You can remove the panel under the glove box and check the "etr switch" located to the left of the expansion valve. Check to see if you have power to the switch and across the terminals. ALSO, with the a/c on try reving up the engine by snapping the throttle above 2500 rpm and see if the clutch kicks in, sometimes the clutches on the r-4 compressors get weak.

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'93 300E 3.2
'05 GMC Yukon XL 4x4
'05 Polaris SP500 H.O. LE