[FC] Problem after installing High Volume Blower Motor

[Andy .]

Andrew - I recently adapted some OEM electric fans to my F150 (replacing the roaring belt driven fan) and ran across some very interesting info in the process.  Like you, I assumed I would run a switch to a heavy duty relay (80amp in this case, as these fans can pull 70 amp peak at startup) and call it a day.   Not quite so simple.  It makes sense though - when power is removed from a spinning electric motor, that motor becomes a generator and backfeeds the system.  This can last a while on something like a blower motor on high speed with a flywheel (fan) attached.  Or in my case, fans mounted behind a radiator that freewheel when going down the road - turned by the incoming air.  
 
Solution is very simple - you need a diode (one way check valve) between your power and ground.  The banded side of the diode goes to the + source (DOUBLE CHECK ME ON THAT THOUGH).   This also eliminates the arcing of the contacts in the relay when the contacts open (switch the relay off).  Many better and OEM relays have built in diodes for this exact reason.  Similar to a condenser in a points system.  The cheap $5 relays at the parts store don't have this feature.  Bosch makes some nice 70 (or is it 80 or 90???)  relays with built in diodes.  I am using an 80 amp (not Bosch) intended for car stereo or battery isolation I got from Amazon for about $16 shipped.  Large, like a starter relay... Speaking of using starter relays for things like this - don't - they are not continuous duty.  http://www.amazon.com/80-AMP-RELAY-12-VOLTS/dp/B001EAWS6O/ref=sr_1_1?ie=UTF8&qid=1319730415&sr=8-1  
 
The diode you need is #276-1144 from Radio Shack.  They are always in stock.  It feels weird connecting your big +12v power wire to ground through a diode, but believe me it works.  Tap in somewhere between the fan and the relay obviously.
 
A google search of electric fan diode will send you to forums that have better explanations than mine - like this:   http://www.chevelles.com/forums/archive/index.php/t-109626.html
 
Oh, and make sure your squirrel cage fan is correct.  Years ago I replaced the stock metal one with a plastic fan and it was backwards as I remember.  Frustrating to have a high volume motor, lots of noise and amp draw, but little airflow - lol.
 
Hope this helps.
 
Andy
 
 

 

> Date: Thu, 27 Oct 2011 07:34:45 -0700
> From: andrew_sego at yahoo.com
> To: corvanatics at corvair.org
> Subject: [FC] Problem after installing High Volume Blower Motor
> 
> I recently installed a high volume blower fan on the heater in my 63 Corvan. I followed the Tech Guide instructions and wired in a relay to control the high speed circuit. The fan works as before in low and medium, and works GREAT on high. The problem I'm having is if I shut the key off with the fan still on high speed the engine will remain running. Sometimes is stays running, other times it will run on for a few seconds and then die. If I shut the key off with the fan switch in the off, low, or medium setting the engine shuts off normally.
> 
> The van is mostly original, except the PO wired in an externally regulated alternator. The regulator has been replaced with a solid state unit, the alternator has also been rebuilt this year. Battery is fresh. Everything was working fine before the conversion.
> 
> Here are some details about the installation. I made a strap to hold the heater in place with the transaxle cover off. The brown/orange wires from the fan switch were pulled from the plug, left connected to each other, and insulated with tape along the harness. I mounted the SPDT 30amp relay to the heater brace. I have a new wire running back from the heater switch to the coil on the relay. The ground for the coil circuit goes to the chassis, and another ground wire is also installed to the blower motor flange for good measure. Fused power to the relay (terminal 30) comes directly from the positive battery cable connection at the starter. I chose this location for convenience. The original wire that ran to the blower motor was combined with a new wire and connected to the relay out. The new wire of those two runs to the blower motor. Thus the 87 terminal is a junction for both wires.
> 
> I'm wondering if somehow the connection at the starter is a problem, since it isn't switched. Also, does is matter which side of the relay's coil circuit is the power and which is the ground? I'm speaking of terminals 85 and 86. I don't think it does, just throwing it out there.
> 
> Aside from shutting the fan switch off before the key, how do I fix this?
> 
> p.s. This reminded of the joke: Man says "Doc, my arm hurts when I do wave my arm like this." Doc says "Okay, that's ease to cure...Don't wave your arm like that anymore!"
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