Can anyone help we are all a bit perplexed as to what this plug is for in the engine bay of my van - its a vw plug - i have accounted for all the wires i need If its not required i will cut it all out or hide it away Dale
VW Diagnosis plug By Jim Mais July 2005 The plug (actually a ‘socket’) was introduced mid-year on 1971 Type 1, 2, 3 and 4 cars. It was connected to various electrical points in the car, which was supposed to enable a computer at the VW dealership to ‘diagnose’ the performance or malfunction of certain engine/electrical characteristics. They were also fitted to Australian-assembled VWs, but unfortunately the specific VW Diagnosis computer equipment never made it to Australian dealers. Many changes were made during the few years the plug was in existence. Check your official VW workshop manual – the ‘Bentley’ manual includes detailed colour wiring charts. If your wiring is not exactly as shown in the diagrams, it is likely that a feature was added or removed. In general, the following items were monitored: Rear lights: (Tail, Brake, Turn) The tail light connection was dropped later. Rear window defogger. Battery condition and charging circuit. Engine compression. Ignition Timing. The diagnostic socket had 24 wiring pins, laid out in four horizontal rows of six pins each. There were numbered 1-6 along the bottom row, left to right, then 7-12, then 13-18, then 19-24 on the top row (left to right). In addition, there were four additional connections in the centre, with the standard VW terminal numbering conventions of 15, 50, + and 31. Pins 9, 10, 11 and 12 are connected to the tail, turn signals, and brake light wiring. The computer could only tell whether voltage was reaching these lights, not whether the lamp was burned out or not. (It would be possible to monitor current draw via the battery ground strap shunt, but I don't know whether this was actually implemented.) Similarly, the connection to the rear window de-froster relay (pin 5) only told whether the relay was supplying power to the heating element. Various checks could be made on the battery circuit. The open-circuit voltage was measured from pin 31 to pin +. With the engine running, charging voltage could be measured. It was also possible to apply external load currents while observing the battery voltage. Generator (or alternator) charging current was obtained by measuring the voltage drop across a specific length of wire. The ends of this wire were connected to pins 6 and 14. On the generator, this wire is the one running from D+ to the regulator. On alternators, the wire runs from B+ to the battery wire splice. Battery electrolyte level was checked by means of a special probe built into one of the battery centre cells. (Try and find one of those today!) The probe is brought out to pin 7. The battery ground strap has two wires attached to its ends and going to pins 8 and 17. These connections measured the voltage drop across the strap when the starter motor was cranking the engine. The computer could operate the starter by applying power to pin 50, also handy for operating the starter from the engine bay. The computer would ‘see’ a series of current spikes corresponding to the compression stroke of each cylinder in turn. The magnitude of these current pulses was translated into rough values of compression pressure, and used as a measure of engine condition. Engine idle RPM could be calculated from pulses received at pin 13 (from the points opening and closing). Point dwell was also estimated from this signal. A hand-held timing light was also used to check the timing setting. In early ‘72 a small pulse transformer was added to the #1 spark plug wire, and produced a signal when #1 plug fired. This signal was fed to pins 19 and 20. The transformer was basically a ring that fitted around the outside of the spark plug wire. There was no physical connection to the actual plug wire; current flowing through the plug wire merely induces a voltage pulse in the transformer winding. Beginning in ‘74, a sensor was added behind the flywheel to detect TDC. As far as I know, this sensor was a variable reluctance type of pickup. The flywheel had a steel pin on its backside, which swept past the pickup face to create the pulse. The combination of the TDC sensor and the #1 plug wire pickup allowed the computer to accurately determine initial timing setting and even to observe timing advance with speed change. The diagnostic program was quietly dropped in the late 70’s; I’m not sure of the exact date, but it probably came about when the new water-cooled VWs began getting fuel injection computers and other sophisticated management systems that didn’t need a separate ‘Diagnosis’ system and plug. There must have been considerable production cost involved in all the special wiring and sensor devices, not to mention the cost to VW dealers for the special equipment to read them. It probably made the dealer’s workshop job a bit easier but it's doubtful whether the program ever had any significant effect on sales.
From reading the above, if you took a simple length of wire and put a male connector on one end and stuck it in to terminal 50 had a switch in the middle attached it to any + feed in the engine bay (coil, battery) you could use it as a simple remote starter? Or not
Glad I found this thread, I've been wondering for years what it was, on and off. I kind of suspected it might be a diagnostic thing, but I didn't know there was such a thing as a fancy engine diagnostic for the T2. Anyway, now I know it's pretty useless I think I'll take it off.
