Engine runs rough when electric fan turns on...

Post exactly how you have it wired up...

I'll bet you need to run a separate circuits for the fan and ignition.

How do you have ignition wired.Is there a relay to the fan?

Could be just load from the alternator? When my Taurus fan kicks in, my idle drops about 100rpm or so. Back when I had the huge stereo, If I had a song where the bass kicked in real loud for a second (an extended low tone, not a "beat") my idle would drop at least 200rpm and almost stall. Much like what you describe.

Might want to try turning up the idle speed a little.

Measure the solenoid "I" terminal voltage running with fan on and fan off. See what you get.

That it. With the switch in Start, the I term pushs 12v OUT to the coil to start, with the switch in Run (or ON) the resisted ign voltage rides on that wire, so anything that draws current away from the ign is going to affect it. If you need switched 12v, use the green with red strip wire going to the voltage regulater. Don't cut it, just get an inline splice and snap over it. Or run a new wire from the ign switch.

Same wire right at the back of the ign sw before it turns into the pink resistor wire. Green w/ red stripe.
I see it's a 66 right? AN easier way would be to take the small nut off the stud coming out th back of the ign switch. THis is the accessory post. HOWEVER! the down side to this is that yor fans will run when you put the switch in the ACC position to listen to the radio with the car off...so most people don't do it that way. These cars are not switched 12v friendly. If you end up with a lot of stuff you need switched 12v for, use the green w/ red stripe wire to trigger a relay fedd by battery voltage. SO the relay keys when the switch is on, then run the output to a post that you can attach any accessory that needs a switched source.

Do you have the ground strap from the firewall to the back of the passenger side head? Good clean connections?
Post up how your headlight relays are connected. Are you still using the individual ground wires for the headlights on the core support on each side? If so, make sure they are clean and havegood contact.
Are there any other wires attached to the coil + post other than the ign wire coming from the factory harness, or the I term?

How low is your idle?
THe load on the 3G may be enough to drag the engine down a bit. Or the 3G alt may need to be checked out.

You try richening up the idle a little bit yet ? If you borderline lean, putting a load on it wouldn't help the idle.

Good day,
I am answering this old thread as I have the same problem, I'm wondering if it was ever resolved.
I have twin Spal fans powered through two Spal relays. The relays are powered by way of a junction block mounted on the rad support. The junction block on the rad support is fed with a #6 wire from a junction block mounted on the firewall. The firewall junction block is connected to the battery and the alternator by way of a # 6 wire from each. The relays' control circuit is fed by switched battery/alternator voltage and switched off and on by the thermo switch in the intake water crossover.

The fans draw 10 amps each at start up and 6 amps each running. I have a 150 amp alternator which, when tested, puts out 90 amps at idle and 140 amps at speed. The distributor and coil assembly draw 4.5 amps. The voltage at the distributor is 14.55 volts at idle fans off and 14.05 volts fans on.

When the fans come on the engine idles rough, when the fans turn off the engine idle smooths out.

It was suggested the power draw (HP) of the alternator when the fans were on was slowing the idle rpm. I disconnected the distributor from the cars power system and powered the distributor with a remote battery, when the fans came on the idle continued smoothly just as it did with the fans off. It was suggested the power draw of the distributor taken from the cars power circuit was the reason the idle didn't run rough. I turned on the headlights, which draw many times what the distributor draws, the idle was smooth with the fans off or on. I can only conclude it is not the resistance to revolution at the alternator (not mechanical).

I reconnected the distributor to the car's power circuit and ran the fans off the remote battery, the idle stayed smooth with the fans off or on.

I connected both the fans and the distributor to the remote battery, idle was smooth with fans off and on.

I reconnected all the cars circuits and installed a new battery, the one I used as the remote battery, the idle was rough when the fans came on. I disconnected the alternator from the cars power circuits and the idle ran smooth with the fans off and on. I concluded the fans are somehow effecting the alternators output and that disturbance is somehow causing the distributor to not operate correctly when the fans are on.

I increased my idle 150 rpm, no improvement, richened the idle mixture, no improvement, the idle was fine with the fans on when the distributor was operating off a remote battery so it's not hot air getting into the intake.

I tested the alternator for AC voltage, I got 0.080volts ac, replaced bridge rectifier still same problem.

Oscilloscope wave form when testing alternator was good.

I drove the car at a steady 30 MPH then cycled the fans off and on with the ground switch mounted under the dash. The car had some surging when the fans were on, none with the fans off.

I'am as stumped as 66 Restomod was.