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The following manual contains detail information about electrical troubleshooting, wiring diagram, cable harness routing and electrical schematics description of 1992 BMW 325i Convertible series.
This electrical troubleshooting manual is divided into following sections: Index, How to Use the Wiring Diagram, Wire Size and Conversion Chart, Symbols, Systematic Troubleshooting, Connectors Views, Power Distribution Box, Fuse Data, Component Location Chart and Views, and Splice Location Views. Further breakdown, this manual covers topics on operation description and systematic troubleshooting for power distribution system, charge and starting system, electronics fuel injection system, SRS, Brake Warning System, ABS, Power Mirrors, Central Locking, Power Windows, Seats, Electro-Mechanical Convertible Top, Body Electrical System, Wiper and Washer, Instrument Cluster, etc.

All wiring between components is shown exactly as it exists in the vehicle; However the wiring is not drawn to scale. To aid in understanding the electrical operation, wiring inside complicated components has been simplified to solid state label designates electronic components.
More on 1992 BMW 325i Convertible Electrical Troubleshooting Manual, find here – 142 pages of PDF file.

The BMW 325ix was produced from 1988 to 1991, and featured all-wheel drive. It was available as a two-door (saloon) or a four-door (sedan) and as touring.The BMW 325iX wiring diagram electrical schematics following article provides detail procedures for maintaining, inspection, trouble diagnose, and repairing the 1990 BMW 325iX Electrical System.
This manual is divided into following sections: Index, How to Use This Manual, Wire Size Conversion Chart, Wiring Diagram and Electrical Parts Symbols, Systematic Troubleshooting, Connector Views, Power Distribution Box, Fuse Data, Component Location Chart, Component Location View, and Splice Locations Views.

This manual also covers topics on: Power Distribution System, Charging and Starting System, Electronics Injection System, Supplemental Restraint System, Brake Warning System, Antilock Braking System, Power Mirror, Central Locking, Power Windows, Seats, Sunroof, Body Electrical System, Wiper and Washer, Instrument Cluster, Active Check Control, Light Switch Details, Headlight, Turn and Hazard Lights, Heating and Air Conditioning, Radio, Antenna and Power Antenna, etc. Here is a quotation from the voltage drop testing section.

“Wires, connectors and switches are designed to conduct current with a minimum loss of voltage. A voltage drop of more than one volt indicates a problem. To test a voltage drop, connect the voltmeter leads to connectors at either end of circuit’s suspected problem area. The positive lead should be connected to the connector closest to the power source. The voltmeter will show the voltage drop between these two points.”

Find more information about 1990 BMW 325iX Electrical Troubleshooting Manual – Harness and Schematics here. (source: wikipedia.com, armchair.mb.ca)