Batteries: Testing the Boost Switch

by Brett Wolfe 1993 U240

RPM should be raised to high idle on all tests of the alternator.   

I don’t know where the boost solenoid is located on your coach. In mine, it is next to the batteries.  

To troubleshoot a boost solenoid:  

It will have two LARGE lugs– one to each battery bank (chassis and house).  

It will have either one or two small terminals. If one, it is the positive/signal wire. If two, one is ground, the other is the positive/signal wire. On the one terminal solenoids, the body of the solenoid serves as the ground.  

With the switch off, the voltage at each of the large lugs should be DIFFERENT, reflecting the difference in voltage of their batteries. And the positive signal wire should have no voltage.  

With the boost switch ON, the signal terminal should have 12+ VDC and the two large lugs should have the same voltage.  

If voltage is still different on the two large lugs, and you do have 12+ VDC on the signal terminal, the solenoid is bad. If no voltage on the positive signal terminal, you have a problem with a fuse, switch or wiring. To verify that both this AND the solenoid are not bad, remove the wire from the signal terminal and take a small jumper wire from either large lug to the signal terminal. A small wire is all you need as you are only talking about 1-2 amps. The solenoid should not “CLICK” and the two large lugs should have the same voltage.  

Brett Wolff