New to coding need help on for loop [on hold]

I am trying to code an activity we did in class the first 3 if statements work but the last 2 shows an error statement has no effect [-Wunused-value].

#include <iostream>
#include <cmath>
using namespace std;

char getSign()
{

    int grade;
    char Sign;
    cout << "Enter grade: ";
    cin >> grade;

    if(grade == 100)
    {
        Sign = 'A';
    }
    else if(grade == 95)
    {
        Sign = 'B';
    }
    else if(grade == 90)
    {
        Sign = 'C';
    }
    else if(grade == 85)
    {
        Sign == 'D';
    }
    else
    {
        Sign == 'F';
    }
        return Sign;
}


int main()
{
     cout << "Your card grade is: 0" << getSign();
    return 0;
}

The result is if I do enter the number 85 or any other number it show 0w