ASCO Series 300 ATS No Transfer

I have an ASCO series 300 ATS 3 phase 120/208 400 Amps. When utility power fails it sends the signal to start the generator as it should, but it does not transfers to emergency. I have check voltage line to line and to neutral and hertz and everything seems correct. Even the emergency power available light at the membrane is lit. I manually transfer to emergency and when utility returns it will retransfer and shut down the unit properly. I removed the control board and installed on another ATS same voltage and amps, and it works perfectly. Just in case I also replaced the rectifier bridge with no results. The coil seems to be working since it retransfers. I have talk to ASCO but they pretty much want to send a tech to verify the unit. What am I missing?


My guess is a defective or out of adjustment microswitch, or a wire pulled off of same.

The operator’s manual is not going to be much help in this situation. The actual schematic may be far more helpful if you can lay hands on it. Once you do so, you will see that if you eliminate the control board and rectifier/actuator as suspects, there are few components left. Two of them are microswitches mounted on and activated by the switch mechanism. I have not called on ASCO recently, but a couple of years ago, an electronic copy of the schematic could be had for the cost of asking for same and having the model and serial number of your switch on hand.

Above is a snippet.