When AC is on, Trans in D, car stopped at light (foot on brake) the instrument panel digital readouts will sometimes dim and then cycle back to normal, seems to coincide with an intermittent engine rough idle. I have a new battery, alternator and voltage regulator so it's likely not one of those. Also, when the car is moving the ac blows cold but seems to "hiccup" for like a second where the blower slightly pauses, then goes right back. No problems with cold air really, just these odd issues which all go away if the AC is turned off. 2002 ML320 111,000 miles

The most likely cause is you've got an idle problem. When the engine speed drops the alternator + battery is not enough to keep up with the high electrical load and the lights should dim.

The compressor puts a relatively high demand on the engine in idle and so marginal problems can become more obvious.

As there are quite a few problems that can cause rough idle troubleshooting can take a while so be prepared to spend time or money finding the cause. Personally I would start with cleaning the MAF (a good idea in any case if you haven't done it in a while), then check the fuel filter. If these are the original plugs and wires it is time to change them anyway. If they've been replaced look for loose wires. Once past these items then check the fuel pump.

If you still haven't found a cause I would purchase one of the better OBD scanners that will both record and display multiple sensor readings simultaneously, then leave it hooked up until you duplicate the problem.

Good luck!

Thanks so much! The plugs have been changed but not the wires. That may be the next step, not too $$ and a DIY. I cleaned the MAF a couple of weeks ago, no change. Will keep chipping away at it.

does your ac compressor cycle on and off often? this is a sign of low refrigerant, and will have the symptom you described especially in a stop and go traffic.

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98 ML320 (gone but not forgotten)