I have an electric central heat sytem(Amana). There is a big difference between the temperature coming out of the vents. The highest temperature is about 118 degrees and the coolest is about 88 degrees. I had the installer back out to check it and all they did was turn the damper down on the warmest vent.
Any idea what is causing this big temperature difference?

Some vents are closer to the furnace than others,, the longer the supply the cooler its going to be, I might think about running a second vent to the coolest room

I agree completely with wmproop. And I agree with the installer. Adjusting the registers should help balance the temps in the rooms (not the temps from the register, but the actuall temps in the rooms). If the closer registers are putting out hot hot air, you can shut them down a bit, as you won't need as much to heat the room. This will force more warm or hot air to the further registers, allowing them to heat the space adequately. This is one of the reasons for having adjustable register, to balance the system. Totally normal, let the installer do what he does and you should be fine.