"How I Do It"

Easy Fix For Inaccurate Temperature Gauge Reading
by Carl Bilter

If your Chrysler 300 temperature gauge reading is very high with a new temperature gauge sending unit in an otherwise properly functioning cooling system, there is an easy fix to provide a correct gauge reading.

First, it is important to validate that everything else in the cooling system is functioning properly, including the radiator, cooling fan, fan clutch (if so equipped), voltage limiter, and temperature sending unit. On the 300J, the voltage limiter is located inside the fuel gauge. Introduced on the F in 1960, the voltage limiter was initially housed in the oil pressure gauge, and remained there through 1962. From 1963 to 1965, it was housed in the fuel gauge. It is a primitive mechanical device that is prone to failure. If the voltage limiter malfunctions, the typical symptom is both the fuel and temperature gauges will rise or fall in unison (and the oil pressure gauge as well, if so equipped). The ammeter will not be impacted. If the limiter functions correctly, the gauges should function correctly.

To determine the actual operating temperature of the engine, an infrared thermometer should be used. Aiming the laser pointer of the thermometer at the thermostat housing usually gives a reasonably good indication of the actual coolant temperature. On our 300s a 180 degree thermostat should be used. That means that the thermostat begins to open and allow the coolant to flow through the system when the coolant temperature at the thermostat reaches 180 degrees F. The thermostat is designed to keep the temperature of the coolant at least 180 degrees. The temperature can and does go higher depending on operating conditions. Never rely on the temperature gauge to assess engine operating temperature until you have checked the actual temperature with an infrared thermometer and correlate that reading to what the gauge is reading.

The temperature gauge is a thermal type gauge that operates on the principle of constant voltage being applied, which is supplied by the voltage regulator (or limiter) housed in the fuel gauge. The temperature gauge operates by registering the current flow which is determined by the amount of resistance in the sending unit located in the water manifold.

When the engine is cold the resistance of the disc in the temperature sending unit is relatively high (around 130 ohms in the 300J). Low temperature will be indicated on the gauge.

As the engine temperature increases the resistance of the sending unit disc starts to decrease. A resultant increase in the current flow will occur causing the temperature gauge pointer to swing towards the right and indicate the increase in engine temperature.

If the temperature gauge itself reads consistently very high or low at normal 180 degree operating temperature, the first suspect is the sending unit. A failing sending unit often has very high resistance resulting in low current flow and a very low gauge reading. On the other hand, replacing the sending unit with a new aftermarket unit may result in a very high gauge reading, such as 3/4 to 7/8 full gauge scale at normal 180 operating temperature. The reason for this is that the modern aftermarket sending units often have too little resistance compared to the factory OEM unit, because they are designed for many applications, not just Chrysler 300s.

The solution is to add a small amount of additional resistance between the sending unit and the gauge, which in turn reduces the current flow to the gauge and provides a lower gauge reading just as the original OEM unit did. To accomplish this, we add only about 10 ohms resistance. In the case of my 300J, I found that 10 ohms was not quite enough for my particular gauge (they do vary some) and so I soldered a 2.2 ohm resistor in series with a 10 ohm resistor to yield 12.2 ohms as shown below.

It is important to use resistors that can handle the heat of the engine compartment, such as the type suitable for use in old tube radios which can run very hot. I selected these resistors from my stock that I use to restore old radios. They are high quality resistors designed to operate up to 311 degrees F with a +/- 5% tolerance; so they are more than adequate. I soldered on lead wires with a connector for the sending unit on one end and a terminal to connect to the sending unit lead from the wiring harness so that this modification is easy to remove to return to a stock setup. I placed a short length of rubber hose around the resistors for insulation, and the adapter is complete. It would be possible to install the resistors behind the instrument cluster as well and be completely hidden from view, but the underhood arrangement is easier and convenient.

With the resistors in place in the circuit, my gauge reading went from reading 7/8 full scale at normal 180 degree engine operating temperature down to reading just above the first mark on the temperature gauge, which corresponds very closely with the reading that was intended when the car was new with a properly functioning OEM sending unit.