If it's a 2.4ghz Tx, then it has to be bound to the receiver. And you have to use the receiver specific to that transmitter. If it's an old crystal type transmitter, make sure the crystals are of the same frequency. If they are, and if the batteries in your Tx are new and the receiver pack is charged in the car, then you probably have a bad set of crystals. ebay would be your best bet for those.
Without seeing the components, it's going to be hard to help on this. But I will keep trying my best to help out.