Relay sticking for cooling fan, WAIT it's running markVIII fan?

flynbrd

Registered User
Here's my Frankenstein cooling system setup

1. IRCM has a new high speed relay installed. No other modifications
2. The stock cooling fan was replaced with Lincoln mark VIII motor. Hot side going to the blue wire and the end wiring going to ground. Middle terminal is not being used. The motor is running the stock sc fan blade. The Mark VIII motor should be about 300+rpms faster than stock.
3. I'm running bosche 75 amp relay to switch the fan.
4. The fan is wired with battery hot to 40amp fuse through 75 relay to the motor. The fan motor is grounded straight to the battery. I using the high side of the fan terminal(stock connector going to ircm) to switch the relay on off.

Everything seems to work however when I turn the car off the cooling fan was still running. I disconnected the switching ground at the relay and the fan was still engaged. Just for giggles I consulted the manly manual which instructed me to tap on the relay. Sure enough after lightly tapping the relay it clicked shutting off the motor. So why is my 75 amp relay sticking on?

Link to the relay
http://www.ebay.com/itm/Diesel-Glow-Plug-Relay-BOSCH-0-332-002-156-12-Volt-75-Amp-/160819801936?pt=Car_Audio_Video&hash=item25719b6f50&vxp=mtr#ht_572wt_1139
 
looks like I wired it wrong! something I found on the net

""I spoke with an Auto Electric Repair Shop today and the guy told me the problem could be from having the main supply wires reversed. I did not think this should matter. Can anyone confire or refute this? I'll try it since it will only take a couple minutes but I doubt it will have any effect.

Many relays are designed with a diode wired up across the coil. This is to prevent back emf from harming the rest of your circuits when the power is removed. If the relay is wired up backwards, it will first blow the diode and then, over time, start to degrade the rest of your components. This kind of relay must have some kind of polarity indicator to make sure it's wired up correctly. ""

The web site

http://www.chevelles.com/forums/archive/index.php/t-53426.html
 
Here's my Frankenstein cooling system setup ...
3. I'm running bosche 75 amp relay to switch the fan.

It's a 75A relay switching a 70A motor. Guaranteed to stick and then weld shut, because the back EMF when the relay drops out will spike the instantaneous current to > 75A (my math says closer to 150 to 200A).

Swap for a Echlin ST34 Relay from Napa, a 200A continuous duty relay, and that problem should go away.

Here's a wiring diagram for adding a fan to a 5.0HO equipped car:

http://forums.tccoa.com/showpost.php?p=1426361&postcount=10

RwP
 
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