Cooling fan power/relay failure question

[Quarterwarrior]

i completed my MK IV roadster two years ago. This winter I changed my carb, for a MSD Atomic FI system. My cooling fan always worked before the change. The system has its own water temp sensor, so i had to remove the one in the intake that controls the cooling fan. The Atomic FI has a wire and a built in control temp for the cooling fan. I am told it is a ground from MSD and the one i took out of the manifold was also a ground. So, I connected the Dark Green Fan Thermo Switch to the atomic and set it for 180 in the Atomic settings. Nothing would work. I had a one wire thermo switch in the manifold before, not the two wire Painless switch. I took the Atomic out of the picture, and just grounded the Dark Green wire to get the fan working and it still does not work. I tried different relays and fuses and still nothing. My question is, I do not get any power to either side of the Cooling fan Fuse? I was told, i wouldn't because it is ground fuse? I have never heard of anything. I get power to one female prong on the relay. I am not using a switch, i have the Orange Cooling fan and the Brown Guage feed tied together. I am totally at a loss. Should I get power to either side of the Cooling fan fuse with a test light? Sorry for the long thread. Thanks for the help.

[tirod]

Be careful what get screwed into the manifold to detect temperature - there are switches, that turn on at a set temp and back off when it cools, and sensors, that have a variable voltage output for a computer.

Switches work well activating a relay for fan power - sensors don't, as the trip voltage won't be high enough to pull in the relay windings. Sensors are variable voltage to send a signal about temperature for the computer to interpret. It can't use a on/off signal, it needs to know when it's 32 degrees as much as when it's 240 to set the "choke" or not.

When a computer is set up to activate the fan the signal is controlled by output from it, not the sensor. And what confuses the situation even more is the nomenclature used to describe that coolant temperature affected device screwed into the manifold - part numbers get lumped into a one size describes all series of numbers, which they aren't.

Switches are on/off and control the fan independently, sensors feed a signal to the computer and it controls the fan.

If that's not the case here, my bad, but there are two completely separate ways to do it and the switch/sensor looks the same. How it's wired is different, too.

[Quarterwarrior]

tirod, Thanks for the info. i pretty much just changed the one wire temp switch that came with the kit, to the wire that is in the MSD Atomic to control the Cooling fan. The instructions say it is ground wire, so i thought it would just replace the factory temp switch.

I have no power to the fuse panel for the cooling fan fuse now. Should there be 12 v power to the fuse panel? I called Factory five and they said i will not see power at the fuse panel for the cooling fan, that is a ground fuse? I have never heard that before. If it should have power to the fuse panel, why would it not have power, what could be wrong? I am totally confused on this. I could just wire in a switch and be fine, but I have no power to the fuse.

Thanks again for the help.

[CraigS]

Is there a wiring diagram for the harness available?

[Quarterwarrior]

42734

CraigS, Enclosed is the wire diagram from MSD Atomic, at the top, I am using the Tan Cooling fan 1 ground wire, instead of the FFR One wire temp switch that was in the manifold. Thanks