to other heat sources (i.e. exhaust headers) will cause the temperature gauge to read high. This is because the sender is being affected by the significantly higher temperature of its surroundings.

Bushings should not be used to make a temperature sender fit. Bushings could cause the sender probe to not be fully submerged in the coolant inside the engine (this could cause the sender to measure coolant vapor temperature, which is usually higher than the actual coolant temperature).

Classic Instruments carries five sizes of temperature senders that are all electrically identical (they operate in the same resistance range).

The sizes are:

SN12MM (12mm thread for LS engines)

SN22 (1/8-inch NPT, typically fits Chrysler engines)

SN23 (1/4-inch NPT)

SN24 (3/8-inch NPT, fits most Ford engines and newer GM engines)

SN25 (1/2-inch NPT, fits older GM engines)