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Nuts and Bolts: Cummins Swap Wiring

Posted in How To: Tech Qa on November 26, 2018
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I’m in the process of putting a 12-valve 5.9L Cummins into my 1987 Chevy Blazer. It’s in and I am down to the little details for the most part. The engine is all mechanical, so there are no computers or electronic controllers to mess with. I understand how the fuel shutoff solenoid works, but how am I supposed to wire it? There’s a pigtail with three leads on it on that solenoid; which does what? Also, I’m not sure what to do about the grid heater. It’s my understanding it was controlled by the PCM on the truck, but obviously that is long gone. This thing is plumbed and ready to fire except for this. Help!

Travis T.

The P-pump on your 5.9L Cummins engine has a fuel solenoid that controls starting and stopping the engine. While gas engines are usually shut off by removing spark, diesel engines are stopped by shutting off fuel. At rest, the solenoid is in the closed position, with no fuel flow. It has two other positions: “pull,” in which the plunger in the solenoid is pulled open, and “hold.” Think of pull as start and hold as run. It takes nearly 50 amps to open the plunger controlled by the solenoid in pull mode, so the circuit needs to be a large one and preferably controlled by a relay. The hold position requires much less amperage and can be wired with normal 14-gauge stuff. You can make up your own harness to connect to the solenoid fairly easily, but we found an inexpensive harness that includes a connector to plug into the solenoid’s pigtail from LarryB’s ( It includes a relay as well as a fusible link to wire up the high-amperage circuit safely.

The grid heaters are needed for startup in cold weather. Positioned right at the entrance to the intake manifold, the grid heaters raise the temperature of incoming air in order to make it easier for the engine to start when it’s below 60 degrees Fahrenheit. These heaters draw a tremendous amount of current (around 100 amps each) and need to be wired with heavy-gauge battery cable and special high-amperage relays. We would recommend using the factory Dodge relays to control current to the heaters because they are inexpensive, reliable, and readily available. You are correct that the PCM originally controlled the heaters on your engine, and we weren’t able to find an aftermarket standalone system that works like the factory did. You could wire in a time-delay switch if you want (15 seconds would be adequate), but most people that have done a Cummins swap simply control the relays with a momentary pushbutton switch somewhere on the dash. When it’s cold outside you just press and hold the button for about 10 seconds before cranking the truck. You’ll want to keep an eye on voltage while doing this, as grid heaters can suck weak batteries flat in no time. They are among the reasons most diesel trucks are equipped with dual batteries.

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