Article and images: Andrew Atwood
 
With the wiring harness and exterior lights installed, I could proceed to rewire the car. Most of that is routine – simply reconnecting the cleaned and rebuilt components – following the wiring diagram, photographs taken during disassembly and tags on the harness. However, I mentioned that during my electrical rewiring of the car I was going to relocate the battery to the trunk for considerations of safety, weight distribution and engine access.

Moving the battery to the trunk

Like other modification projects, planning ahead before starting the work is critical. Relocating the battery to the trunk is not as simple as bolting it in and running wires. First I needed to decide exactly where to locate it. After considering the center of the trunk behind the seats (not very attractive) and the wheel well opposite the driver (already occupied by the spare tire) my only obvious choice was the left-rear wheel well opposite the spare. The four-corner self-leveling system I was installing would compensate for the added weight and the battery would be tucked away in another unused portion of the trunk.

To mount it, I removed the sturdy factory battery bracket from the donor car for the W117 engine, the tired-old and venerable W126. It was a simple job to fabricate two small brackets that bolted to both the inner side and bottom of the wheel well, keeping the battery level and above the curved metal floor of the wheel well.

With my battery mounted firmly, I had to run an entirely new positive-charge battery cable all the way to the front of the car to both the starter and alternator, taking into account current flow and the voltage drop that occurs when running large amounts of electrical current over long distances. With no fuse drawing more than eight amps, some of you might wonder what would be considered a big-amp draw on this old car.

Actually, the starter for the newly installed W117 V-8 can draw up to and sometimes exceed 250 amps on very hot days. In addition, with the electronic ignition and other fuel-injection components such as the fuel pump and warm-up regulator, one might see close to 275 amps.

For that reason, a stock battery cable or even one or two sizes larger would still not be adequate and could cause hard starts, not to mention blown fuses and melted wiring. There is a formula to help calculate the amperage to ensure the correct wire size is used for the corresponding wire length and current draw; most wire distributors’ websites such as www.bulkwire.com have the necessary calculator.

To be extra sure, I calculated a 300-amp draw as a worst-case scenario, taking into account my new custom air-suspension compressors. I estimated the length to be 20 feet to take care of bends in the wiring. I like to run no more than a half-volt drop from battery to starter, which I find will allow most starters to operate comfortably down to a minimum battery voltage of 11.5 volts and not create any damage. I ended up with a 2/0 wire, wire speak for double-0 or “double ought,” as it is commonly called. This wire size would give me about a .477-volt drop, which is about what I wanted. Perhaps I was being overcautious, but this will prevent many electrical issues and give me room to add more modifications later. I like to think ahead.

Thinking about safety

While wiring the battery, I decided to install a battery-safety cutoff or battery-disconnect switch in case of an accident that could pinch and short out the battery cable, which could easily lead to electrical sparking and damage to precious components or even worse, fire. So I installed a resettable battery-disconnect switch. In case of a direct short to ground on the battery’s positive cable, it will mechanically disconnect the battery as well as shut off the fuel pump to prevent fires. Standard equipment on racecars, these disconnects are easy to buy and install. I usually install mine on the ground-cable side, that way even if the car is hit on the positive battery terminal, the disconnect will still function correctly.

With the battery and cables installed, I could now run all auxiliary wiring for my additional modifications, including the air-suspension compressors, suspension controller and modified fuel-pump circuit. This did require a bit of engineering as the factory pump on the original engine only drew 4-5 amps and the twin Bosch-pump setup for the big W117 engine can draw up to 12 amps. Again I turned to the sad-looking stripped W126 and she gladly donated a 20-amp relay with holder and bracket, as well as an auxiliary fuse block for my cause. I will always remember that car for all that she donated to this project.

A relay and auxiliary fuse block was the common way that the Mercedes-Benz factory made modifications or additions to its cars for either customer-requested enhancements or even electric windows, such as on the early W108s; by doing this, it looks factory original. I used the original fuel-pump wire to activate the new fuel-pump relay, which then supplied battery power through the fuse-block addition with a 16-amp ceramic fuse. This prevents me from pulling too many amps through the old W112 wiring harness and eliminates the need for new wires up to the ignition switch.

For those not familiar with four-pin relays, this is a simple job. Referring to the wiring diagram, the relay needs to be grounded on either the No. 85 or No. 86 circuit; the signal wire or ignition-switched wire is connected to the other side. Then a wire is run from the No. 30 pin to the fuse block and from the fuse block directly to the battery’s positive terminal. To finish the job, the No. 87 pin gets connected to whatever item you’re trying to activate, such as a fuel pump. This means that the switched wire only has to carry half an amp or less to keep the relay energized, thus saving the old wiring.

Restoring the electric antenna

Moving on to some restoration work, this car had an electric antenna installed when I bought it, and through some research I found these were options for these cars for the 1965 price of $45, about the weekly paycheck for the average worker in 1963! The specific Hirschmann antenna that was in my car was first optioned in 1963 for all W112s as well as the W100s that started production the same year. Finding one still on my car was cool: Most eventually failed and were tossed in the trash and replaced with more modern electric antennas or old-school mechanical units. The latter were a bit of a pain as one needed to exit the vehicle to extend it or ask a passenger to reach out and pull it up.

Because I don’t like to replace parts unless absolutely necessary, I decided to tear this one down and rebuild it. What was I getting into now? After disassembling it, I found that at one time it had about two inches of water in it, causing some extensive damage: I cleaned everything to see what was salvageable. To my surprise, most of the damage was from corrosion to the antenna mast reel. Correcting this just required a lot of cleaning and then careful reassembly and lubrication.

Next, I turned my attention to the electrical side of the assembly. By jumping the motor with a battery, I found much to my delight that it ran perfectly with no noise. All that was left was the hard part: synchronizing the motor and shut-off switches. This took about an hour of fiddling and then I was able to get it to work.

The antenna mast was my next obstacle to tackle. They are no longer available for these assemblies so I modified a mid-to-late 1970s Hirschmann smooth antenna mast and got it to fit perfectly: It looks stock. But why spend an eight-hour day rebuilding something hidden in the fender that no one will see? I could have installed a modern replacement and not many would be the wiser. But I’ve always believed that people who restore cars to their own satisfaction are always the happiest. Please yourself and the work will always be rewarding. But enough of my philosophy; now that I’ve got the electrical system buttoned up, it’s time to install the windows and then turn to the interior trim and rubber moldings.
 
Regular contributor Andrew Atwood owns Atwood European Repair and Service (atwoodeuropean.com) in Phoenix, Arizona.
 
 


 
The completed battery installation, showing red battery cable and one of the air-suspension compressors. Mounted on the wheel housing between battery cable and battery is the 20-amp relay with holder, bracket, and auxiliary fuse block scavenged from the stripped W126. Of course, the battery will remain disconnected so long as I am working on the car as opposed to driving it.



Detail of battery cable routing and tie down.



Three views of the cool Hirschmann antenna available as an option for my car from 1963:
unrestored, restored, and fully reassembled. The original motor ran perfectly with no noise.



With motor and shut-off switches synchronized, it is ready to install.



A close up of the installed antenna.



The classy Hirschmann on the fender of my ride.