Thread: Electric fans drop engine rpm

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I just installed a new radiator with 2 14" fans. I used a relay to power up the fans. When the fans turn on, the motor rpm at idle drops like 150+ rpms. It appears to be a stock OEM alternator, brackets, and pulley. I'm guessing it was the stock 55 amp alternator. It's a slightly pumped up 350 and it idles a little lumpy but gets real rough when the fans turn on. It's got a pretty fresh Optima battery. Can the fans pull so much that the alternator struggles so much that it drags down the motor? Would a bigger alternator solve this?
thanks for any suggestions....

The short answer is yes. Dual 14” fans will draw upwards of 50 amps and your stock 55 amp alternator is probably taxed to provide sufficient output. Once the engine is running, the battery is no longer in play here. I’m making an assumption that this is in your 37 Ford and as such you do not have room/clearance to use the standard clutch fan and shroud. As such, you will definitely need to upgrade your alternator. I have used Powermaster 67293 alternators on multiple builds and have been very satisfied in terms of appearance (polished aluminum) and performance (150 amp). It will set you back north of $150, but is a great one-wire replacement unit.

Make sure to install a bonding strap directly from the back of the alternator (lug is provided) to bare metal on the frame or engine.

Regards,
Glenn

Install a #6AWG stranded copper cable directly to the battery https://www.summitracing.com/parts/sum-810002 or similar).

As a side note – it is NEVER a good idea to replace a GM OEM clutch fan and associated shroud with electric fans unless absolutely necessary for clearance.

i run a single flexlite syclone fan , draws less and cools just fine. i have used this combo on 6 builds. the 56 big window sat in the sun for an hr with air on . engine temp would rise to 215 , fan on temp dropped to 180 then fan shut down. it cycled for an hr.

I should have mentioned that this is in my 1970 El Camino with a 350 sbc. I need to update my profile. The 37 has been gone for a long time.
Glenn, contrary to your advice, I removed the stock radiator, fan and clutch and installed the 3 row aluminum radiator with the dual fans. Doing this dropped the temp 10-15 degrees but this drop in rpm is something that I don't like. I'll try to identify this alternator set up before I do anything else.

Gotcha - did the new radiator necessitate removal of the shroud and stock fan due to clearance issues?

Can you post a picture or two?

if i have room i run a summit 5 blade fan.

No clearance issues, I just thought the dual electrics fans would be better. I'll take some pics when I get home and post them if I can figure out how to download them on this site.

I don't know what carb you're running but there are idle compensation solenoids available that will increase the idle under load. They are intended to compensate for air conditioning load, but should work for this as well. I agree that you will need to upgrade your alternator. If you don't now, you will later. Running at max load will fry it eventually.

Originally Posted by 1carguy

No clearance issues, I just thought the dual electrics fans would be better. I'll take some pics when I get home and post them if I can figure out how to download them on this site.

If you get stuck, e-mail them to me at mfmike1@comcast.net and I'll post'em up for you.

no electric fans will outperform one of those 5 blade fans. they move a ton of air. 1 16 in electric will do better than 2 14's .

I was going to mention the AC solenoid, but was beat to it.
Another option is to have the fans on separate temp sensors, about ten degrees apart.

The engineers at GM (or any car company really) responsible for the peripheral devices attempt to satisfy the majority of the stated design requirements in an “affordable” manner that will allow production of components that are:
• well suited to the task
• transportable among multiple platforms (i.e., Camaro, El Camino, Nova, Impala, etc. – entry level V6 – big block engines)
• suitable for many production years

In regards to cooling “systems” GM has done a very good job with fan clutches and shrouding. The factory setup brings cool air into the radiator and the shrouding provides near perfect efficiency in ducting said air across the radiator and back across the engine. In stock configuration (clutching fan, 180 degree thermostat and 50/50 glycol-water coolant), a factory car can transverse Death Valley or climb the Grapevine at 80MPH and the temperature would not exceed 200 degrees.

As our vehicles age, it is not unusual to encounter problems related to overheating. The first place to look is the radiator. A good reverse flush, new thermostat, and 50/50 coolant mix cures most problems.

The next most likely problem is the fan clutch. They’re a bit tricky to diagnose but telltale signs of a bad fan clutch are:
• fan spins when the engine is shut off
• wobble (could also be water pump)
• poor air conditioning at idle
• oil leaking around fan clutch
• vibrations

When in doubt, change the fan clutch. The Hayden HD thermal 2747 fan clutch is the best $50 you can spend along with $25 for a Mr. Gasket 4364 180 degree thermostat.

In all but extreme engine builds, the stock fan and shrouding is the best way to cool the engine. If vehicle modifications or larger engine installation causes a clearance issue I would always try a flex fan and then electric (in that order). Any after-market fan system should incorporate a shroud if at all possible to maximize the air flow across the radiator (as well as provide some modicum of safety).

Well said Glenn. As always!