Troubleshooting electric cooling fan performance

Electric cooling fans became popular with the introduction of front-wheel-drive and the relentless goal of improving fuel economy. Besides eliminating the hassles of driving a mechanical fan with the engine mounted sideways, electric fans also reduce weight and improve the performance of the air conditioning system.

In spite of these virtues, electric fans can present a diagnostic challenge when they fail. Control circuits vary from the basic to the complex, and so do troubleshooting procedures. Here are some key points to keep in mind when troubleshooting electric cooling fans.

Be sure to check out more on troubleshooting electric fans in our Diagnostic Review.


To maximize airflow, some vehicles use multiple fans to best control airflow. Be sure you understand the normal operating conditions for each fan circuit before you try to pinpoint the exact problem. This will help you isolate which fan circuit needs diagnosing.

Before you tear into the fan circuit with an assumption that something major must be wrong, be on the lookout for visual signs of problems. More than one technician has gone off on a witch hunt, chasing a high-tech ghost when the real culprit was a low-tech problem like a dislodged connector or a blown fuse.

Inspect the battery connections, fuses, fusible links, and all grounds to make sure they're OK. Check all connectors for proper seating, including those at the fan, thermostatic switch, relay, controller (if equipped), the A/C clutch cycling pressure switch and the A/C compressor. Then, check the wiring harness sections in between these parts for breakage or worn insulation. Repair or replace any bad wiring.


Symptom 1: Fan doesn't run. The first step is to trip the relay with the ignition on and see if the fan runs. Look up the wiring diagram for the fan relay in a shop manual (sometimes it's right on the relay) for this step to see which terminal grounds directly to the coolant switch or computer. Some systems have two relays: one low-speed and one high-speed. If the fan runs with the relay tripped, you know the fan, the relay, and their connecting circuits are OK.

If the fan doesn't run, unplug it and connect jumpers directly to the battery. If the fan still doesn't run, replace it. However, if the fan runs, the relay, its supply circuit, or the wiring between the fan and relay are at fault. Check a shop manual for specific voltage and circuit information. If all circuits check out OK, replace the relay.

The last thing left is the coolant switch and its circuit, or the computer/controller and its circuits. You can easily check the switch while it's still in the engine with a temperature probe (digital pyrometer). Please note that the following switch check only applies to switches that open and close to ground, not variable coolant sensors as used for computerized fans.

Connect an ohmmeter to the switch and look for continuity as the switch reaches its closing temperature. The switch should show continuity when closed and infinity when open. If not, replace it.
If the switch checks out good, make sure its circuit has continuity back to the relay. With the ignition on, ground or jump the switch to ground. The relay should close and engage the fan.

Symptom 2: Fan engages with the A/C only. With the ignition on and the A/C off, unplug the coolant temperature switch connector. Connect a jumper wire between the connector's terminal and ground or across both terminals on two-wire connectors. If the fan engages, the coolant switch is faulty; replace it.

Symptom 3: Fan engages when the engine is hot, but not with the A/C. Remove the connector from the cycling clutch pressure switch. Jump both connector terminals with a jumper wire. If the fan kicks in, check the charge level of the A/C system. In the event that it's low on charge, repair any leaks and recharge the system. If the charge is OK, replace the cycling clutch pressure switch.
If the fan fails to run with the switch connector jumped, check a shop manual for voltage specs at the connector and at the dash control switch and use your digital multimeter to take the readings. Replace any bad parts.


Computer-controlled fans engage according to inputs like the engine coolant sensor and vehicle speed sensor, just to name a few. This allows closer control of engine temperature, but also complicates the diagnostic process because the computer itself is now involved.

Some things to keep in mind:

• Ford uses integrated controllers on some of its models to control a number of functions, including the cooling fan. Rather than a stand-alone relay, the controller may contain a number of relays inside. For the exact layout and designation of the inputs to and outputs from the controller, consult a reliable shop manual.

• It's wise to understand the exact fan strategy designed into a given vehicle. For instance, some fans turn off automatically at a certain road speed because airflow at that speed makes fan operation no longer necessary. If you didn't know this, you could wind up trying to track down the reason a fan doesn't work at road speeds.

• More than ever, a scan tool can play a vital role when pinpointing circuit woes. By commanding the cooling fan through the powertrain control module (PCM), you can quickly determine that much of the circuit is working properly. This can eliminate some of the required testing using with jumper wires and other circuit bypass techniques.

Once you've made necessary repairs, leave nothing to chance and run the vehicle to make sure the fan operates properly in all modes. That way, you know the car and your customer will both keep their cool.

Be sure to check out more on troubleshooting electric fans in our Diagnostic Review.