electrical and ignition Thermo switch / electric fan troubleshooting

[lobf]

I replaced my water pump this weekend, which meant pulling out my radiator and electric fan- and just for a bit of extra room, my battery. 

 

Everything is put back in, but now the fan isn't coming on when the temp gets high, but it will come on when I bridge power to it through the relay. 

 

I wired up this fan on a budget when I was 20, and never bothered to install a diode- now having done some research, I'm thinking that when I unplugged the battery it blew up the thermo switch. I'm going to wire up a manual switch tomorrow so I can drive to work at least. 

 

My questions are-

 

1) Do you think my lack-of-diode hypothesis is probable? I'm sure this isn't the first time I've disconnected the battery but maybe this time just did it? 

              1b) How do I find non-radioshack 1n4001-comparable diodes?  

 

2) Where do I order a new thermo switch? I don't recall where I got this from or how exactly to search for it. Also, can I choose the sensitivity of my next one? 

 

Thanks for the help everyone, love you guys. 

[Simeon]

The thermoswitch is unlikely to be damaged by the fly back voltage from the relay coil. Certainly not by disconnecting the battery. If it was going to be a problem it would be due to the relay being energised and the the thermoswitch cutting the feed to the relay coil. 

 

You can always test the switch by suspending it in water on the stove top and seeing measuring the resistance across it as the water heats to boiling. 

[lobf]

 

Good to know, thanks. I don't have any gear to test resistance at the moment, so I'll have to see if I can troubleshoot the switch other ways. 

 

4 minutes ago, Simeon said:

If it was going to be a problem it would be due to the relay being energised and the the thermoswitch cutting the feed to the relay coil. 

 

Can you help me understand what you mean by this? Or under what conditions this may happen? 

 

Thanks again for your help! 

[Simeon]

The reason diodes are sometimes used across relay coils is to suppress the 'flyback voltage' that is induced by the magnetic field collapsing in the energised relay coil. This is a short but high voltage transient that will appear across the contact of whatever switch you use to open the circuit. In your case it *could* see the contacts in the thermoswitch burn out but in reality they are sized to switch a much greater current than a single relay. 

 

If if your relay had been energised and the thermoswitch contact closed when you disconnected the battery then you would see the effect (amongst other more obvious ones) at the battery terminal. 

 

A very practical example is your ignition coil which is an inductive coil that is charged when the points are closed and releases a flyback voltage which is also transformed to a high voltage when the points open. Similar principle but we use a capacitor (condenser) to suppress the spark at the points. 

[lobf]

 

I understand the purpose of the diode, specifically I'm wondering how I might know:

6 minutes ago, Simeon said:

If if your relay had been energised and the thermoswitch contact closed when you disconnected the battery then you would see the effect (amongst other more obvious ones) at the battery terminal. 

 

So the thermo switch would have been more likely to fry if it was closed when I disconnected the battery? Or you're saying I would have seen some other indication? 

 

Now that I'm thinking of it, I dropped a wrench that unfortunately landed making contact with the + terminal and the body of the car, causing sparks and the wrench to attach itself to the body and make a little divot in each... Could this have done something? 

 

Thanks again for your help, real layman here. 

[Simeon]

Was your fan running when you disconnected the battery?

 

If your fan was running it would be presumably because your engine was hot. 

 

If so, the contacts would be closed and therefore you couldn't get a damaging spike across the contacts which could burn them out. There would be no (or little) voltage differential across the switch as it would be closed. 

 

The act of the contact opening causes the collapse of the magnetic field and so the flyback voltage would be present to arc at that contact. 

 

If if your fan wasn't running then the relay wouldn't be energised so there wouldn't be any magnetic field. 

[lobf]

26 minutes ago, Simeon said:

Was your fan running when you disconnected the battery?

 

If your fan was running it would be presumably because your engine was hot. 

 

If so, the contacts would be closed and therefore you couldn't get a damaging spike across the contacts which could burn them out. There would be no (or little) voltage differential across the switch as it would be closed. 

 

The act of the contact opening causes the collapse of the magnetic field and so the flyback voltage would be present to arc at that contact. 

 

If if your fan wasn't running then the relay wouldn't be energised so there wouldn't be any magnetic field. 

 

Right, okay thanks. It wasn't running, so probably no issue with the switch. If I touched both leads going to the thermo switch together, that should turn on the fan right? That'll help me figure out where my problem is. 

 

I'll check to see if I disconnected anything, which is an equally likely problem. 

[Simeon]

2 minutes ago, lobf said:

 

I'll check to see if I disconnected anything, which is an equally likely problem. 

 

This is your most likely candidate. 

[lobf]

11 hours ago, Simeon said:

 

This is your most likely candidate. 

 

Indeed. Gave it another look today, everything seems to be connected still. 

 

Again, I can turn the fan on, so it works. Going off the diagram: http://imgur.com/a/grtWd

 

According to this, if I connect 85 or 86 to ground, it should turn the fan on, correct? Neither seems to be working. 

 

[lobf]

Let this be a lesson to slow down and not forget the basics. The whole issue seems to have been a blown fuse. Glad to know it was as easy as that, though! 

 

Now back to this fucking pump belt...