Cooling fan relay installation

[manoa matt]

I have a 78 spider that did not originally come with a cooling fan relay. My
wiring at the sensor to fan connection has burt out twice, and blows fuses
about once every 2-3 months of daily driving. I've installed new wiring,
cleaned connections and fuse contacts with limited results.

I recently did the headlight relay installation and love the results. My 76
parst car originally came with a cooling fan relay in the circuit. I removed
the relay and associated wiring from the harness.

The question is: Do I graft in the relay from the 76 into the 78 circuit,seams
simple enough. Or do I install a whole new relay and wiring? Anybody have
a diagram for inserting a cooling fan relay that I can compare against the 76
wiring diagram?

[racydave]

Matt, A common 4 or 5 terminal Bosch relay would go like this. Terminal(s) (edit! he was wrong! ) 30 to grnd, 85 to batt or ign, 87 from sensor, and 86 to blower negative. Of course, I could be wrong again? The fan is fused on the pos wire.

[manoa matt]

The 76 fan relay is triggered by the fan switch in the rad, which is a ground interupter, not a switched power. I'm guessing if it worked in a 76 and if I hook everything up as per diagrams then it should work on the 78.

[danaspider]

Matt, while you are doing this, maybe add an extra relay from a switch in the cockpit where you can turn the fan on manually. Sometimes nice to have if temp switch fails. You are right the temp switch is switching the ground.

If you are blowing fuses on a regular basis, I would wonder if the motor in the fan is going bad and pulling too much current? Adding the relay will not help in that area.

Just some ideas.

[manoa matt]

The 110 watt stock electric fan should only pull 9.17 amps...in theory. The fan is on a 16 amp fuse with the horn, which drivers don't seem to use in Hawaii.

When I got the car, I pulled the fan, opened the motor and cleaned all contacts and brushes. Just over 2.5 years ago. I'll check again this weekend and see that the actual amp pull is, before and after cleaning. Fortunatly I have two more fan motors I can also use/check.

I'm still trying to figure out the 76 diagram, but it seems backwards for a relay activation circuit, unless both wires are power to the fan with the rad switch making the switch from power to ground.

[danaspider]

Matt, just did a little research between your 76 and my 79. On the 76, when the thermal switch in the radiator closes, it passes a ground to relay 39. This ground causes your relay to pull in. The closed contact (pin 87)passes 12V to your fan. The 12 v for pulling the relay in comes from Fuse 1, and the 12V to the fan comes through the relay from F10. So I am assuming that you are blowing F10, correct?

So first I would look at replacing relay 39, since this would be the same as adding another relay. Have you tried that yet.

On the 79, there is 12V to the motor at all times via F10, which is hot always. And the entire control of the fan on and off is from the thermal switch closing to ground. So no relay involved. You might consider that type of setup.

Hope that helps and let us know.