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Fan noise.

The '77 911 engine in my 914-6 conversion was recently reassembled and runs very well. Part of the refurbishment included overhauling the alternator including replacing the bearings. The fan/alternator assembly has a typical high frequency whine that begins to increase in volume from 2500 engine rpm's, is loudest at 3000 rpm's (intolerable in long drives) and quickly declines to almost no whine above 3100 rpm's. I assume issue is "unique" to this typical 11 bladed fan blade or possibly poor replacement bearings. The noise has been so consistently with no worsening or change for some 5000 miles of driving, thus suggesting to me that it may not be the bearings. Any advice will be welcome.
Old 03-06-2018, 07:14 AM
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I would think if it were bearings that the noise would persist and not dissipate above 3100 rpm. Has any finish been applied to the fan or housing (to where the fan may be out of balance)? You might also check the electrical output of the alternator, and see if there is any difference in performance that is related to speed and noise. Also, make sure everything is properly torqued down. I had a problem vibration one time, kind-of in the same RPM range. It turned out to be that my battery and spare tire were loose, and a minor, imperceptible vibration, which would normally be handled by the engine and trans mounting system, was significantly amplified by these loose items.
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Old 03-06-2018, 10:47 AM
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Thanks for your suggestions. I have a voltmeter which shows charging is steadily at 14.2. I will look for fan wobble and vibration noise.
Old 03-06-2018, 01:56 PM
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Through my firewall hatch, I was able to inspect the fan - saw fan blade tip "scraping" on left third of the cowling. I spun the fan/alternator and attempted to move the shaft in an out and up and down - spun free and no shaft movement. It was suggested to me that I may have set the fan belt too tight. So I loosened the tension by moving one shim into the pulley, reset the pulley with a "thumb push" test on the fan belt deflection of about 1/2 to 5/8 inch (very subjective!). Went for a test drive - whine sound is reduced and is intermittent/not constant in a reduced rpm range. So better but not eliminated - probably live with it.
Old 03-09-2018, 08:34 AM
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Was the fan powdercoated? The tips of the blade and the housing can come into contact with heavy powdercoat. The tips should be masked off.
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Old 03-09-2018, 09:03 AM
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Quote:
Originally Posted by 75 911s View Post
Was the fan powdercoated? The tips of the blade and the housing can come into contact with heavy powdercoat. The tips should be masked off.
This. But you should be able to see the scrape marks on the housing. It is possible, since you had the alternator out, it has not perfectly lined up in the housing at re assembly.
I had this issue when re assembling and always check as I tighten it down - feeler gauge between the blades and housing will tell you. I shimmed mine until even all around. They should be anyway, but.....over time things change. The belt flexing will put a bit of wobble in to it also.
Alan
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Old 03-09-2018, 10:47 AM
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Thanks for your info/advice.I am going back in there to use feeler gauge while reseting the alternator in place and gently filing the blade tips if necessary. Will report back.
Old 03-09-2018, 11:27 AM
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You should be able to see where it has been touching - scuff marks. If it is not touching, not the problem.
But I powder coated one of my fans and it left a rub mark on the housing in one spot. The clearances are quite close. If it is touching, then you need to try and re align the fan/alternator, and then the feeler gauge check will help you get it squared up.
Alan
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Old 03-09-2018, 12:00 PM
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I removed the fan again, rechecked the alternator shaft - no movement laterally or longitudinally and spins freely by hand with no noise - check. Inspected the fan carefully - it had a clear coat of paint on it and a few of the blades showed possible contact with the cowling. I used a fine flat file to remove the paint and oxidation on the blade tips down to clean metal and removed a little metal (1/1000"?) on each blade tip with a fraction more on the three blades that had some scuff marks. The cowling showed two opposing areas about 1/4" wide by 4" and 7" long, where the clear coat was scuffed. I sanded all the cowling circumference in line with these two opposing scuff marks, also down to bear metal. Checked the alternator securing bolts -good and tight. then reassembled the fan/alternator back into place. Went for a test drive - whine noise is diminished considerably - lower volume and narrow rpm range - 2800 t0 2900 engine rpm's. So noise is still there but much more tolerable - will live with it. Thanks again for all your advice.
Old 03-11-2018, 12:28 PM
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The extent to which the housing clamp bolt(s) is torqued is important, since the magnesium housing is really quite fragile. Over-tighten the bolt (my SC has one only) and you distort the housing and leave it out of round. Such damage could also have been inflicted by a previous tech(s) over the years. The one on my car was thusly distorted, had score/witness marks at the 7 o’clock area and made the noise you describe. Replacing it with a good used example, with the clamp bolt torqued “just enough” solved the problem. Best, John in CT
Old 03-14-2018, 09:18 PM
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The fan housing might be cracked.
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Old 03-15-2018, 06:56 AM
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I checked for cracks and could not see any. The scuffing was in two areas, from 6 to 10 and from 2 to 4 , about 1/4" to 1/2" wide. the single clamp bolt was not too tight, but I loosened it a bit more.
Old 03-15-2018, 07:46 AM
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Quote:
Originally Posted by Porsche 2 View Post
I checked for cracks and could not see any. The scuffing was in two areas, from 6 to 10 and from 2 to 4 , about 1/4" to 1/2" wide. the single clamp bolt was not too tight, but I loosened it a bit more.
You checked the thin "alternator supports"? Its hard to do without removing the alternator. They are quite fragile and can crack if the fan belt is too tight.
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Old 03-15-2018, 08:15 AM
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I looked carefully with the cowling and housing in place on the engine and fan removed - not ideal viewing.
Old 03-15-2018, 09:30 AM
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Methinks you need a replacement fan housing.
Old 03-15-2018, 05:29 PM
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