Carrera AC System connected to DME pin #29

[scarceller]

I have been trying to find information on Carrera DME Motronic Box Pin #29. Pin 29 is directly wired to the AC Compressor Clutch signal, the same 12V signal that activates the AC Clutch is also feed to pin #29 on the DME. I can not find any description of how this Pin 29 affects idle but I know it does from tests I have performed. I have applied 12Volts to this circuit with the AC turned 'OFF' and the harness unplugged from the AC Compressor Clutch. I did this with the engine running and fully warm on my 84 Carrera 3.2L and this circuit does indeed have an affect on the idle, it appears to bring the idle up a few 100 rpm, more like 200-500RPM, can't really tell from just the tach but you can hear the idle come up.

I'm not surprised to find this pin 29 connected to the AC as it makes sence that the engineers thought about the AC Compressor mechanical load as well as the added electrical load from the AC Blower(s). It seems that the engineers recognized that the Idle Control Valve alone was not enough to compensate for the AC load. So they added some sort of circuitry to the DME for helping in bringing up the idle.

Does anyone know exactly how this circuitry works? How much should the RPM change?

I'm very intrested in these details because if you have a Carrera with non-stock cams (as I do, I have WebCams 20/21) this little secret may just be a really nice way to move up your base idle from 850RPM to 900RPM.

I can't find any mention in the manuals I have about this link between the AC and the DME pin #29. The wiring Schematics confirm the link, here are few scans:

AC Schematic (from 85 Carrera):



DME Schematic (from Carrera DME Test Plan):



Any input on this would be helpfull.
Thanks in advance.