Hybrid Comet - Electric Fan Wiring Problem

[MercuryMarc]

I have a 14 inch pusher fan in front of my 3 row radiator, swtiched on by temp sensor sending switch plumbed into the radiator hose. I hooked up an indictor lamp on the dash that turns on when the fan is running. The lamp wire is spliced into the hot wire from the fan to the temp switch relay and is grounded to the dash. It works great.

However, when I go over 55 mph at niight, the lamp turns on! It looks like I have set up the fan to act as a windmill generator, since it spins fast enough to send current back though the lamp circuit. This occurs when the teme gauge is very cool so I know the temp switch relay is off.

A friend told me I can put a diode on the hot wire from the fan to the temp switch relay to prevent the current from going the wrong way. Has anyone had this issue? And what diode would I use?

Thanks!
Marc in SF
63 Comet Ragtop
69 250I6/C4
Dual Cupholders
http://mercurycomet.net

 

[DaGr8Tim]

I have a stupid thought about your sitution & I hope somebody can explain to me why this wouldn't work.

First off is the fan grounded through a frame to the radiator or support, or does it have a seperate ground wire? If it's a seperate ground wire, could you run the ground through the firewall, to an LED or LED bank, and then ground it under the dash?

In my mind, I know this is wrong, but it still seems like it should work. Could somebody explain to me why it would be wrong?

 

[mustang6]

The issue Marc is having has nothing to do with the ground, although he could switch the ground as well as the hot with a multi-pole switch and cure his "problem". That way even though there was current on the hot side as generated by the fan, it couldn't ground through the indicator light and cause it to go on. Or a diode placed inline to the fan power, on the fan side (as opposed to the battery/power source side) of the power wire compared to the indicator light connection, would act as a one-way gate and not allow the light to be energized unless it was receiving power from the switch. The diode must be big enough to handle the wattage of the fan, and multiple diodes can be run in parallel if necessary to achieve the required rating. I've done this before, and Radio Shack was my source for the large diodes.

http://www.radioshack.com/product/index.jsp?productId=2049728&cp=&kw=diode&parentPage=search

 

[addo]

Couldn't you just run a little diode immediately before the light globe?

Or, a double pole relay that grounded the power wire to the fan when not in use.

 

[MercuryMarc]

Thanks for all the suggestions...
The fan is a 14 inch Derale Tornado unit rated a at 10.5 amps.

http://www.derale.com/electricfans.shtml
So I think I want to go the diode route at my local Radio Shack.
Do I get one rated slighly below the 10.5 amps (10) or slightly higher (12)?
Seems like the easiest and least expensive solution.

Thanks-
Marc

 

[StrangeRanger]

Instead of a diode, I used an industrial bridge rectifier in a similar situation; I just used one pair of terminals. They are available with male spade connectors and wattage ratings well above anyhting you'd ever need.

 

[StrangeRanger]

http://www.nteinc.com/specs/5300to5399/NTE5322.html

I used P/N 5322. It's rated for 25 amps (@200v like that matters on a 12V system.) One screw to attach and (2) 1/4" inch spade terminals to connect. No need to even heat up the soldering iron. Use either "AC" pin as the "DC voltage in" and the "+" pin as the "DC voltage out." ALL reverse flow is blocked. IIRC about $15

 

[MercuryMarc]

Thanks a bunch! I got it fixed...

I had to do a little research on the web (for example - what the heck is a rectifier?) and realized that a rectifier is nothing more that 4 diodes hooked up in a cirlce and 4 terminals (2 are AC in, 1 DC +, and 1 DC -).

Radio Shack had only 3 amp diodes but I picked up a 25 amp, 50 volt Full-Wave Bridge Rectifier for a grand total of $3.29.

http://www.radioshack.com/product/index.jsp?productId=2062584&cp

So I just hooked up the "DC -" terminal to the incoming hot wire and the "DC +" terminal to the outgoing hot wire to the fan. That way I am using the diodes in the rectifier but not really rectifiying anythng.

It works great - lights up when the fan is powered from the temp switch but no light when I'm cruising at 70 mph. I used Radio Shack part number 276-1185 which is just what Strange Ranger describred - about the size of a postage stamp, hole in the middle for easy mounting to the fan bracket, and connected with spade terminals.

Thanks again- I'll post a pic of the fan setup soon.

Marc in SF
63 Comet Ragtop
Dual Cupholders
69 250I6/C4
http://mercurycomet.net

 

[MercuryMarc]

Here is the wiring diagram for how I finally got it to work.
I am new to the electrical symbols so if anyone sees any errors please let me know.

Marc in SF

http://www.angelfire.com/falcon/comet0/fan.html

 

[69.5Mav]

MercuryMarc

Your diagram is in error. The way it is drawn the switched relay contacts are shorting out the battery and your fan is on all the time. I suggest you redraw it with the positive from the radio going to one side of the relay coil and the other side of the relay coil going to one side of the temp switch and the other side of the temp switch going to ground. Then wire the positive side of the fuse to one side of the relay contacts and the other side of the relay contacts to the bridge rectifier. This is most likely the way it is wired otherwise it would not work right.

You could also modify your setup to make it work differently. The first one is to change your bridge rectifier setup to have only one diode in the chain by reconnecting the wire connected to the positive or + terminal of the bridge to the correct AC terminal of the bridge. How do you find the right AC terminal? Well since there are only two I'd say trail and error. This will waist a little less power in one of the diodes. Next you could wire the relay coil to the 25 Amp fuse instead of the battery. This will allow the fan to run after engine shut down and cool a hot engine that would spike in temp otherwise. This should extend the life of under hood components by keeping under hood temps down.

By the way as long as you have a relay in this circuit I would have gotten a two circuit one (double pole single through) and wired the indicator up to the unused circuit.

Good Luck
69.5 mav

 

[MercuryMarc]

Like this?

http://www.angelfire.com/falcon/comet0/fanwiringa.jpg

And thaks for the upgrade suggestions!

Marc

 

[69.5Mav]

THAT'LL WORK

69.5 MAV