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M, 400 fan rpm

K

karl f

Well-known Member
does anyone have handy the rpm or drive ratio of the radiator fan for a M or 400 compared to the crankshaft rpm? Or a the way to calculate it?

thanks
karl f
 
The speed of the fan relative to the crankshaft will vary with the adjustment of the fan pulley. The more the belt wears the higher it will ride in the fan pulley and the lower it will ride in the crank pulley, both of which will work to slow the fan.

To find the actual speed you can measure the outer diameter of the belt in each pulley and then apply that ratio to crankshaft speed to find the fan speed. The most precise way is to measure the diameter at the midway point of belt contact on each pulley but the outer diameters will get you extremely close to the actual figures.
 

Having more tools sure eliminates a lot of questions. I know I thought of a tach right away, but I do not have. If only it was something i could add to my wedding registry :)


I tried to be all exact this evening with a micrometer, and found the crank pulley much larger than my measuring capacity. so eyeballing with a tape it will be.

karl f
 
Use a piece of stiff wire, a coat hanger will work, bend it in a U until it just clears the belt. remove it and measure between the ends. Poor mans caliper.
 
Mark both pulleys with a marker so you can see them against a piece of tape, or a fan blade with tape and tape on the shroud. (pull the plugs if needed) and crank the engine over counting ten turns of the large pulley. (and counting the turns of the small as well) Slow hand crank turning is easiest!
then it is again simple math to set the ratio. Jim
 
If your doing all this for sizing a different alternator pulley, just get the smallest one you can get. Your not gonna exceed the limit running it on a M or 400
 
That alternator will begin to charge when it gets 12v on the #1 terminal. At almost any RPM. It gets its exciter current from that source. If it will not charge till near full throttle, it is not operating correctly. (guaranteed) Putting a resistor of 10 watts 100 ohms in the wire coming from the ignition switch, going to #1 terminal, allows it to avoid back feeding the distributor, thus staying running when you shut it off. Use a small pulley, but the issue is actually the internal workings, not the RPM. Jim
 
even jumping the #1 directly with 12v will not excite the alternator until high rpm. #2 is getting 12v at all times.
based on the input i"ve gotten,
looks like it is a defective alternator out of the box. I"m afraid the tester at work will "pass" it since it is not variable speed.

thanks for the input.

karl f
 
even jumping the #1 directly with 12v will not excite the alternator until high rpm. #2 is getting 12v at all times.
based on the input i"ve gotten,
looks like it is a defective alternator out of the box. I"m afraid the tester at work will "pass" it since it is not variable speed.

thanks for the input.

karl f
 
Is it a one wire alternator? This is a common problem with the one wire alternator. That is why many guys here recommend a GM 3-wire alternator set up. mike
 
mike. It is a 3 wire.
The most frustrating thing about it is I've done it "right" to eliminate my concerns and it's still biting me in the rear. Also the fact that I have coached maybe 10 people in person on a conversion and theirs were all successful.

and to everyone else, thanks for the input. According to the card that comes with the alternator, the light off/turn on rpm is around 250. so 975 alternator rpm at idle should be sufficient. maybe it's a poor connection? I'll add a reply when i get the desired results.

I have not messed with it yet in hopes of cooling off and looking at it fresh. rather than cooling down about it, it's making me madder every day ;)

karl f
 
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