added cooling fans dragging the engine idle rpm down


The Grumpy Grease Monkey mechanical engineer.
Staff member
I just installed a new aluminum radiator with 2 14" fans and when they kick on, the draw on the electrical system drags my motor down by about 150+ rpm at idle. This is in a 1970 El Camino with a 350 sbc that has a lumpy cam and idles a little rough but feels like it wants to stall when the fans turn on. I don't want to just kick up the idle RPM to overcome the fans. It appears to have a stock GM alternator and has a fresh Optima battery. Is this normal or do I need to upgrade my electrical/alternator?

if your running a stock 55 amp-88 amp alternator its very likely that the addition of several electrical cooling fans could and more than likely would drain considerable current from the system, most cooling fans will run at several speeds(RPM RANGES) and a 14" fan is very likely to pull well over 15-20 amps on initial start up, so the typical OEM alternator on a 1970 el Camino will be seriously over loaded if both fans turn on, making use of relays, heavier gauge wire, and a larger capacity alternator mandatory

reading these links might help
swapping to a 150 amp alternator might be the quick solution especially if you wire the fans with relays correctly

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I cant see how upgrading the alternator or wires would cure the rpm drop in idle? It loads the engine in either case with the same load.
The engine needs to be stronger at idle, maybe recurve the distributor for more initial advance can help a little, or in worst case of a lumpy cam, ad like an old type of throttle kicker solenoid like on old AC systems with carburetor?
the rpm drop is partly caused by increased drag loads (as the fans kick in they drain electical energy and loss of ignition current energy