Im using a vixen air compressor, When it gets to about 120psi the air compressor sounds like it is working a lot slower, from 0-120 the sound of the compressor is constant then starts fluctuating at higher psi. Then I shift the car to neutral and it gets full power again, and when i shift back to drive the compressor slows down again. Sometimes in drive it goes back and forth between full power and less power. Im not sure why it sounds like the voltage drops in drive and gets raised again in neutral. Anyone else ever had this?
Welcome to the Forum! For starters, what kind of vehicle do you have? Next, question, Have you checked Grounds, Positive connections for the Compressor, Battery Connections, are the connections all corroded? Is there maybe an issue with with power wire for the compressor? Is The Ground for The Compressor clean and tight? Does this problem happen when the Headlights are on or off? Check Alternator connections and ground. Have you tried turning the key to the “On” Position but engine not running and then shift to Drive? Just to name a few, Hope this helps 
Hey Repeatman! My car is a 2005 Honda Accord v6. My ground is bolted into the frame which seems pretty good to me. The positive is a short 12 gauge inline-fuse wire that came in the kit which I soldered into 10 gauge wire that leads to the trunk where the relay for the compressor is. Im not sure if the 12 gauge section is causing an issue as this is the part thats closest to the battery. I recently did a thorough cleaning of both terminals. No corrosion, this is a new build i put in less than a month ago and its been doing it ever since. The headlights dont need to be on for the problem to happen. I have not checked the alternator connections/ground but i did do a multimeter test to make sure its charging the battery, which went from 12.8v to around 14.6v when the car is on. My compressor ground is connected to the pressure switch, and the switch is connected to a frame bolt. I feel like the problem is definitely a power issue since switching the car into neutral seems to boost the voltage/amps to the compressor. I hope its not an alternator issue 
What size battery is in your car?
How old is it?
What is your alternator output?
These three questions get overlooked when dealing with medium to heavy load situations like using a DC compressor. What you need to find out is what kind of load you’re putting on the battery and alternator. It might be that it can’t handle all the electronics (PCM, transmission, all the rest of the electrical load the car has) plus the compressor kicking on and running.
You might want to bump up the battery size, or add an extra battery to help with the load. The worst case scenario is you might need to add a slightly larger alternator … you’re prolly in the 120 amp range and might need to go to a 160.
-Kris
The only replacement alternators i can find for my car are 110 amps. I can probably bump the battery to an 800 range cca, mines is currently 550 cca. battery is about 2.5 years old.
Then your best bet would be to add a second battery to ease the load, then the alternator can play catch up while the primary battery feeds the rest of the car.
-Kris
It sounds like too much work getting the alternator wired to charge the trunk battery. I might just leave the issue since the compressor seems to fill the tank eitherway. Next time i replace my battery ill just get the fattest one that fits in my battery tray lol
Actually, it’s not that bad.
Using some 2/0 welding cable, you can run it your positive from the main battery right down the side of the frame to the trunk, and then the negative to the frame.
You can source it from Amazon/eBay, or a welding shop. If you can find a long set of 4 gauge jumper cables, you can go that route as well.
-Kris