I have a 1990 Ford E150 van.I can usually do my own repairs,but this one I can't pinpoint.With the engine running,it shows 13.5 to 14 volts,from the battery taken with a multimeter.When I turn on any ONE accessory,it jumps to 16-18 volts.In other words,turn on only the heat,the voltage jumps high.Turn on only the lights,it jumps.Step on the brakes,same thing.It causes the temp gauge to spike high(I assume from the extra voltage),the headlights to flicker,the volt gauge needle to jump.So far the alternator and the voltage regulator both been change with known good units. Any help would be appriciated.