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jdr37 · 12-04-2014, 05:04 PM

This question is one of curiosity NOT an issue that I am concerned about at this point in time. I recently installed a new serpentine belt tensioner pulley on my Ford F53 '95 7.5. and under load I can hear a very slight "whirring" sound, then no load (foot off the gas) the sound goes away.

I had a conversation with a good friend and retired master mechanic who thinks it is due to engine "load/no-load". I am not convinced because the accessory equipment being powered by the belt are still placing the same load on the flywheel and belt regardless of the drivetrain load/no-load. How/why would the tensioner pulley be under a different load?

As I said, this is just a matter of curiosity at this point -- all thoughts appreciated.

Dave

Paul R. Haller · 12-04-2014, 05:14 PM

Could it be that the tensioner pulley has a bearing that is a little noisy? Different engine speeds can create a noise in the bearing. Can you listen to it with a mechanics stethoscope?
-Paul R. Haller-

jdr37 · 12-04-2014, 06:04 PM

Thanks Paul. I can listen to it and will do so. Even if it may be a bearing noise why is it different at no-load vs load at exact same engine speed (overdrive with torque converter in lock-out)? This is the question I am so intrigued with.
Dave

BFlinn181 · 12-04-2014, 06:36 PM

I thought "load" refers to when the engine is doing work, i.e., climbing a hill, driving at speed down the highway, etc. You seem to be defining it as increasing RPM. When you suddenly increase the RPM, vacuum decreases when the throttle plate is opened. When you let the throttle plate close to idle vacuum increases again to a steady amount. Inertia of the alternator rotor, water pump, power steering pump will change as RPMs change, perhaps that's causing the difference in sound.

TeJay · 12-04-2014, 07:28 PM

This is a bit confusing but here goes. Load means just that. Asking your engine to work. That requires more throttle. Now if you encounter a hill and keep the RPM's the same you will have to open the throttle. If you don't open the throttle you will slow down as you go up the grade. If you don't open the throttle the load on the engine won't increase because you are not making it do more work to keep the speed the same. You should not hear the noise you described because the load has not increased. This can't go on for to long because the transmission will down shift.

If you depress the throttle to maintain the speed and RPM's you are asking for more alternator output as a result of a higher demand on the ignition system. That load on the alternator will increase the magnetic field which makes the alternator rotor harder to turn. Your noise may be the result of that increased load on the alternator bearings. When the load on the alternator increases that will also place more strain on the belt and new tensioner bearing.

BF181 alluded to this happening if and when you increase the load. I kind of broke it down a little more with the increase in thealternators magnetic field.

TeJay

jdr37 · 12-04-2014, 07:33 PM

When in overdrive the torque converter is "locked" and there is no slip in the TQ so that, as an example, when driving at 60 mph with throttle open and engine RPM at appx. 2200 the engine is "under load". If foot is removed from throttle, now no load, the engine is coasting and if on a slight downgrade the speed will remain at 60 and the engine RPM will remain at 2200. In this instance the engine RPM is being maintained by the drive train as opposed to the engine when "under load" is maintaining the drive train speed and MPH.

Under this "no load" scenario the accessory equipment (alternator, AC, PS pump radiator fan, etc.) is still requiring the same power as when engine is under load. In short, when I refer to "load" I am referring to the engine powering the drive train and no-load is the drive train maintain engine RPM which is in turn powering the accessories.
DaVE

jdr37 · 12-05-2014, 05:34 AM

Again let me describe the situation. At cruise speed and engine RPM AND when the accelerator is held steady to maintain that speed on level ground the NEW tensioner pulley is making a slight whirring sound -- it is NOT the alternator. When the accelerator is released the sound immediately stops before speed and RPM decrease. Why?

subford · 12-05-2014, 07:38 AM

As you know the alternator does not have a tensioner pulley so we know that is not it or the belt that drives it along with the AIR pump.

That leaves the power steering/brake pump and the A/C compressor.

It is cold out right now so I do not think you had the A/C clutch on so its pulley just turns on its bearing. They can lock up but do not think I have heard one make noise unless it locked up.

That leaves the power steering/brake pump and the tensioner pulley itself.

When you have noise that comes with normal power and goes away with no power I think of the pinion bearing in rear end. But I do not think you can hear that in a class "A" RV.

So is the power steering/brake pump making the noise?
Maybe, is it low on fluid (it uses type "F")?
If you any other fluid beside type "F" (ESW-M2C33-F) in it it can make a noise.

jdr37 · 12-05-2014, 07:42 AM

Bill,
This started IMMEDIATELY after I replaced the tensioner pulley. There is no reason to think that it is coming from any other of the accessory units. What I am really wanting to know is WHY does it make the sound only when engine is under load.
Dave

Lt46 · 12-05-2014, 08:11 AM

Why not put your mind at ease and just replace the tensioner with another new one? It is not unheard of for new parts to fail right out of the box.

TeJay · 12-05-2014, 02:27 PM

Here's some comments.

I think you got your answer in post # 4 & 5. When cruising on level ground a certain load is needed to maintain speed. That load also requires some extra energy to spin belt driven items. It matter not which spinning device (alternator, PS pump, air pump or what required more energy to keep it going. I happen to believe the ignition system requires more current to keep the spark plugs firing and that means the battery is requiring a greater charge. Therefore the magnetic field in the rotor/stator increases and it is harder to turn the alternator rotor. The extra force needed by the belt is tweaking that new bearing on the tensioner causing it to make a different noise. It does not mean that the bearing is bad.

The PS pump won't effect the belt unless you are turning. The water pump I believe will require roughly the same energy during cruise or no-load. The air pump would be the same as the water pump. The air compressor on some makes and models does lock up occasionally to pump oil throughout the system but that would not be a steady thing.

If you really want the answer then just change the tensioner (as suggested) again and see what happens.

TeJay

BFlinn181 · 12-05-2014, 02:45 PM

From the first posting I didn't immediately see that Dave (jdr37) was describing a noise while moving. That's why I explained my definition of 'Load.' The fact that the sound can be heard while driving down the highway means it's more than a 'slight whirring sound.' Since you are convinced it only became evident after you changed the tensioner pulley, I suggest a couple of things to try. First, as has been suggested, use a mechanic's stethoscope to listen to each pulley/bearing in the belt system. Next, it could be that the other belt is too tight, creating the whirring sound from a bearing under, uh..., load. Check the tension of the other belt(s) and listen for changes in sound again with the stethoscope. Any noise that can be heard while underway has to be a warning of a future issue if it's making itself known over the suspension, wind, tire, and engine sounds.

subford · 12-05-2014, 02:54 PM

Note that the alternator is not on the same belt as the tensioner that was changed.

TeJay · 12-05-2014, 03:13 PM

OK, It's time to ask that dumb question. If he has a serpentine belt and a tensioner does that not mean that everything is controlled or driven by that same belt???? That was what I always ran into when working on the one-belt does it all.

I guess there are situations when you might have a S-belt and also a V-belt but I never ran into any.

TeJay