Hi
When the jeep is idling (at traffic lights) with the air conditioning on the oil pressure drops and the check guages light comes on. When driving along the pressure is fine. Also if i turn the A/C off at the traffic light the oil pressure is still lo w but not low enough to cause the check guages light to come on. Any ideas ?

The A/C is putting a strain on your engine which is lowering the idle a little.That little bit is enough to lower your oil pressure enough to trip the light.You can raise the idle a little to compensate for this or try a little thicker oil for the summer.It can also be your oil pump is tired but try these to things first.

what is the idle speed with the ac on and the car in drive? it should be 450, if it is lower than that i recomend changing the Idle Control Valve on the back of the throttle body.

http://www.partsamerica.com/PPF/SourceArea//DestinationPage//PartType//PTSet//year/1996/make/JEEP/model/GRAND+CHEROKEE/submodel/LIMITED/SearchFor//VehicleEngine.asp

click 6 or 8 then fuel injection then idle control