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Loss of 12v.....NOT caused by a "salesman switch/solenoid"


D Nelson
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My hope is to put my experience here so that it may save someone else the time I wasted looking, AND having to cancel a trip. It was a 20 minute fix... but took a solid 2 days of researching, tracing cables/wires, and checking voltages before I "stumbled" on to the fix.  Background... The coach is stored indoors connected to a 30amp service when not on the road. We were about to leave for a trip, and I had turned on the fridge to pre-cool before filling, and turned on a roof fan and opened a few windows because it was actually cooler outside than in. The next day (trip day), we take the cold items to put in the fridge, and find the coach completely dark. No house lights, no 12v to the dash, no fridge, no Gen. The only thing working is the 110v wall outlets, and the inverter. 99% of 12v loss posts/threads, on every message board, point to the "Salesman Switch/Solenoid". Mine had been bypassed. So, now I fall into the 1%. Which translates into less that 1% remaining applicable information. In checking voltage at the battery banks, I find the house at 13.2v and the chassis batteries at 8v. Keep in mind the house is charged by the inverter, which IS working. Since the chassis batteries were dead, I had them checked and they were good, but needed to be charged. Im puzzled by this because there is a maintainer, and the coach is on shore power. I charged the chassis batteries. I now have ignition and dash lights but engine will not turn over. I decide to work on one problem at a time.  I  start checking voltage at the rear power distribution bay. Im getting the same volts, as at the house battery bank. When I check the chassis side, I get multiple false voltage readings. In the power dist. bay, negative leads for both sides, come in between the 2 main disconnects, and are connected by a fused block.  The fuse is intact, and I was using it as the ground to check voltage at every terminal in the bay. The false readings prompted me to look in the direction of a ground issue. After tracing main cables and wires, front to back, I found no smoking gun. I took a piece of Romex and connected the ground wire directly to the chassis and touched the other end to the neg post of battery bank... all systems came up! A trip to the auto supply and about $15, 12v problem is"neutralized". But, the engine still will not turn over. 😠! After a short mental break,  I realized that when it wouldn't start before, I had tried to do a rear start... thinking it might be the ignition tumbler causing the start problem. I had left the switch set to "rear". Flipped to front and all is right with the world.  I spent a lot of time tracing bundles of cables and wires. Many of them are impossible to follow as they disappear behind frame and shields and have way too much excess. Its been my experience, grounding is the most common component of a 12v circuit to fail. I knew this, but overthinking the issue clouded my thought process. KISS(keep it simple-stupid)

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Bad grounds are what cause many members headaches beyond comprehension. They seem to be one of the most posted about and causes for problems on this forum.

Some members have posted to check all grounds at least once a year for cleanliness and how secure they are.

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Cool post, glad you got it fixed and thanks for the information. I had my bus move at the storage place to do some work on it. I got a call saying it wouldn't start. I had to go to the storage area and figure it out. I just put in new batteries, so was confuse. The place had 3 guys working on it and nothing. All the switches were on. I looked at the rear start an it was switched to the rear. Flipped the switch and it started with maybe 2 revolutions. Had 3 embarrassed lot guys and a good laugh. Apparently one of the got switch happy.

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Tom, your Diplomat may not have this switch which would be located in the engine compartment.

It was mainly installed there to be used by mechanics so they wouldn't have to go back to the front inside to start the engine after working on it for what ever reason.

I use it to start the engine after the service techs have done routine oil and filter service each year. if there is any fuel or oil leak I can shut the engine off quickly back there.

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