Emergency Start Switch

KatanaPilot · 02-20-2016, 10:42 AM

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.

bob dirt · 02-20-2016, 12:16 PM

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".

KatanaPilot · 02-20-2016, 12:20 PM

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.

backdoctor · 02-20-2016, 03:58 PM

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.

bob dirt · 02-20-2016, 04:51 PM

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...

MNtraveler · 02-24-2016, 02:50 AM

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.

backdoctor · 02-24-2016, 07:05 AM

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.

AquaMan · 02-24-2016, 07:16 AM

Quote:

Originally Posted by MNtraveler

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.

KatanaPilot · 02-24-2016, 07:50 AM

Quote:

Originally Posted by backdoctor

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.

AquaMan · 02-24-2016, 08:30 AM

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.

backdoctor · 02-24-2016, 09:04 AM

That was my thinking. Once the rig was running, no further need for the connection to be live. Bob, you took power from the BCC fuse block? Did you disassemble the panel to install? Can you post a schematic and photos please? So you would power the coloured wire and run the other to ground?

bob dirt · 02-24-2016, 09:17 AM

The wires are already there. The missing fuse (F-1) powers up the circuit. You can also just jump the ES terminal to the coach battery connection on the isolator manually. About 15 seconds after the coach starts, you can hear the isolator power up thus, charge all the batteries.

bubbles · 02-24-2016, 09:24 AM

Quote:

Originally Posted by backdoctor

That was my thinking. Once the rig was running, no further need for the connection to be live. Bob, you took power from the BCC fuse block? Did you disassemble the panel to install? Can you post a schematic and photos please? So you would power the coloured wire and run the other to ground?

Yes, switch should be momentary. Actuate for start and then release. Switch can be any type (rocker/press/toggle etc.).

chengdave2003 · 09-20-2017, 07:46 PM

I found this thread after going into the battery control center to check out what was in there, and I realize it is quite an old thread. I found the emergency start circuit almost completely wired up. I did not know about the two wires underneath...I'll have to check that out.

What I understand from what I've read is that I have to find the tan and the gray pigtail underneath the motorhome; I have already found the wire loom, I just haven't opened it. To these two wires I need to run them to wherever and connect them to a 12vdc momentary switch. Also a fuse or circuit breaker needs to be installed in the appropriate spot in the fuseblock. What size fuse/circuit breaker would be recommended in the circuit?

Is this correct????

backdoctor · 09-20-2017, 08:16 PM

Mostly. There is already a fuse location in the block of the BCC so you just need to add a 10 amp fuse to make it safe. The wires are in a black bundle under the area beside the step. There's not a lot of space to work in. You'll need to use a shrinkwrap to protect the connections.

I used a static switch instead of a momentary one, so I could switch the solenoid on when I'm driving and charge the coach batteries if they are at 80% or above. Normally the BCC won't close the solenoid to connect the systems to charge the house batteries unless they are below 80%.

Eric Jackson of the frozen North

chengdave2003 · 09-21-2017, 09:17 PM

Thank you. I've got heat shrink and everything I need to do the job, except for the switch. I ordered a resetable 10 anp circuit breaker. Now to go ID those pigtails....

bubbles · 09-21-2017, 09:42 PM

If your BCC is like the Georgetown BCC a wire to a momentary switch common terminal is spliced to the pigtail from the BCC fuse and from the switch N.O. terminal a wire is routed back to the BCC and spliced into the other pigtail. In other words the switch is wired in series with the pigtails to the previous mentioned terminals. The switch supplies 12 volts to the interconnect relay within the BCC to tie the coach and chassis batteries together.

chengdave2003 · 09-21-2017, 09:44 PM

That makes sense. I just need to go look for that wiring...

Thank you!

bubbles · 09-22-2017, 06:55 AM

Quote:

Originally Posted by backdoctor

Mostly. There is already a fuse location in the block of the BCC so you just need to add a 10 amp fuse to make it safe. The wires are in a black bundle under the area beside the step. There's not a lot of space to work in. You'll need to use a shrinkwrap to protect the connections.

I used a static switch instead of a momentary one, so I could switch the solenoid on when I'm driving and charge the coach batteries if they are at 80% or above. Normally the BCC won't close the solenoid to connect the systems to charge the house batteries unless they are below 80%.

Eric Jackson of the frozen North

I may be completely wrong here but I believe the chassis battery has priority for the charging of both coach and chassis batteries when the engine is running not the coach battery above 80%. I think the chassis battery has to be above 13 volts to trigger an interconnect relay paralleling the coach and chassis batteries. If the chassis battery voltage falls below about 12.5 volts the interconnect relay will de-energize. I think just the opposite is true for the batteries if connected to shore power. You diesel guys are a different breed from us gas guys but the BCC functions may be the same, or not. Oh well just a thought. I am on uncharted waters.

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