Diagnostic Plug
The Diagnostic Plug is a multipin socket in your engine bay, allowing you to perform some basic engine functions.
Contents
Overview
When things start going wrong in an engine, a technician needs quick and easy access to the fundamentals of the engine to diagnose where the problem lies. However, as engines get more complicated, accessing all those parts becomes more of a headache. For that purpose, car manufacturers in the Eighties started fitting Diagnostic Plugs to their vehicles, to aid problem-solving.
Of course, the rudimentary electronics fitted back then are nothing like the modern OBD-II system cars use now, but BMW's efforts weren't a complete waste. With a simple piece of wire you can directly operate the starter motor to turn over the engine, as well as perform basic service functions such as resetting the service lights. With the appropriate scan tool, you can even read fault codes to ascertain exactly where the problem lies. However, accessing live engine data is beyond the capabilities of the Diagnostic Plug.
Pinouts
| Pin | Wire Gauge | Colour | Function | Image |
|---|---|---|---|---|
| 1 | 1.5 | BK | Ground Distribution, G103 | 
|
| 4 | 0.5 | BR/VI | Gauges/Warning Indicators, Coolant Temperature Sender | |
| 6 | 0.75 | GY/BU | Not Used | |
| 7 | 0.75 | WT/BU | Service Interval Indicator, Service Interval Processor (Reset) | |
| 8 | 0.5 | YL | Ignition, TDC Sensor | |
| 9 | SHIELD | Ignition, TDC Sensor | ||
| 10 | 0.5 | BK | Ignition, TDC Sensor | |
| 11 | 2.5 | BK/YL | Starter, Start Signal (50) | |
| 12 | 0.75 | BU | Charge, Alternator (D+) | |
| 13 | 1 | GN | Ignition, Ignition Coil | |
| 14 | 2.5 | RD | Charge System, Alternator (30) | |
| 15 | 1.5 | GN/YL | Idle Speed Control, Idle Speed Control Unit |
| Pin | Wire Gauge | Colour | Function | Image |
|---|---|---|---|---|
| 1 | 1 | BK | Ignition Coil, Motronic Control Unit | 
|
| 6 | 0.5 | WT/BK | SRS Connector (Not Used) | |
| 7 | 0.5 | WT/GN | Service Interval Indicator, Service Interval Processor (Reset) | |
| 11 | 2.5 | BK/YL | Starter, Start Signal (50) | |
| 12 | 0.75 | BU | Charge, Alternator (D+) | |
| 14 | 2.5 | RD | Battery | |
| 15 | 0.5 | WT/YL | Motronic Control Unit (RXD) | |
| 16 | 1.5 | GN/WT | Oxygen Sensor | |
| 18 | 0.5 | GN/BU | Motronic Control Unit (Programming Voltage) | |
| 19 | 1.5 | BR | Ground Distribution (G103) | |
| 20 | 0.5 | WT/VI | Motronic Control Unit (TXD) |
