Car radiator cooling fan testing with multimeter

Thread Starter

john2k

Joined Nov 14, 2019
209
My cars cooling fan has stopped working, I checked and changed the fuse and that seems to be fine. There are two relays powering the fan, it appears the fan is activated by two different relays. So chances of both relays failing at the same time I would think would be slim. Is there a way with a multimeter to test if a fan motor is faulty or not?

Another thing I guess I could try is to hook up a halogen 12v light bulb to the connector going to the fan and see if the bulb lights up.
 

Jon Chandler

Joined Jun 12, 2008
294
Some car fans depend on the ground connection being made by connection to the chassis by the mounting screws or by a jumper wire from the fan body and chassis. Checking for a solid ground connection is a good first step.
 

Thread Starter

john2k

Joined Nov 14, 2019
209
I disconnected the connector and used multimeter to read voltage across the pins on the connector when the fan is supposed to be on and I get a 14ish volt reading which indicates the connections all fine and the fan motor itself is knackered. I'm assuming electric fan motors are moving components so do eventually fail?
 
While i'll agree it probably is the fan one good thing to do is back probe the connector with the fan still connected. You may have corrosion or damage that will show a proper voltage at the connector with no load, but as soon as you reconnect it won't be a good enough connection to power the load. Depending on the car fans can get expensive!
 

Thread Starter

john2k

Joined Nov 14, 2019
209
While i'll agree it probably is the fan one good thing to do is back probe the connector with the fan still connected. You may have corrosion or damage that will show a proper voltage at the connector with no load, but as soon as you reconnect it won't be a good enough connection to power the load. Depending on the car fans can get expensive!
I cleaned the connector on the fan motor itself and tried to power it with direct 12v from battery but it doesn't work. I got a second hand replacement from lower mileage car same age and that works when I give it direct 12v power.
 

Thread Starter

john2k

Joined Nov 14, 2019
209
Ok so I replaced the fan assembly with a second hand unit. Gave it a good clean before installing. Also tested it by plugging directly to the 12v battery and was working. One thing I noticed is that when I powered it via the 12v battery it just ran at a reasonable speed but not super fast. But after installing on the car and when the cars ECU turns the fan on when it needs maximum power, the fan goes super fast and loud. How is it the car is able to speed up the fan compared to when it's directly connected to the battery? It's only 2 pins. I would have thought plugging it directly to battery would have given maximum speed. Just a thought
 

MisterBill2

Joined Jan 23, 2018
12,697
It may also be that there is more to the control than ON/OFF. And it is likely that the connection from the controller was better than your test connection. As those fans may draw 20 amps or more there may be quite a voltage drop in a temporary hookup. And for an education, try connecting your voltmeter across the old fan and spinning it. Most of those PM fan motors are fairly good generators as well.
 
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