Below is a description of how the 120 VAC system functions on our 1984 310 Classic Motorhome.
There are two time delay transfer relay boxes, the first is located in the rear electrical compartment where the shore power enters. The second is located under the passenger side rear closet in the compartment with the fold down door. The purpose of the two transfer relay boxes is to isolate the shore power from the coach when the generator is running. The arrangement that Airstream used allows for shore power to be plugged in at the same time the generator is running.
There are two scenarios when it comes to providing 120 VAC to your motorhome. The first is from shore power and the second is from the generator. Normal mode would be from shore power.
When you are plugged into shore power (generator not running) both time delay transfer relay boxes are de-energized. In this condition the timer coils inside the transfer boxes are de-energized so power flows through the normally closed contacts.
In the first transfer box power flows through the normally closed contacts and feeds the main circuit breaker panel located in the rear credenza in the bedroom. From there one of the breakers feeds the outlets, the second one feeds the Univolt and the third one feeds the rotary switch in the galley. The galley switch then controls which device is going to be receiving power. In this situation only ONE air conditioner can run at a time. This is due to the shore power feed only being 30 amps which is only enough for one air conditioner (or other device) at a time.
Now lets say while you're still plugged into shore power the generator is started. Once the generator starts producing 120 VAC the coils in both of the transfer boxes are energized. After a period of time (45 seconds to 2 minutes) the contacts change state. The normally closed contacts open and then the normally open contacts close (break before make).
At this time the rotary switch no longer has control over the rear air conditioner. The rear air conditioner is now isolated from the rest of the 120 VAC system in the motorhome. It is now running off of its own 30 amp circuit breaker fed by its own separate power feed (L2) from the generator.
Our generator is rated at 7000 watts. This is just enough to run both air conditioners at the same time. However the generator can not handle starting both air conditioners at the same time. So the way to operate the system when you want to run two air conditioners is to turn one air conditioner to FULL COOL so that its compressor is always running. You then use the second air conditioner to modulate the temperature in the coach. If it gets to where the one on full cool starts cycling its compressor because the coach is cool enough you should turn off one of the air conditioners.
I know the transfer system works because I inadvertently started the generator one day while still plugged into shore power. There was a brief loss of power while the relay contacts changed states and then things were back to normal inside the coach. I personally haven't had a chance to try both air conditioners at the same time because our generator had a bad voltage regulator and could barely handle one air conditioner. The regulator problem has since been replaced and I plan on running both air conditioners when I get home in a couple of weeks.
I have made one wiring change to the rear air conditioner circuit that allows me to use a separate extension cord from the rear air conditioner to shore power so that both air conditioners can run at the same time while connected to shore power.
The second sketch shows the details of how I made the change. I used a standard duplex receptacle. I removed the ties between the two sockets which isolated the sockets from each other. I then disconnected the cable between the rear air conditioner and the transfer relay box and then put a male 120 VAC plug on the end coming from the rear air conditiner. At the point where I removed the rear air conditioner cable from the transfer relay box I added a cable from that point to one of the duplex sockets.
The other duplex socket is connected to an extension cord that can be plugged into a separate 20 amp shore circuit. I then plug the rear air into one of the sockets depending on where I want it to be powered from. I used a duplex receptacle because thats what I had handy. Someday I want to change it to a 2 pole switch to make it easier to use.
A previous owner wired an auxiliary land line on our '83 310 and bypassed the relays that would isolate the shore power from the generator. I hoped to find out how to restore them because, you never know when you're going to fire up that generator without remembering to disconnect!
Thanks!
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Airstream OCD...there is no cure!
1983 Classic 310 Motorhome.
AIR 15765
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