Is there an allowance for donating a car to charity or whatever?

Only if it has some value to the charity. Is it driveable, or roadworthy with certificates saying it is? Can it be sold as an antique and in good shape. I don't know, what is it you have ? I am assuming you can't get rid of it any other way other then trying to give it away, and probably junk. So, no, junk is not any good to a charity.

There are several agencies that will take even junk cars and give you a receipt that can be used for a tax deduction. If the charity accepts the donation, you should get a receipt.

The charity will give you a receipt, they must be a real charity. They are to give you receipt for actual value, not a higher value

A junk car HAS salvage value, if for nothing else than the metal in the car.

The charity to which you donate the car WILL eventually send you a receipt that reflects what they got for salvage on the car or for they sold it dfor at auction. If you get nothing from them, you can safely claim $500, which the IRS will NOT question.