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Appears to be the additional demand for amps from the alternator is causing the alt to put a little drag on the engine. Do you have a 60amp or 70amp alt on it now?How high is the idle if you turn everything on, adjust the idle to 800, then turn it all off? Is it livable with it running a little higher w/nothing running but the engine?
In the absence of any idle-up circuits, this seems to me like normal and expected behavior. The idle on our '93 with a 22RE drops when I turn on the headlights. On my 85 it does not, however the idle is around 1100 RPM. What is the idle set at on your other 85?To check this, start the truck and then let the battery reach full charge. Unless I am mistaken, the alternator output at this point should be zero amps. Measure it just to be sure! Call this number A0. Now turn on the headlights and measure the current draw at each headlight. Call these H1 and H2. Now measure again the current output of the alternator, and call that H1.In an ideal world, H1+H2 = A1-A0. A tiny difference is likely not be a problem; check against a known good system for reference. A huge difference means trouble...Make good and sure you are not charging the battery while this is happening. The amps going into the battery will change with time and potentially throw off your measurements.
I'm curious... what were the results of the multimeter check? Did you post them and I missed it?
The amp output and draw when the components are on is the issue, not the watts. Did they bench test it or on the vehicle?