Fan Circuit Wiring

My 330 has always been prone to overheating.  I've done a number of items to reduce this problem but I recently read a comment on Tom Yang's bulletin board that one should check to voltage at the fan motors to see if there is excessive drop along the way.  When I did this, I measured about 9.5v, quite a bit from the 12.6/13.5v that could be there.  This means that the fans aren't turning as fast as they should, thus less cooling air through the radiator.

I thought that I had straightened out all of the wiring when I first got the car but the A/C relay had been replaced and wasn't operational.  It took a while staring at the electrical schematic to understand how it was supposed to be wired.   The fans are supposed to run in parallel when the water is hot (radiator temperature switch is on), but the left fan comes on with the A/C regardless of the water temperature.  The schematic is quite busy and not very clear around the A/C relay area.  But knowing how they were supposed to work allowed me to figure it out and draw the circuit below.

When the A/C is off, the A/C relay has the left fan connected to the same wire coming from the fan relay that goes to the right fan .  So the fan relay and radiator switch will run both in parallel.  The red shows how the circuit is connected when the radiator fan switch is on.

When the A/C is turned on, power is supplied by that relay to the left fan.  The right fan then runs depending on the water temperature.

The A/C relay that was in the car was a SPST (single pole/single throw) instead of a SPDT (single pole/double throw) that was needed.  So a trip to the local NAPA dealer yielded the right relay.  Of course, it doesn't come with any wiring diagram, so I sat with a volt meter and a 12v supply to figure out what leads did what.  After that, it was just a case of wiring everything up according to the diagram above.  I had one mistake.  The left fan would be on when the A/C dash switch was off and then come on when the switch was turned on.  Just the opposite of what was desired, but that was easily fixed by swapping wires on the relay.

I also cleaned the contacts on the fan relay as these are the most likely place for a voltage drop.  After all of this work, I still only measured 10.4v at each fan.  That's better than the 9.5v I started with, but still not as much as I would like.  Every item and contact along the way drops the voltage some, but the most voltage drop is in the wires from the relays to the fans.  This is the original wire and looks to be 14 gauge, plenty heavy enough as the fans draw 6 amps each.

We measured the voltage on Lowell Brown's car the other day and it was 9.25v.  So Lowell is going to go through this same process (though it'll be easier as no there's A/C on his car).  I'll wait and let him figure out what else to do to raise the voltage.

One of the things that is not shown in the above diagrams is that the +12V comes from fuse C2, which is one of the fuses that draws power through the ignition relay.  This adds another set of relay contacts that can lessen the voltage to the fans.  One of the changes I would recommend is to separate the power used to actuate the relay from the power used to drive the fans.  For the power to the contacts, I would put in a 30 amp inline fuse, connected to 2x2 junction point directly above the fan relay.  The fan relay coil would still be controlled with the ignition switch and relay, so the operation would not be any different, i.e. when you switch off the key, the fans will stop.  The advantage is that there is much less power being drawn through the ignition relay and the fans have one less set of contacts to draw power through.  You can see the difference in the two fuses/wires at the lower left.

Copyright 2001-2014, Kerry Chesbro