AEM 30-6905 Universal Programmable EMS-4 - Manual (Page 120)

Page 120 of 279 EMS-4 Install and Tuning Guide_Rev 1.6
Water Temp Sensor
The coolant temperature sensor (CLT) is a variable resistance thermistor (thermal resistor) that
sends a return voltage back to the ECU based on the temperature of the engine coolant. This
sensor can also be used for virtually any other type of automotive fluid if it is located in that
liquid‟s respective reservoir. Unlike other types of sensors that emit voltage signals to the ECU,
with CLTs the ECU outputs a +5V signal to the CLT and the sensor‟s resistance determines the
return voltage. The AEM EMS is compatible with virtually all CLT sensors.
The CLT provides the ECU with engine temp information. The AEM EMS modifies the base fuel
calibration during start and warm up based on the CLT input. In addition to adding fuel, a CLT
can signal to the ECU to modify ignition timing at varying engine temps for optimum
performance. On racecars, ignition timing is typically advanced during warm up to heat the
engine faster. Racecars use an ignition based warm up because usually idle speed motors are
not used. On streetcars, where clean emissions are important, we reduce timing based on CLT
input to light the catalytic converter(s) off sooner.
O
2
(Oxygen) Sensors
There are many types of O2 sensors that are employed by vehicle manufacturers, and it is well
beyond the scope of this manual to describe all of them. An O2 sensor provides a reading of
the air/fuel ratio
(AFR) to the ECU so that it can make the necessary fuel calibration corrections
to achieve a desired Air Fuel Ratio (AFR).
An O2 sensor works by sensing whether there is an abundance or lack of oxygen in the exhaust
gases, depending on whether the gas mixture is too rich or too lean. If there is excess oxygen
and the mixture is too lean, output voltage from the O2 sensor to the ECU will be high. The
ECU may then compensate by adding fuel. The converse is true of rich mixtures.
Common O2 sensors include 3-, 4-, and 5-wire heated or wide-band sensors. Three- and 4-
wire sensors are ideal for determining whether a vehicle‟s AFR is at the optimum stoichiometric
ratio. Stoichiometric ratio refers to the ideal mixture of fuel and air by mass to completely
consume both reactants (gas and air) with nothing left over. Based on the properties of most
pump gasoline used today this ratio is typically a 14.64:1 air/fuel ratio. Although this ratio
provides the best combustion characteristics with the least emissions output and optimum
catalytic converter performance, it is NOT the best AFR for maximum power at full throttle or
under boost. This mixture is too lean and may cause engine damage.
AEM currently offers a line of high quality, inexpensive UEGO gauge controllers. Because the
AEM gauge utilizes the internal AEM UEGO controller and Bosch UEGO Sensor, it is accurate
and repeatable to 0.1 of an air/fuel ratio point! With this, there is no abrupt oscillation as found in
many competitor gauges, which utilize a narrow band oxygen sensor detecting only
stoichiometry.