Electric fans cause bad idle

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wazoo64

wazoo64

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I installed an MSD box today. The engine fired right up but had a rough idle ...almost like a small vaccum leak. Set up a the vaccum gauge to tune the carb and it was all over the map until I turned off the electric fans. As soon as the fans turns off the idle was perfect.
I am running a 75 amp powermaster alt and was thinking I need to get one that is 100amp to solve this problem. I think the MSD box is not getting enough juice when the fans are running
Anyone want to chime in on this one?
 
May have to run the fans thru a solinoid or heavier wire or both,pfogel commented about this same issue the other day to me and we are pondering this
 
So a 100amp alternator won't solve this problem??? Will 100 amp alt fry the bulkhead wiring? No problems with 75amps now
 
I would first find out how much current your fan is capable of pulling and also check your positive wire source to the fan. I would run it through a relay and have the relay source near the battery and not drawing main current from the ignition circuit. It sounds like you're pulling current from the ignition circuit thus lowering its voltage and causing a misfire. You can verify this by checking voltage at the ignition module with the fan on then off.
 
If you still have the stock starter relay there is a stud and nut connection there that is a easy pick off for another circuit. Ring terminal with a suitable gauge wire to supply a relay in the same general area ( near the fan ). I would place a maxifuse in that supply wire too. Then out of the relay to the fan motor. What ever is turning the fans on now will signal the relay to close instead.
 
You might have electrical noise generated by the fans that is creating false triggers on the MSD. A simple digital voltmeter test at the fan circuit and MSD will indicate if the alternator is putting out enough voltage. I would think 13.8 to 14.5 VDC would indicate good. You could also use a clamp-on DC ampmeter. If you place the meter on AC and measure the DC voltage you can measure the ripple and perhaps the noise. I would think anything over a few tenths of volt is not good. Do this at both the fan and MSD circuit.

A solution may be to feed the fans by a relay controlled fused circuit from the alternator output. Then feed the MSD from a circuit sourced at the battery + terminal. The battery is like a big capacitor and will filter noise.
Perhaps use the meter on 2V scale VAC, and see how much ripple is at the battery.

There are also filter circuits that my help.
 
First, have you done any modifications to the stock harness with the upgraded ALT? if not, it would be wise to do - Redfish had an excellent suggestion to use the starter relay post as a distribution point to power some of your accesories.

run the fans off a relay - possibly 2 relays if needed. I would run 10g wire from the starter relay stud to the fan relay, and just a 12+ signal wire from a switch in the passenger compartment.

I re-wired my Charger w/ a new, internally regulated ATL, and eliminated the ATL gauge and charging through the headlight switch. Basically, I ran a new 10g charging wire from the alternator to the starter relay post - now that acts as my main distribution point. I ran a separate "sense" wire to the same post. So now my headlight realys and fan relay are powered by their own 10g B+ wire from the starter relay post. The old "charge" wires that passed through the bulkhead, through the headlight switch, and through the ALT gauge are now fed from the starter relay post.

It may sound like a lot of work, but the only new wires I ran were the ALT charging wire, and the new wires for the relays - you can easily update your electronics in a weekend.

I am currently running a single electronic pusher fan to supliment the viscous fan, and I am running 3 sets of true HID lights (heads, Highs, and Fogs) and at idle, all can run without dimming lights, or slow windshield wipers, etc - and I'm only running a 60amp ALT.

Oh yeah, I run a Mallory 6A box and my car runs like a champ.
 
KitCarlson said:
You might have electrical noise generated by the fans that is creating false triggers on the MSD. A simple digital voltmeter test at the fan circuit and MSD will indicate if the alternator is putting out enough voltage. I would think 13.8 to 14.5 VDC would indicate good. You could also use a clamp-on DC ampmeter. If you place the meter on AC and measure the DC voltage you can measure the ripple and perhaps the noise. I would think anything over a few tenths of volt is not good. Do this at both the fan and MSD circuit.

A solution may be to feed the fans by a relay controlled fused circuit from the alternator output. Then feed the MSD from a circuit sourced at the battery + terminal. The battery is like a big capacitor and will filter noise.
Perhaps use the meter on 2V scale VAC, and see how much ripple is at the battery.

There are also filter circuits that my help.
Click to expand...

If you really suspect any AC ripple on the line you can check it with any cheap oscilloscope.
 
OK, fans pull 60-75 amps your MSD pulls 11 amps other misc. 15 amps. Do the math your Mopar alter. puts out 90-100 at high rev., 40-45 at idle. Your MSD NEEDS min. 11 volts to run. Your might be running right at that point or just under for rough idle. We put in a 160 amp alter. which is 90 at idle. More the amps the better, but volts must be between 12-16. Don't forget to bring up the gauge of the wire to handle the amps.
 
Here's the power junction you need. We mounted ours under the battery plate close to relays up front and battery.

cn-1_s14.jpg
 
67gtx said:
OK, fans pull 60-75 amps your MSD pulls 11 amps other misc. 15 amps. Do the math your Mopar alter. puts out 90-100 at high rev., 40-45 at idle. Your MSD NEEDS min. 11 volts to run. Your might be running right at that point or just under for rough idle. We put in a 160 amp alter. which is 90 at idle. More the amps the better, but volts must be between 12-16. Don't forget to bring up the gauge of the wire to handle the amps.
Click to expand...

Where did you find 160 amp alt? I can only find 140 GM stlyle at Summit.
 
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