1990 problems air,headlights,horn,
 

1990 AIR compressor will not engage,,,,,,2016 new a/c hose , compressor, orface value, dryer, a/c man hose , ac oil, Help,,,,,,,,,,,,,,,ok headlights have new motors and open and function ,,,but do not close.............horn does not work fuses all good,,,,,,,HELP ANY IDEAS

Sounds like my car after I left it sitting for a couple years under a cover.

Be absolutely sure to start with a FULLY Charged battery. Measure the voltage with engine off it should be 12.5-7 Vdc Check the ground connections, one dirty ground will make all kinds of problems.

Using a good volt-ohm meter check the voltage at the fuse block at the end of the right dashboard. Find the best ground you can get, and measure the battery open circuit voltage. Then do the battery test at the battery. If they are different then you probably have a voltage drop like mine was when I first started trying to figure it out.

First check the little post behind the battery, it is easy to trace as the red wire from the battery (+) goes to it and supplies voltage and current to several parts of the car. Corrosion likes this spot, this is where several "Fusible Links" get their power from. I suggest removing and cleaning this connection if there is any sign of corrosion.

The main problem I had was that when I looked at the fuses they were "perfect" or at least looked perfect. I took my meter and checked the resistance and that way the fuse was "open". I replaced all the fuses and then I use a old point file and cleaned the connections at each fuse. It looked okay but after cleaning the voltage jumped up to full battery voltage! Last but not least is the mini fuse block behind the dash. Here were a couple grounds that went bad and once cleaned all the accessories in my car were working including the radio, it had full battery voltage. Suddenly everything worked like a charm.

Several of the problems you mention all tie to this one point, the fuse panel. I would look hard and find the electrical gremlins you are messing with.

Our Cars have several ground straps on them, if one is damaged or missing it can affect many parts of your car.

You did not mention whether or not the car was or is running. The more info we get the more we understand your plight and help you resolve it.

Good Luck and be careful with that old electrical system!