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To Shutoff Solenoid Or Not To Shutoff Solenoid: Answering A Common Cummins-Swap Question

Written By, Mike McGlothlin


How do you shut down a diesel engine? By cutting off its fuel supply. In the busy world of Cummins swaps, and in particular where the Bosch P7100-equipped 4BT and 6BT are king, owners are faced with two primary means of turning off their engines. One comes through the use of a fuel shutoff solenoid. The other comes from foregoing the use of a solenoid and manually cutting off fuel flow to the injection pump. To some, it’s a minor detail. But to many others it’s the kind of thing that can make or break a project.

After all, using a cable-activated kill switch on a show-worthy Cummins conversion doesn’t quite fit the bill. But on the opposite end of the spectrum—where a purpose-only trail-rig, rock crawler, sled puller, drag racer, or farm vehicle doesn’t exactly warrant any such sophistication—having factory-like fuel shutoff operation isn’t necessary, or worth the costs associated with adding one. If you’re tying up the loose ends on your own Cummins swap or are finishing a P-pump conversion and you reach this point, this article is for you. We’ll discuss the good, the bad, and the ugly for either option, along with how each method of operation works.

What The P7100’s Fuel Shutoff Solenoid Does

Before diving into which method of engine shutdown (and startup) is right for you, it’s helpful to understand exactly what the fuel shutoff solenoid does. The solenoid mounts at the rear of the P7100 near the AFC housing and is activated by the starter. When it’s triggered it moves a small, vertical governor lever, which allows fuel to flow into the P7100. During shutdown, the lever travels in the opposite direction, stopping fuel supply from entering the pump. In a pinch, where the shutoff solenoid has ceased to function, you can maneuver the governor lever by hand (in conjunction with having the key in the ignition turned to the “on” position) to get the engine started.


The Benefits Of A Fuel Shutoff Solenoid

Perhaps the biggest advantage of running a fuel shutoff solenoid is the convenience it provides. After all, it doesn’t get much simpler than key-on, key-off accessories. But on top of that, a shutoff solenoid offers OEM-like operation—something that can be a priority on a high-end Cummins swap project or a vehicle that’s daily driven. Even on ’98.5-’02 versions of the Dodge Ram 2500 and 3500, where the 24-valve Cummins has been treated to a P-pump swap (i.e. ditching the factory VP44 in favor of the P7100, shown), most owners opt for a fuel shutoff solenoid rather than a manual, cable-operated shutdown mechanism.


3 Ways The Solenoid Fails

The electric shutoff solenoid calls for 12-volt power, predominantly for the pull coil operation, which is used (briefly) for startup. It requires several key components to function: the solenoid itself, a 70-amp solenoid relay, and a fusible link. Not surprisingly, a burnt up solenoid, a bad relay, or a corroded or burnt fusible link are the three primary causes of a malfunctioning fuel shutoff solenoid. As with anything automotive, age and long-term exposure to the elements takes its toll on these parts over time.


Which Shutoff Solenoid Should You Buy?

A fuel shutoff solenoid for a Bosch P7100 will never be cheap through your Cummins or Mopar dealer. However, when you buy from an OEM you’re also paying for a warranty of some kind—and a warranty that your supplier will likely stand behind. With that said, at $580 to $700 a pop, most buyers turn to an aftermarket solenoid for their personal projects. And while most (if not all) solenoids are manufactured in China these days (aftermarket and OEM), some aftermarket versions do yield OEM-like quality.


LarryB’s All-inclusive Solenoid Fix Kit

One aftermarket fuel shutoff solenoid that enjoys a reputation for offering OEM reliability yet retails for $110 to $115 is the aftermarket unit sold by LarryB’s. But in addition to offering a durable solenoid, the company also sells a complete kit for mounting a new unit (and that retails for less than $180). The comprehensive parts list includes a new solenoid (with the required 2-1/2-inch bolt spacing), a bracket with 2-1/2-inch bolt spacing, a new governor lever, and all-new bolts, washers, and cotter pin. The kit also comes with a new relay and fusible link, thereby ruling out all the potential failure points of the P7100 fuel shutoff solenoid.


The Manual Solution

Now for the alternative to running a fuel shutoff solenoid: the kill cable. This is a common work-around for those on a budget, Cummins owners that’ve experienced solenoid failures in the past, or DIY’ers who are trying to keep things as simple as possible. When the cable is pulled toward the driver, the vertical governor lever on the P7100 stops fuel from entering the pump. When the cable is pulled the engine will crank but not start, which makes for a great poor man’s anti-theft mechanism. With the cable pushed in (away from the driver), the governor lever allows fuel to enter the P7100 and the engine will start.


Why The Kill Cable Makes Sense

It goes without saying that this method is more affordable than buying a fuel shutoff solenoid, as most required parts can be sourced at a local hardware store. However, simplicity is a major driving factor behind the kill cable option. With no shutoff solenoid in the mix, there is no wiring to worry about. This means no solenoid to burn up, no relay to fail, and no fusible link to corrode or burn. For budget builds and dedicated-use projects such as rock crawlers, trail rigs, pulling trucks, and drag racers, the kill cable method suits most owners just fine.


Kill Cable Ingredients

As seems to be the case with most things in life, there is more than one way to skin a cat for any given situation. Installing a kill cable on a P7100 injection pump is no different. However, the following list of parts resembles what we’ve seen an abundance of. In most vehicle applications, approximately four feet of cable is enough to reach the P-pump, and a cable clamp, an L-shaped bracket, and minimal hardware is all that’s needed to finish the job—except for the handle that’s affixed to the cab-end of the cable. For that, T-handles seem to get the nod most often.


Kill Cable Installation

First, the cable is navigated through the firewall (typically by utilizing an existing grommet). On the engine side, the aforementioned L-shaped bracket attaches to the P7100 via the rearmost bolt of the AFC housing and the cable clamp is tied in with the bracket to steady the cable near the point where it connects to the governor lever. Without question, the most important part of the install is ensuring that the governor lever’s travel is 100-percent in either direction when you push and pull on the cable. In the cab, some installers mount the pull handle to the bottom of the dash (shown) or near the kick panel while others prefer it be integrated within the dash itself.


Daily Driven Vs. Purpose-Built

One vehicle shown here has a fuel shutoff solenoid onboard and the other doesn’t. Can you guess which one is which? The second-gen Dodge Ram, which conceals a P-pump swapped 24-valve Cummins, is street-driven and used every day of the week—reason enough to stick with a solenoid and the convenience of simply cycling the ignition to either fire up the 5.9L Cummins or shut it back down. Over in the mire, a ’32 Ford Victoria serves only one purpose: its job is to blast through 200-feet of slop in 3-second intervals or less. In this purpose-built application, the compound turbocharged 4BT Cummins that propels it gets by just fine with an old-fashion kill cable.

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