Fuse 5 blowing when Otterstat triggered

[flux3d]

Following successfully sorting the valve clearances and other problems the car straight away throw up another fault for me to fix. As soon as I finished doing the clearances I took it out, only to have it start to overheat because the fans weren't turning on. Using the AC to start up the fans I got the temp down and limped home.

Now, jumping the otterstat wires brought the fans on, no problem. So I figured it must be the otterstat. So I ordered one off ebay and decided to also fit a switch to override the otterstat and bring the fans on when I want them. I'd fitted three illuminated rocker switches before to the coin tray, two of which are not being used, so one of these was wired up as the override switch. The switches have three posts, live in, live out and an earth to allow for illumination. I wired it so that the live in was the live to the otterstat (solid green) and the out was the black/orange. Flicking the switch brings on the fans, plus illuminates the switch, so job done right?

Now, if I flip the wires so that the black/orange is the live in and the solid green as out as soon as I turn the key in the ignition it blows fuse five. Also, if wired correctly as before (solid green as the live into the switch) and the switch is in the off position as soon as the otterstat closes fuse five blows again.

Everything points to the switch allowing a short to earth, but how can this be so? Surely the earth only allows enough current through to illuminate the switch? Have I got the wrong type of switch maybe?

[DeLorean03]

Try it without utilizing the earth(aka, ground here in the States). Just hook it up to where the switch enables the fans - leave the switch bulb turning on and off disabled for now, and let us know what happens.

[flux3d]

Tried it without the earth attached and all seems ok. I've switched it out for a standard, non illuminated switch and everything now works. Maybe the illumination pushed the load over the 10A limit, but I'd be surprised. Seems odd, like it was shorting to earth through the switch, but somehow not damaging the switch. Can it do that?

[DeLorean03]

Where is the wiring for the earth located in the car? I am wondering if it was shorting out on something and blowing your fuse.

Either way, it is fixed now :D!

[flux3d]

Just taken the car out for a drive and all seemed well. Fans came on and it stayed nice and cool. BUT, just as I got home I heard a click from the relay compartment and the fans went off. The switch didn't bring them on either. Investigating further the fan relays seemed very hot (too hot to touch for long) and it would seem the thermal trip is tripping out. Well, I presume I've identified the thermal trip for the fans correctly. To the right of the relays and before the fuse board. The one on the left (black/orange wire at bottom, brown/yellow at the top) had a very warm connection at the top. The brown/yellow wire was warm to the touch.

Case of one of the fans starting to fail? Poor earth? I have just cleaned and changed the bolt on the earth on the drivers side up front.

Any ideas? Is it likely to fail soon?

[Bitsyncmaster]

Just taken the car out for a drive and all seemed well. Fans came on and it stayed nice and cool. BUT, just as I got home I heard a click from the relay compartment and the fans went off. The switch didn't bring them on either. Investigating further the fan relays seemed very hot (too hot to touch for long) and it would seem the thermal trip is tripping out. Well, I presume I've identified the thermal trip for the fans correctly. To the right of the relays and before the fuse board. The one on the left (black/orange wire at bottom, brown/yellow at the top) had a very warm connection at the top. The brown/yellow wire was warm to the touch.

Case of one of the fans starting to fail? Poor earth? I have just cleaned and changed the bolt on the earth on the drivers side up front.

Any ideas? Is it likely to fail soon?

You need to measure your fan currents. If you can borrow a clamp style current meter that is best. Most multimeter can not handle greater than 10 amps. The clamp style also will not introduce voltage drops. OEM fans can draw 20 amps each when they get old. The fan relay is rated for 30 or 40 amps.

[Rich]

Case of one of the fans starting to fail? Poor earth? I have just cleaned and changed the bolt on the earth on the drivers side up front.
Any ideas? Is it likely to fail soon?

When you cleaned the fan ground connection it likely reduced the resistance of the fan circuit, creating higher fan currents at a given voltage.

Agreeing with Dave that the next step is to measure current. If only one fan is running high amps then maybe you need just one fan. If fan currents are not too high it could be a bad circuit breaker.

Speaking of voltage, put a voltmeter on a main 12V+ connection and a good ground with engine running. Be sure the alternator/regulator isn't running >14.5V, which could cause trouble all over the place. Do not use the dash voltmeter for this test.

[David T]

When you cleaned the fan ground connection it likely reduced the resistance of the fan circuit, creating higher fan currents at a given voltage.

Agreeing with Dave that the next step is to measure current. If only one fan is running high amps then maybe you need just one fan. If fan currents are not too high it could be a bad circuit breaker.

Speaking of voltage, put a voltmeter on a main 12V+ connection and a good ground with engine running. Be sure the alternator/regulator isn't running >14.5V, which could cause trouble all over the place. Do not use the dash voltmeter for this test.

Check the rating on the circuit breaker. The two of them (the cooling fans and the heater fan) are both supposed to be uprated. New uprated ones come in the relay upgrade kit. Your earlier problem with the switch blowing the fuse sounds like you did not wore it correctly.

[flux3d]

Thanks everyone. All the relays were changed out by the PO in 2011 for DMC ones (they all have the green 2011 DMC label on them). As for the CB's I'm not sure. However, I think I've found the problem. The brown/yellow wire from the relay to the CB was getting quite hot, while the brown/grey wire out of the CB seemed fine. On closer inspection the nut and friction washer fixing the post for the brown/grey wire to the CB had worked itself quite loose and was spinning. I imagine this would probably cause quite an increase in resistance on this side of the CB, so raising the current pull.

I think Rich is right about the cleaning of the ground having an effect. I think this allowed for for current to flow to the fans, therefore highlighting the problem. I'd also like to know what the fans are pulling, like you said David, but without the equipment that's going to be hard. A job on the list is to pull the fans and inspect/overhaul them as soon as.

Not sure if the above has fixed everything. I'm in the middle of a very protracted house move so the chances for testing everything out are limited, but hopefully I can soon and report back.

Thanks for the pointers again guys. As always your help is very much appreciated.