Mono Valve Coil?

[Cummmins]

I replaced the mono valve plunger a while back in my car which helped with the vents blowing hot air when set on cold (ac off). The air when set to cold is still warmer than it should be. I found that coolant is leaking by the mono valve into the heater core. If I put pinch-off pliers on the heater hose just before the mono valve the warm air issue goes away. I just inspected the rubber on the valve yesterday and it is in good shape. What I did find is that when the valve is actuated outside of the chamber is that it has very little force keeping the plunger down, or closed. I'm sure that coolant is just easily forcing it open. I have confirmed that I have a good 12v at the connector, and a good ground. Can I get a new coil, or do I have to get the entire $300+ valve assembly?