CVI Calculation

An important function of the Powertrain Control Module (PCM) is to monitor Clutch Volume Index (CVI). CVIs represent the volume of fluid needed to compress a clutch pack.

The PCM monitors gear ratio changes by monitoring the Input Speed Sensor (ISS) and Output Speed Sensor (OSS). The ISS, or turbine speed sensor sends an electrical signal to the PCM that represents input shaft rpm. The OSS provides the PCM with output shaft speed information.

By comparing the two inputs, the PCM can determine transaxle gear ratio. This is important to the CVI calculation because the PCM determines CVIs by monitoring how long it takes for a gear change to occur.

Courtesy of CHRYSLER GROUP, LLC

Gear ratios can be determined by using the scan tool and reading the ISS/OSS values in the "Monitors" display. Gear ratio can be obtained by dividing the ISS value by the OSS value.

For example, if the input shaft is rotating at 1000 rpm and the output shaft is rotating at 500 rpm, then the PCM can determine that the gear ratio is 2:1. In direct drive (3rd gear), the gear ratio changes to 1:1. The gear ratio changes as clutches are applied and released. By monitoring the length of time it takes for the gear ratio to change following a shift request, the PCM can determine the volume of fluid used to apply or release a friction element.

The volume of transmission fluid needed to apply the friction elements are continuously updated for adaptive controls. As friction material wears, the volume of fluid need to apply the element increases.

Certain mechanical problems within the clutch assemblies (broken return springs, out of position snap rings, excessive clutch pack clearance, improper assembly, etc.) can cause inadequate or out-of-range clutch volumes. Also, a defective ISS/OSS or wiring can cause these conditions. The following chart identifies the appropriate clutch volumes and when they are monitored/updated: