Relay Troubleshooting?

OFOF · November 23, 2004, 11:52:23 AM

On our trip home from Scottsdale in my '34 Mickey and I noticed that during the last two gas stops that the electric fan stayed on even after the key was turned off and removed from the ignition. The fan is controlled by a manual switch on the dash. Shuting off the switch had no effect. The switch activates a relay that provides battery power to the fan motor. Could the relay be frozen closed (on)? I can't think of any other way juice could get to the fan motor after the ignition is turned off.
Any ideas?
Don.

58 Yeoman · November 23, 2004, 12:51:55 PM

With the fan switch turned off, pull the hot wire off the relay and check the wire for 12 volts. If there is no power there, I'd be looking at the relay. If there is power, your switch wire could be shorted. You could also use an ohm meter to check the hot terminal with the output terminal on the relay; if it reads ohms, the relay could be stuck.

WZ JUNK · November 23, 2004, 02:20:55 PM

Quote from: "OFOF"On our trip home from Scottsdale in my '34 Mickey and I noticed that during the last two gas stops that the electric fan stayed on even after the key was turned off and removed from the ignition. The fan is controlled by a manual switch on the dash. Shuting off the switch had no effect. The switch activates a relay that provides battery power to the fan motor. Could the relay be frozen closed (on)? I can't think of any other way juice could get to the fan motor after the ignition is turned off.
Any ideas?
Don.

I wire the fan relay so tha it will trigger on the ground side of the circuit. That way I can use a sensor in the block to turn on the fan or I can add a toggle switch that is to ground and when either of these sees ground the fan comes on. If this is the way your relay is wired, I would look for a short in the trigger lead to ground. First I would replace the relay and see if this cures the problem as normally the fan would shut off when the key is turned off even with the circuit I described.

rooster · November 23, 2004, 02:42:15 PM

Quote from: "OFOF"I can't think of any other way juice could get to the fan motor after the ignition is turned off.
Any ideas?
Don.

Stuck relay my guess too! Some rods are wired to continue cooling after ign turned off, just like the newer cars. Bad part about this is, you walk away from your car after parking it thinking your fan is just doing its thing, if it sticks closed you got a dead battey when you come back. thats way I dont do it that way. Dont know how yours is wired.

HOTRODSRJ · November 23, 2004, 09:01:38 PM

May I give some advice based on my years of designing ac/dc power supplies many moons ago.

My bet is "welded" relay contacts too. This is the most common failure mode for cheap relays or relays that are underated for the continuous current and/or put in "heated" locations making for the same or relays that are not really made for a highly inductive application.

I know what your thinking. I have a 30 or 40 amp relay and my fan is 25 amps (continuous) or less. So what's the big deal and why did it crap out? First, the rule of thumb in relay design/application is always pick a relay that will take your peak continuous current X 150%. This generally covers you for most all conditions including elevated temperature environments. This means if you are running at 20 amps continuous which most good cooling fans are.... a 30amp relay is reeeeeeaal marginal. But wait, there is another problem that most companies don't tell you or just don't bother with. Most of these relays are designed for lighting systems and/or only very moderate inductive circuits and general light duty...even the 40amps models! Even if the relay meets the "continuous" rating, A big 18+ amp fan motor is a pretty sizeable inductive load for those relays. The exacting problem is that when these relays cycle on and off they create a huge inductive spike of voltage when the contacts open and try to disconnect your fan from the source. This is when you would see a big spark across the contacts as they open. This "inductive spike" caused by the motor itself becoming a generator of current (because it has stored energy stored in the form of rotating mass....converted to electrical current as a generator....only in the opposite direction) will try to "pump" the arc and start welding your contacts with each operation closer and closer and closer until they fail in the "stuck together" mode. You can run and I recommend a "freewheeling" diode ( a "bucking diode" across the contacts of the motor that offers another path for "opening" current to dimish safely without frying the relay contacts) which will save your contacts and make your relay last for hundreds of thousands of reliable operations! All professional/quality and long-lived motor circuits have them and only in the automotive realm (which is not exactly considered rocket science)will you find that for some reason they leave them out. However I have seen alot of OEM fan designs lately designing them in now.

Now....you may want to forget the diode suggestion and that's okay. In that case.....here is the answer. I only use 70 amp relays anymore. With fans typically exceeding 25amps or more....it's the best thing since sliced bread and they can take much higher inductive loads and are an overkill for continous current. These are about the same cost and will take anything you can throw at them in the fan department.

Check out Newarks at http://www.newark.com/NewarkWebCommerce/newark/en_US/endecaSearch/partDetail.jsp?SKU=26C0334&N=0

I don't suggest the common relays from any electrical kit manufacturer and especially not Radio Shack relays. These are NOT designed for your fan application per se. There are some OEM relays that you can look for that are very good, but pricey! The link above is the best thing you can buy for reliability for your cooling system.

I hope this helps some of you next time you want to change that relay out.

rooster · November 23, 2004, 09:18:38 PM

Thanks for that info, im gonna get one, then place my old one in the survival kit!

OFOF · November 24, 2004, 01:13:40 AM

Thanks for all of the tips guys! I'll run a couple of tests in the morning and if it is, as I suspect, the relay I'll buy one of the hefty ones that SRJ recommends.
Don.

OFOF · November 28, 2004, 06:24:59 PM

Quote from: "OFOF"Thanks for all of the tips guys! I'll run a couple of tests in the morning and if it is, as I suspect, the relay I'll buy one of the hefty ones that SRJ recommends.
Don.

Yep, it was the relay. I got a new 30 amp relay, installed it. and the fan shuts off when the key or switch is turned off. I'm going to order one of the 70 amp jobs to install during the winter. Thanks to all for your help!
Don.