Phooey

For reasons beyond my control, Pumpkin hasn't gone anywhere for quite a while.  So, a couple of weeks ago I went out to do something out there, hit the "unlock" button on my key fob and...nothing.  I unlocked it with the key and got in.  When I put the key in the ignition, nothing happened.  Usually, the instrument panel lights up, but nada.  Dead battery.  No problem, though.  Most motorhomes, including Pumpkin, have two sets of batteries.  One is the main engine or chassis battery, the other is one or more deep cycle batteries for the "house" portion, the power for "stuff" in the back.  Because motorhomes are often parked in the boondocks for a while, having a dead battery when you get ready to leave can be a major inconvenience.  So, in their vast wisdom, the RV makers decided to put a special "Battery Boost" switch on the dash that, when energized, connects both battery sets together which effectively boosts the chassis battery enough to get the engine started.  It's like a built-in jump start.

So, finding a dead battery, I hit the Battery Boost switch to give it a jump start...nothing.  Well, almost nothing.  I could hear the solenoid click that indicated the Battery Boost solenoid was energizing.  When that happens, the house batteries should be connected to the chassis battery for a boost.  But, nothing.

The house batteries (there are two connected together to act as one big battery) were fully charged.  Pumpkin had been plugged into power from my workshop.  When plugged in, the house batteries are kept charged by the converter.  But, not the chassis battery.  For whatever reason (cost, no doubt), most manufacturers don't include the charging of the chassis battery in that process.  But, they do include the Battery Boost switch to take care of this...when it works.  Pumpkin's Battery Boost switch did not work.

I put a charger on the totally dead chassis battery.  It took about 24 hours for my 10 amp charger to get the battery back up enough to start the engine.  After I got the battery charged, I tackled the no Battery Boost problem.  It wasn't too difficult to determine the problem, but getting to it was another story.

The Battery Boost solenoid has to connect a big heavy battery cable from the house batteries and a big heavy cable from the chassis battery.  Its location is hidden under the front passenger seat.  So, first, I had to remove the seat.  Of course, Mercedes used unusual bolts.  Well, at least, unusual enough that it sent me to O'Reilly's to buy some special sockets.  They are called Torx heads, but I don't think Mercedes calls them that.  I had a large Torx bit set, but not a Torx socket set.  That's all I need, more tools.

Seat removed.  Once I located the four bolts and also realized the seat had to be swiveled a little to come off, it was done.

Now to expose the Battery Boost solenoid.  The solenoid is the round silver thing with the battery cables connecting to it.  When it is energized, the two large terminals on either side are shorted together - or, at least, supposed to be.

Here's what the solenoid looks like.  The smaller center terminal is where the wire connects that comes from the dash Battery Boost switch.

Here's a photo of an "exploded" view of the defective solenoid.  The copper bolts are the heavy terminals.  One bolt (on the right) has a black coating on the side of the bolt head while the other (on the left) I have cleaned up.  The round thingy with the spring on it is the plunger with a copper washer that was also blackened (I have already it cleaned).  For normal operation, when the solenoid is energized, the plunger is pulled down so the copper washer connects with both bolt heads.  I probably could repair this solenoid by cleaning it up good, but since it is not easy to get to, I'll install a new one.

Now it's time to start reassembling.  I received the new solenoid in the mail yesterday.  But, what about the original problem - chassis battery run down?  Well, there's a gizmo available that is connected across the two battery systems and will maintain trickle charging for the chassis battery so when the RV is plugged into "shore power", the chassis battery is trickle charged.  The device I got is named Trik-L-Start.

A convenient place to connect the Trik-L-Start is right there on the Battery Boost solenoid, so that will be part of my reassembly process.  I've elected to mount it on the outside of the seat base where I can see the three monitor lights on it.  I'll run the three wires from it through a hole in the seat base into the area where the solenoid is located.  Easy-peasy.

Here's the finished product.  Trik-L-Start mounted on the outside next to circuit breakers.

The solenoid is mounted and connected with the Trik-L-Start yellow and blue wires connected to appropriate sides of the solenoid and the black wire connected to ground.  The two heavy battery cables coming together on the right post of the solenoid are the house battery cables.  The short one of the two going to the right connects to the battery disconnect solenoid which is controlled by the Aux Batt (battery disconnect or "salesman") switch in the coach.  This solenoid disconnects most of the devices that use 12-volt power in the coach, but not all (co2 sensor, LP sensor, etc.).

The Battery Boost switch works and so does the Trik-L-Start.  Will wonders never cease?

Now to reinstall the seat and it's done.  But first, a nap.

Okay, that's done - both a nap and seat reinstalled.  But, before reinstalling the seat, I made one more voltage check.  While plugged into shore power, the coach battery measured 13.86 volts and the chassis battery measured 13.31 volts.  The measurement of the chassis battery indicates the chassis battery charge is being maintained by the Trik-L-Start.  Otherwise, it would measure 12.7 volts or less.  Now if this dead battery episode hasn't done too much damage to my chassis battery.