Blower Motor Relay info (Keeps blowing fuses)

[Jstan78]

Hello all, I switched out my blower motor relay because mine was working Intermittently. So after i put the new one it kept blowing fuses, swaped back to the old one and it works fine. I believe it may be the wrong relay, but my parts store says its the right one. What do you guys think? Thanks in advance.

[78 Chevyrado]

look at my posts in this thread:
http://forum.73-87chevytrucks.com/smforum/index.php/topic,24232.0.html

All parts stores give a "correct part number relay" for my truck that only fries itself and blows fuses...  the new relay you just bought is now fried, return it, get another just like it and modify it like in the thread i posted...

[Jstan78]

78 Chevyrado, Is this the same one you are talking about? That is the only resistor I see on there.

[78 Chevyrado]

That's it man!  I had to cut that resistor out of my new relay for it to work.    Is the resistor in the pic one you already tried or is it totally new/untried?  if you already tried this relay the resistor should be burnt up. 

I had checked out All the truck wiring and connectors and it was fine so i finally pulled the cover off the 4th new relay and the resistor was burnt up, so i snipped it out of the 5th new one and its worked ever since.

I did have on hand at the time the orignal relay that came in the truck new, and it was identical EXCEPT the OE relay did not have that resistor in it.

[Jstan78]

That is the one I fried, thanks for all your help, I will give it another shot sometime this weekend. I will let you know how it goes.

[Jstan78]

Well I tried what you said, and it works like a champ!!! Thanks for the advice.

[bd]

  FYI - The diode doesn't hurt anything on older trucks that didn't originally use them.  They are there to suppress arcing across the fan switch when high blower is switched OFF.  They actually prolong switch service life (as well as protect delicate electronics on the newer trucks).  But, diodes are polarized and only conduct current in one direction.  The end of the diode with the silver band is the "cathode" or negative end of the diode.  The diode conducts only if its 'cathode' is connected to negative voltage.  When used as a "clamping diode" (a fancy name that means the diode limits or "clamps" transient voltage spikes) the diode is connected into its circuit with reverse polarity (also called reverse bias), which means the 'cathode' connects to positive voltage.  When the 'cathode' is connected to positive voltage the diode cannot conduct. 

Now, regarding GM high blower relays, the 'clamping diode' is wired across the relay coil leads in such a way that when the relay is energized, the diode is reverse biased, thus doesn't conduct.  But, the current flowing through the relay's coil builds and supports a strong electromagnetic field around the coil.  When power to the relay coil is cut off, the electromagnetic field collapses and generates a strong reverse polarity current backed by a momentary voltage spike of several hundred volts.  At that moment, the clamping diode becomes forward biased (the cathode becomes negative) and the diode conducts, effectively shorting the relay coil leads together.  The energy stored in the magnetic field dissipates through the coil instead of arcing across the switch contacts.

The problem with service replacement relays is that too many applications are serviced by too few parts.  Essentially, the relays are constructed for applications that use opposite polarity wiring than exists in our trucks.  Hence, if the orange (+) and black (-) relay control wires are reversed in the relay connector, the diode won't 'fry' anymore and it will serve an added benefit....

Or, you can just cut it out of the relay and not worry about it.