Alternator Belt Tension & Check

[Beachdrifter]

Hi all,

Well today I received a new alternator from DMCH. Interestingly enough, it had a service bulletin that stated something to the effect:

"For a full proof way of checking the belt tension, put a socket on the alternator pulley and turn it clockwise. The belt should not slip, in fact you should feel the alternator wanting to turn the engine"

Never had heard that before, was wondering if anyone tried? I tried it and it looked like my belt didn't grab hard enough and slipped. Although it also mentioned that alternator belts can still be loose and not squeal. I guess maybe mine fell into this category as I thought it felt and looked tight enough, but seems like it didnt pass this suppose test.

Any thoughts on this test?


Thanks,

[Ron]

For years, a lot of alternators came with similar instructions (both ways if no spring loaded tensioner). A rule of thumb I never heard anything against, except some tend to over-tighten them and ruin a bearing.
...I've never seen one that would slip when turning it manually and also pass using a belt tension tester.

From memory, the manual says 1/4" deflection when pressing with your thumb at the longest run and recheck after running. (another old rule of thumb).

[David T]

I do that but instead of using a socket wrench I just grab the alternator by the fan and try to rotate it. For a new belt you get it REALLY tight because it will stretch. You don't want to overtighten it because you will ruin the bearings in the water pump.

[D Knight]

Belts don’t stretch, that wear on the sides, which is what that run in, and fall deeper in the groove of the pulley. There is nylon on the core of the belt that keeps it uniform. Overtightening will wear it out faster and put unnecessary load on all bearings.


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[David T]

Belts don’t stretch, that wear on the sides, which is what that run in, and fall deeper in the groove of the pulley. There is nylon on the core of the belt that keeps it uniform. Overtightening will wear it out faster and put unnecessary load on all bearings.


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Call it whatever you want, wear, stretch, seating in the pulley. There is a spec for a belt that has never been used and there is a spec for a belt that has been used. You want to tighten the belt just to the point past where it slips. Any more and you risk damaging the bearings in the water pump. Under normal operating conditions the belt does slip a certain (tiny) amount and it causes it to heat up. If it slips too much it gets too hot and the hotter it gets the more it slips and the more it slips the hotter it gets till it stretches and flies off. Too tight and you overload the bearings causing them to get hot, lose their lubrication and seize up. Typically there is a deflection spec to use but most just make it tight enough so it won't slip and stop there.

[Helirich]

I do that but instead of using a socket wrench I just grab the alternator by the fan and try to rotate it. For a new belt you get it REALLY tight because it will stretch. You don't want to overtighten it because you will ruin the bearings in the water pump.

This is what we do on aircraft engines before flight. My Delorean has a alternator with an internal fan. I find if you start the car with the AC on, it will squeal if the belt is loose.

[JBaker4981]

From memory, the manual says 1/4" deflection when pressing with your thumb at the longest run and recheck after running. (another old rule of thumb).

I've always felt that 1/4" is too tight. Safer to be too loose than too tight. I've normally gone by 1/2" of play when pressing with your thumb but hell, what do I know.