Circuit Description

Vehicles equipped with RPO LC8 - compressed natural gas (CNG), have an intake air temperature (IAT) signal modification module that is wired in-line between the mass air flow (MAF)/IAT sensor and the engine control module (ECM). A jumper harness is used to connect the module to the MAF/IAT sensor and the engine harness. The IAT signal circuit is routed from the MAF/IAT sensor to the IAT signal modification module, to the engine harness connector, and on to the ECM. The other MAF/IAT sensor circuits are routed directly from the sensor to the engine harness.

The IAT signal modification module is integrated with the fuel pump disable module. The module is supplied 12 V from the CNG relay. The fuel injector control module (FICM) activates the CNG relay when in CNG Mode.

There is a control circuit from the FICM to fuel pump disable module. In gasoline mode, the FICM grounds the control circuit, voltage is less than 1 V. In CNG mode, the FICM applies a +12 V low current digital signal on the control circuit.

In Gasoline Mode, the internal IAT signal modification module relay connects the input circuit from the IAT sensor to the output circuit to the ECM, through the engine harness. No change or modification of the signal is made in gasoline mode.

In CNG Mode, the activated internal IAT signal relay connects a 2.4 KΩ +/ - 1% fixed resistor between the input circuit from the IAT sensor to the output circuit to the ECM, through the engine harness. When the resistance is added to the IAT signal circuit the voltage signal is changed and the temperature reading is reduced.

The ECM calculates the ignition timing based in part on the IAT sensor signal. By reducing the IAT temperature output in CNG mode, the ECM may change ignition timing up to an additional 12 degrees.

The following table illustrates the difference between temperature, resistance, and voltage: