For the past 6 months I have had cooling fan issues. For awhile, the engine would overheat, when I was idling, or just slowing down in general, (unless I put it in neutral and turned on the heater). Now no matter what I do, the fans will not kick on. I'm pretty positive my temp switch maybe is faulty, but not the cause of the whole problem, because the fans don't turn on, even when the ac is all the way up. I recently changed the thermostat, and it didn't help at all. I'm completely lost on this matter, and need to fix this asap. Any help would be greatly appreciated.

Radiator Fan Doesn't Run

Radiator fan failure is one of the leading causes of engines overheating, often resulting in warped aluminum heads, blown head gaskets, and damaged valves.

1. Check all under hood and under dash fuses with a test light or multi-meter:

a. Multi-meter. Set multi-meter to DCV, turn ignition switch to ON (Position II), touch the black test lead to ground, and touch the red test lead, in turn, to each test point on the fuse. The meter should record 12+ volts. Voltage must be recorded at each test point on a fuse; otherwise, the fuse is bad.

An alternative method, which is very fast, is to set your multi-meter to audible ohms, DISCONNECT THE NEGATIVE BATTERY TERMINAL (or you could blow the fuse in the multi-meter), and touch the red and black test leads to the test points on each fuse. When you hear the meter buzz, you know the fuse is good. If your meter doesn't have audible ohms, but it does have ohms, you can still use this method--just read the meter instead. The alternative methods perform a continuity test on the fuse. Just remember to de-energize the circuits, by disconnecting the negative battery terminal, and record any radio activation codes first.

b. Test Light. Turn ignition switch to ON (Position II), connect alligator clip to vehicle ground, and touch the probe to each test point on the fuse. The test light must come on at each test point on a fuse; otherwise, the fuse is bad.

These methods allow you to accurately and quickly check all under hood and under dash fuses in several minutes, even in dark, cramped locations. They eliminate the possibility of removing and replacing the fuse in the wrong slot. Should a fuse test bad, use the fuse puller, provided in the under dash fuse/relay box, to remove it.

The radiator fan fuse is usually located in the under hood fuse/relay box.

2. Jump the radiator fan motor and ensure it runs. Disconnect the radiator fan motor connector and attach jump leads to the battery. Better yet, use a Power Probe II--it's safer, since it is fused.

3. Check/replace the Engine Coolant Temperature (ECT) Switch, not the ECT Sensor. To test the ECT Switch, remove the electrical connector. Using a multi-meter, set to ohms, check for continuity across the terminals of the switch with the engine cold. The switch should not have continuity, while the coolant is below 189 degrees F. Start the engine and allow it to reach normal operating temperature. Stop the engine and check for continuity again. The ECT Switch should show continuity, when the coolant temperature reaches 196 to 203 degrees F, and above. If the switch fails to show continuity above this temperature, replace it.

If the radiator fan runs all the time, the ECT Switch will likely show continuity below 189 degrees F. This situation will, also, require replacing the switch.


4. Check/replace the radiator fan relay, which is usually located in the under hood fuse/relay box. It's easiest to use a Power Probe II, in conjunction with a multi-meter. Power Probe II will power up the relay, while the multi-meter will test for continuity (ohms).

This additional information may help.

Engine Overheating

Most engine overheating problems are directly attributable to poor preventive maintenance or improper coolant filling procedures. Attention to detail makes a big difference. It's best to change coolant every 3 years with a 50/50 mixture of auto maker recommended antifreeze and distilled water. It's good practice to remove engine drain plugs during this process. To refill Hondas, it's important to open the bleeder bolt, add coolant until it comes out of the bleeder bolt, tighten the bleeder bolt, turn the heater control to high, turn the fan off, run the engine until the thermostatically controlled radiator fan comes on, add additional coolant until the radiator is full, and rinse out and refill the coolant reservoir. On all cars, it's important to purge air from the cooling system, since it can cause overheating or no heat in the system.

Today, it's important to only use the antifreeze recommended by the car'smanufacturer. There are huge differences in the types of antifreeze and using the wrong one can have disastrous consequences. Don't take a chance and use the wrong antifreeze. Honda, for instance, recommends using an anti-freeze that doesn't contain silicates or borates. Silicates are abrasive and cause premature water pump bearing seal wear, which greatly shortens the life of the bearing. Once antifreeze gets into the bearing, it will not last long. Other manufacturers have differing requirements and it's not wise to experiment with the chemistry.

It's also important to use distilled water, because tap, well, and softened water can cause unforeseen problems. Tap and well water are loaded with minerals that can build-up and block radiator and heater cores. Minerals found in tap water tend to combine with coolant additives. When this happens, they form a chemical complex called “phosphate scale,” which coats the heat-transfer surfaces of radiators, heater cores, and heads, resulting in decreased heat-transfer efficiency. When this occurs, your vehicle will overheat and run much hotter than it was designed to. This scale can also lead to the destruction of water pump bearings. When this happens, the radiator or heater core must be removed and roded or replaced. Soft water is known to weaken solder joints and cause corrosion, due to salts in the water. Keep in mind that chemical reactions increase with temperature. With the high heat of modern engines, it doesn't take long for caustic steam and heat to corrode, overheat, and damage an aluminum block engine not properly maintained.

It's also important to regularly change radiator and heater hoses. Most experts recommend changing all hoses every 4 to 7 years. Inspect hoses regularly for softness, bulges, and deterioration. Pay particular attention to the top radiator hose. Internal deterioration may be almost impossible to detect. Therefore, don't be "penny wise and pound foolish" and try pushing things too far. Many aluminum heads and valve trains have had to be replaced, due to hose failure. The repair bill can easily be $3,000 or more. All it takes is allowing an aluminum head engine to overheat once. Never drive a car that is overheating--have it towed.

Common causes of overheating:

1. Insufficient coolant and/or air in the system. When the system is cool, check the radiator and overflow tank coolant level. If the level is low, start looking for leaks. Check for deteriorated or damaged radiator and heater hoses; e.g., cracks, pin holes, and clamps that leak under pressure. Also, look for bulges and collapsed hoses. Attach a cooling system pressure tester to theradiator, if necessary, to help locate leaks. Pressurize the system to 16 psi. Learn how to properly purge your cooling system of air--this can be crucial in some vehicles. On many Hondas, for instance, fill the radiator with coolant, until it comes out of the opened bleeder bolt. It's important to follow the exact procedure for draining and filling the cooling system in the Owner's Manual--they do vary and it's very important. Failure to do so can result in a warped head, blown head gasket, and the engine seriously overheating.

2. Air flow problems--leaves, bugs, dirt, feathers, or plastic bags blocking the flow of air through the radiator. Gently clean the radiator with a gardenhose.

3. Radiator fan not operating properly--check fuses; Engine Coolant Temperature(ECT) Switch, not the ECT Sensor; and radiator fan relay. If the coolant is low and/or there's air in the system, the ECT Switch may not turn the radiator fan on.

4. Radiator shroud does not seal tightly enough for the fan to draw air through the radiator. This can cause the engine to overheat, while the vehicle isstopped.

5. Faulty radiator cap--doesn't maintain proper pressure or one or more valves are bad. Test cap relief pressure with a radiator cap tester (KD 3700 or Stant12270). On non-expansion tank systems, which are most systems besides Ford's, there are two valves inside the cap that can fail. One allows hot coolant to flow into the overflow tank, the other allows coolant to flow from the overflow tank to the radiator, as the engine cools. If the coolant can't flow back into the engine, there won't be enough coolant and the engine can overheat. It's best to use a high quality, preferably OEM, cap.

6. Faulty thermostat--sticking closed. Improperly installed thermostats are one of the leading causes of Hondas overheating.

7. Radiator core blocked--mineral build-up (phosphate scale) and corrosion are the most common causes.

8. Inoperative water pump--pinch the top radiator hose closed with your hand, while the engine is idling, then release it. You should be able to feel a surge of coolant, if the pump is working properly. Change the water pump every time you change the timing belt.

9. Improper grade of engine oil--higher viscosity oil generates more internal friction and heat. Synthetic engine oils have higher coefficients of heat than conventional oils, which allows bearings and engines to run cooler--use them.