Mark VIII Fan/DCC Controller Combo Relay Size?


Mar 9, 2019
Shakopee, MN
So, I've decided to add an oil pressure switch to my Mark VIII fan with DCC controller. I wanted to get my fan to not continue running after the engine is turned off. It does that presently, but not for too long. I messaged the company, but didn't hear anything back, so I assume they must be defunct or something, so any solution is going to be up to me to figure out. Looking at the wiring for the DCC controller, there is nothing that tells this thing whether the engine is running or switched power is turned on, or anything. Maybe it can draw its own conclusion based on the radiator temp sensor starting to see a decline in temp to tell it the engine is off or something; I don't know. And the company was always pretty mysterious, so we may never know. There are a bunch of unused pins on the controller, I have no idea what they do.

ANYWAY, the oil pressure switch will go to a relay that will cut the power to the controller when the engine is off. The Mark VIII fan is notorious for pulling a crap ton of amps, but the DCC controller uses soft start and stop to mitigate that. So that leads me to my question. What amp rating relay would you use for this? I remember my old Dakota Digital controller used 75 amp relays, maybe in parallel or something I don't remember for sure.
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Honestly, I couldn't say. The FK45 that I have is not the current model, but the directions are identical to what I got. The website info about the FK75 (current model) says that it "has higher continuous current of 75a ... " so maybe I can assume that means mine is rated for 45? Maybe? This company tells you VERY little about their product, and most of what they do say is greek to me ("current density"???). All I know is the first version of the FK45, whatever that was, was designed specifically for the Mark VIII fan.
Yep... It's 45 amp. I would match that if I were to install a relay in line with the controller.

A couple of other things:

You also have the option of installing a relay in line with each fan individually. So, don't sweat finding a specific relay. Also, if you're looking for a heavy-duty relay, ignition solenoids are just that.

Additionally, using the oil pressure switch is as good as any other trigger but keep in mind that to do what you are trying to do here will work with any [ignition on] power source as a trigger for fan the fan relays. When the ignition is off, the fans are off... Simple.
Yeah, I actually picked up the oil pressure switch for something else, but thought maybe I would use it for this too. We'll see how it goes.

I was wondering if going to a higher than 45 amp capacity would be an okay thing to do here. I know it's not something you want to do with circuit protection, but a relay isn't really that, so a higher capacity one would be added insurance, I'm assuming? I actually bought a 70 amp Dakota Digital relay to use, although if that's not the right way to go, I could return it. I've had experience in the past with my fan controller setup not being up to the task, and I'd really rather not repeat that. Most of the relays i can find are $10 Chinese made cheapies, and I just don't want to trust though. Dakota Digital is at least a reputable company, and that's the largest one they offer.