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Old 02-20-2016, 10:42 AM   #1
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Emergency Start Switch

We own a 2015 Forester MBS, similar of course to the Solera, Sunseeker MBS, etc. One thing that we found lacking was the capability to use the coach batteries to jump start the vehicle on the off chance the chassis battery died.

There have been multiple posts about this topic in the past, with the general response from either FR or posters that "Mercedes Benz does not allow this." So I think most of us have just taken that at face value and either moved on, or come up with their own solution - like long jumper cables.

Recently I noticed that the Dynamax division of FR has begun producing an MBS based RV, namely the Isata 3. One of the "features" listed in the brochure is an "emergency start circuit". This of course, peaked my curiosity as to how this could be allowed, given the previous apparent prohibition by MB. I posted on the Dynamax forum and Mr. Clemens did confirm that (1) the Isata 3 does indeed have this feature and (2) the "prohibition" by MB was really a misunderstanding - coach builders could indeed install this functionality.

I had asked for a schematic of this circuitry so I could potentially design and install something similar on our Forester. Turns out I don't need to design anything. If your Forester (or I'm guessing any FR motorhome) that has the Precision Circuits Inc. Battery Control Center installed, all the functionality already exists. All that is needed is two wires to connect to the existing pigtail and a switch.

I did talk to Technical Support at Precision Circuits and they confirmed my reading of the wiring schematic. Once I complete my install, I will post pictures and a description of what was involved in the installation.

In summary - those of us with MBS based FR motorhomes can easily install an emergency start circuit if desired.
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Old 02-20-2016, 12:16 PM   #2
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Thanks for the info. I was just looking at that yesterday. I am also going to hook up a brake control. There are several unused wires under the seat of my "W".
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Old 02-20-2016, 12:20 PM   #3
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The emergency start switch pigtail wires are not located in the cab - they are in the wiring bundle that exits the back of the Battery Control Center. This is located at the footwell on the left hand side as you enter the RV. The actual wires are just tucked into the wire loom located under the RV in between the BCC and the storage compartment to the left of the door. There is a tan and gray pigtail - one is the 12V trigger and the other is the return to the solenoid. The switch has to go between these two wires. I will locate mine in the storage compartment for protection and since I don't expect to use it except on very rare occasions.

Pictures will follow when install is complete.
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Old 02-20-2016, 03:58 PM   #4
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I looked at the bcc and saw this solenoid and switch and wondered if this wasnt wasn't exactly the situation. I even traced the wires down to the loom as it exits through the floor. I considered completing the circuit to "see what happened"! And in an earlier day would have done so, but the smell of burnt wiring and arcing circuits kept me from experimenting... Thanks for pursuing this. I'll be very interested to learn the outcome.
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Old 02-20-2016, 04:51 PM   #5
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I just confirmed all said above is true. I located the two wires and stripped them back. I disconnected the engine battery by pulling off the negative cable. The battery is NOT in the foot well but is under the driver's side floor beneath a steel cover. The key will not even turn when there is no power. I touched the tan and grey wires together and heard the dash come alive. I did have to install a fuse to the block. There is a location just for this (f-1).I will install a momentary switch on the left side of the passenger seat base for easy operation. This won't be used much but it will be much easier to use than getting access to the battery in the floor and hooking up jumper cables.

Thank you KatanaPilot for the motivation...
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Old 02-24-2016, 02:50 AM   #6
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Good info! Thanks folks.

PS: If you fire up the generator and turn the ignition key to "ON" (without starting, since you have a dead battery, right?), the generator will charge the chassis battery. I discovered this through simple experimentation. As far as I know, it is undocumented. I have no idea how long it would take to charge to a usable level, and this surely depends on how depleted the chassis battery is. Ability to charge requires the battery to accept the charge.

Another useful experiment would be to d/c the chassis battery, fire up the generator, and see if the engine can be started off of that. However, I would want one of the electrical gurus to comment on whether that would be safe or too excessive a load on the genny/converter.
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Old 02-24-2016, 07:05 AM   #7
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Senor dirt, can you describe why you need a fuse in the circuit? And how you will run the wires to the cab, inside the chassis or outside and back into the cab via the firewall? What kind of switch? Push and hold, or a single pull or rocker? This is mod many will want to make, so the greater the detail the better. Photos always appreciated.
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Old 02-24-2016, 07:16 AM   #8
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Quote:
Originally Posted by MNtraveler View Post
Good info! Thanks folks.

PS: If you fire up the generator and turn the ignition key to "ON" (without starting, since you have a dead battery, right?), the generator will charge the chassis battery. I discovered this through simple experimentation. As far as I know, it is undocumented. I have no idea how long it would take to charge to a usable level, and this surely depends on how depleted the chassis battery is. Ability to charge requires the battery to accept the charge.

Another useful experiment would be to d/c the chassis battery, fire up the generator, and see if the engine can be started off of that. However, I would want one of the electrical gurus to comment on whether that would be safe or too excessive a load on the genny/converter.
The chassis battery is probably being charged from the converter and not directly from the generator. This is just a guess though.
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Old 02-24-2016, 07:50 AM   #9
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Fuse and toggle switch required

Quote:
Originally Posted by backdoctor View Post
Senor dirt, can you describe why you need a fuse in the circuit? And how you will run the wires to the cab, inside the chassis or outside and back into the cab via the firewall? What kind of switch? Push and hold, or a single pull or rocker? This is mod many will want to make, so the greater the detail the better. Photos always appreciated.
There is a wiring schematic on the back of the Battery Control Center (BCC) cover. The fuse provides power to the emergency start circuit 12V trigger. Without the fuse, there is no power and a fuse will protect the start circuit.

It's your choice if you run wires all the way back to the cab - the pigtail is about 6" long. I'm going to install a weatherproof guarded switch inside the adjacent storage compartment. I don't anticipate needing this capability often. The switch needs to be a toggle (on-off) not momentary contact if I read the schematic correctly. It's not a latching relay, so continuous 12 volts is needed to hold the relay (solenoid) closed.

I am reasonably sure that shore power also charges the chassis battery through the 120vac to 12vdc converter. Measuring chassis voltage at the jump start terminals under the hood shows converter output voltage.

Photos will follow when the mod is complete.
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Old 02-24-2016, 08:30 AM   #10
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I think the idea of the momentary switch is that you hold it while starting, that way you don't forget to turn it off.
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