W140 603 idle speed setting

[pokeyarw]

I've got a 1992 300SD with a 603 engine in it.

I recently had a problem with the fuel filter, (probably ice and dirt), and drove it a couple of hundred miles to get home with less than full fuel flow. I changed the filter when I got home and everything was fine except that the idle dropped to less than 500 rpm with the A/C on and is really low (<400) when the engine is cold. The battery light flickers when first started up from cold. I don't know if running with the bad filter has anything to do with the low idle—only that they seemed to happen at the same time.

It runs fine and has normal power, and idle is up around 750 and smooth when the A/C is off and the engine is warm.

I've looked around for how to set the idle without much luck. There is an adjustment on the throttle cable, but the reading I have done suggests that it's not that simple. Do I have to fool with the ALDA unit, i.e., take the manifold off, or is there something else to be done?

Also, as the idle is so low when the engine is cold or when the A/C is on, I think that the A/C cuts out for low RPM. The compressor reengages when I run it in EC for a while or when I shut off the engine. Am I correct that it is likely that the A/C will be fixed by adjusting the idle properly?