Bugpack Mesa oil cooler fuse requirements

[fallingoffalot]

Hi folks, I've been wondering why my oil-cooler never comes on and whilst poking about today found that the fuse had blown. It was 5A, doesn't sound like enough. I tested the cooler with a direct connection and it ran fine. So I went out and bought a 12V 30A relay and a 15A inline fuse, so now the 5A switched supply turns the relay on, which then powers the fan from a separate battery supply through the 15A fuse (when the temp sensor triggers).
Basically I did this:
https://www.thesamba.com/vw/forum/viewtopic.php?t=568998&postorder=desc
I've looked at the fan itself and there's no rating spec written on it, so does anyone know what kind of current these fans pull (I think it's 8").

Cheers

 

[Moseley]

Can you measure it by putting a multimeter in line with it when running? Remember, the fuse is there to protect the wiring, not the fan. The wiring should be sized based on the load on it, and the fuse subsequently to prevent the wire from melting / burning if something were to go wrong with the fan.

 

[fallingoffalot]

Yes good point about the wire, I considered putting the meter inline, but couldn't figure out, with the stuff I have, and not having 4 hands, how to hold it all in place whilst I make a direct connection to the cooler. I've now got 17A wire with a 15A fuse. I chose this as the hard-wired connection to the fan looks like it's less than 17A, I'm guessing about 8-10. I'm assuming the 5A fuse blew when the oil cooler fan started (on that hot day in July last year when I drove down to Newquay). I need to test it, I should go on a fast (70 mph) drive!

 

[Moseley]

What do you mean by 17A wire? Wire is normally measured in gauge (diameter) and for automotive applications, should be multi-strand. The current-carrying capacity is dependent on length, and there are various tables that can be found on the internet to look up what gauge wire is required based on the cable length required, and the draw of the components at the end of it. So a short, narrow cable can carry a similar current as a long, wide one.

 

[fallingoffalot]

It's auto wire from Halfords, that's what it said on the real, doesn't say what length that's over. The longest length I've got is about 2m

 

[Moseley]

Does it have any stamping on it to suggest what the ’AWG’ number is? Alternatively, you should be able to get a reasonably good estimate of the wire gauge if you can get a set of verniers across the core. I appreciate this wasn’t your original question, but I’d want the peace of mind that the cable is sized appropriately for something as important as a cooler fan, as well as the fuse being suitable.

 

[fallingoffalot]

I'll check, there's bound to be something stamped on it. It's multi-cored and I guess I can get an estimate. It looks beefier than the +ve that is already part of the fan, so should be OK.

 

[naskeet]

Electric motors have a start-up current which is higher than the steady-state current at normal running speed. If anything impedes the normal-speed rotation of the motor, such as partially-seized bearings or other increase in load, the motor would take more current and run hotter. This is how some people manage to burn-out DIY electric drills!

 

[fallingoffalot]

Further to my oil cooler solution, it occurred to to me that I've wired it up slightly differently to the diagram on the Samba:


This is what mine was originally like, which blew the fuse

...and this is what I wired up... with the power to the solenoid being switched by the key and the power to the fan being controlled by the thermostat. The problem with the Samba one is that the earth on pin 85 is left floating until a proper earth is made. So my slightly different solution avoids this.


The other way to wire it up would be like this.... so the power to the solenoid would be controlled by the thermostat.

So which is the best way to do it?

 

[sparkywig]

Ideally you want the thermostat to switch the relay.