Engine runs rough when electric fan turns on...

[66 Restomod]

Ok, I'm stumped.

I installed an Old Air electric fan setup last year on my modified '66 and the fan cools great. But when the fan turns on, the idle drops about 200rpm and gets rough. I saw 13.1v at idle with the fan on (and the MSD ignition running), so I bought a 3g alternator thinking voltage drop caused the rough idle.

Now I see 14.2v at idle with the fan running -- but same rough idle.

The car seems to run fine at all other times, and drives down the road fine with the fan on. The spark plugs, timing, fuel system seem fine too.

One item that might help is that with the cam specs the car has only 11inches of vacuum at idle. I'm wondering if a low vacuum cam might cause a rough idle with this electric fan running at idle, but not sure why, if so... Any ideas?

 

[BarnStang]

Post exactly how you have it wired up...

 

[D.Hearne]

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

 

[iskwezm]

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

 

[ForceFed70]

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.

 

[66 Restomod]

Thanks! Here's how it's installed...

Here's an old pic of the fan before i installed it.

fan+wires.jpg (image)

I followed the directions when I installed it , only needing to hook up three wires. A ground wire, a positive to the battery (I have it going to the + on the solenoid so it looks better). Then the last wire is hooked up to the I terminal on the solenoid. That's exactly where I was supposed to hook it up. But I guess the fan is causing a voltage drop so the MSD ignition doesn't run smoothly?

This is a fairly low-power fan. It uses a 25amp fuse and consumes only 10amps when running. When it kicks on the 3g electrical system drops from 14.2v to 13.9v before immediately recovering.

If the last wire is the culprit, where do I install it instead?

 

[66 Restomod]

I think that 25amp fuse I mentioned that was pre-installed with the fan is actually a 25amp relay. Sorry, I'm electrically challenged...

 

[Tim65GT]

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

 

[BarnStang]

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.

 

[66 Restomod]

Thanks, I will run the switched positive wire from the green wire with red strip near the voltage regulator on the firewall. I think I can make it look clean. But if I want to run a new wire from the ignition switch, how would I do that?

 

[BarnStang]

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.

 

[66 Restomod]

Thanks! Behind the instruments in the dash there's a triple connector that has a black/red stripe going to it that I think is switched power and should be 12 volts too. Not exactly sure though. I had planned to plug in my stereo head unit to there with a bullet connector. Can I use that one if there's an opening available?

 

[66 Restomod]

Update: Ok, I removed the switched 12 volt lead from the "I" terminal and soldered it into the green wire with red stripe at the external voltage regulator. When I checked, that wire is switched on with "accessory" and "run", but not "off" or "start". (The fan will run if the temp is high enough when the ignition is set for "accessory" but that's not a problem for me.)

But I can't say that I notice a real difference from before in idle quality when the fan turns on...

I did notice something that was probably this way before -- the lower rpm/rougher idle sometimes seems to continue after the fan turns off, at least for a little while, which has me really stumped. This 3G alternator is pumping out 14.4v at idle. I also noticed something that was this way like before, the headlights get dim for a second when the fan turns on as the voltage briefly drops to under 13 then they recover quickly. The headlights are relayed with a mail order kit, so I thought they shouldn't be inconsistent like that.

Now I'm thinking maybe I'm simply expecting too much from this old car/new fan/3G alternator setup... Or maybe this issue isn't electrical at all?

 

[BarnStang]

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?

 

[BarnStang]

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.

 

[66 Restomod]

Not electrical?

Sorry for the delay in answering. Those are all good suggestions, and I went through them and seem to have them covered.

I wanted to say that after plenty of tinkering and head scratching I think this rough idle is not electric at all... I have reset the air/fuel mixture screws (4) on the carb, bumped the initial timing 2 degrees, and increased the idle about 75rpm. This helps the rough idle when the fan comes on, but not completely.

While doing these changes, I noticed that when the fan comes on, it's very powerful, and pushes a continuous wave of hot air towards the engine (and carb).

So I think I have some sort of "fan wash" condition where the really hot air from the fan gets sucked down the carb. That's why I notice this at idle, but not when driving down the road. Now I'm thinking of making my hood scoop functional to help solve this.

 

[10secgoal]

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

 

[66 Restomod]

Thanks, I have richened the mixture a bit to where the car seems to run great at part throttle. I used a vacuum gauge for the adjustments since I don't have much experience with a carb.

If I go much farther with richening, there's a gas smell from the exhaust so at that point I think I've gone too far. I'm going to drive the car another week or two and pull a spark plug to check the carb adjustment.

 

[Miles428]

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.