Hi everyone!

I've been attempting to diagnose why my Jaguar X-Type 2004 2.5 V6 radiator fan control module burned out (fans kept spinning whenever the battery was connected). While I was checking for a replacement, I learned that a typical reason for the fan control module to burn is too high current draw from the fans themselves. My question is, in what range should the current draw be with the stock fan assembly? I've tried browsing around for specifications, but I've found info with current draws ranging from 10A all the way to 20A, per fan. Currently I've measured roughly 12A from both fans individually when connected directly to the battery; does this sound about right or is the draw too high?

 

IsmoL, the fuse for the fans is rated at 80 amps. If you follow the general fuse sizing standard, the fuse rating should be around 125% of the max load that the circuit would see. Playing with the numbers a little bit, it would be reasonable to say that the fans would pull a maximum of 60 amps (60 x 125% is 75 amps, nearest fuse would be 80 amps). So, that would mean that each fan when spinning at maximum speed would be pulling worst case, 30 amps. If the fan is spinning at a slower speed, the current draw is going to be less. So, having a current draw of only 12 amps is well within what I would expect to be worst case conditions. There is also some give and take added in due to starting surge, but that really starts to get into electronics and motor theory. Trying to keep it simple here.

The issue is that the fan controllers have transistors inside of them. Due to the high current draws, this causes them to break down. When they break down, they allow current to continue to flow through them, even when they are not commanded to allow current to flow. This results in the transistors becoming a source of power and this causes them to get really hot and break down even more till they reach a point that they just fail (therefore do not conduct at all). Keep in mind, during normal use, this is not as much an issue because the controller is switching the fan on and off at a high rate of speed (100's of times a second) and it controls the speed by varying the amount of on and off time. You need a slow speed, the fan is turned on for a very short period of time compared to a long off time. You need a fast fan, the off time is very little compared to the on time. The transistors don't heat up nearly as much because heat is a function of current times the voltage drop across the component. With the transistor conducting at 100%, the voltage drop if very minimal (around 0.6 VDC). Where, when it starts to leak, it may see 6 or more volts. So, even getting a 10 amp current flow, that would make that transistor be exposed to 60 watts of heat (6 volts x 10 amps) where with it switching on and off, it is pulling 30 amps and dropping 0.6 volts (30 x 0.6 = 18 watts). Big difference.

If you want me to explain it differently, let me know. I work around electronics for a living.

 

For other readers:

Somewhere I'd read 150W per fan for this double fan assembly. Don't know which car vendor (similar fan assemblys are used in several Ford cars, Volvo cars, and Land Rover cars, ...),

At nominal voltage (12V) it would be 12,5A.

At my 2006 Jaguar X-Type 2.5 L AWD I'd measured about 15A and 15.5A directly connected to the battery at estimated 11,6V (motor not running).
(TRMS current clamp)
Seems to be too much...


Cheers, catfondler