Q: No heat after replacing dead alternator

The alternator, low battery, and fuel light will have no effect on your heat. Your lack of heat is a separate issue aside from a car that won’t run and cannot produce any heat. Someone has suggested you check the blower motor and relay indicating you don’t have any air flow out of your vents. It’s important to understand there are two scenarios here that can create a lack of heat output. There can be no air coming from the vents, or the air coming from the vents will not get hot. These are two distinctly different problems with different solutions.

If you don’t have air flow, tap on the blower motor with a small hammer while the blower is on. If it begins to work again, you need a blower motor. This is always the first step I take when a vehicle does not have air flow out the dash vents. If this doesn’t work, then it is time to consider other parts of the electrical circuit. The relay is certainly a good place to begin, as well as checking fuses and the blower motor resistor. Blower motor resistors are the most common failure items in these circuits. If all these check out good, the last thing to consider is the blower motor switch on the dash.

If you have airflow with cold air, you will first need to make sure the cooling system is full. Do this when the system is cold by removing the radiator cap. If it is low, you have a leak in the cooling system that needs to be repaired. Top it off if it is low and be sure to fill the overflow jug as well. Run the motor for awhile and keep in mind you may need to bleed air from the cooling system, if it was low, this will affect the heat output in the cab because air bubbles always seem to begin in the heater core first.

If that doesn’t fix it, you will want to feel both heater hoses, assuming the cooling system is full. Both hoses should be hot with one being slightly cooler than the other. If one is cold and the other is hot, you have a clogged heater core. If both are cold, you have an air bubble in the heater core and possible other parts of the system.

If you need further assistance getting this checked, a certified professional from YourMechanic can come to your car’s location to diagnose the heating issue and follow through with repairs.