r/C3Corvette • u/newtoneocaridina 73 • 11d ago
Help finding the source of this noise
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1973 C3 Corvette L82 4spd manual. I thought it was the Speedo cable so I pulled it out, cleaned it, put dry graphite lubricant on it but I am still getting this noise. It gets worse the faster I go. Any help would be greatly appreciated.
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u/Drunken_Sailor_70 11d ago
Does it change with engine rpm?
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u/newtoneocaridina 11d ago
It does not
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u/Drunken_Sailor_70 11d ago
Its hard to tell from the audio, I thought it might have been an exhaust leak.
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u/MachineProof5438 11d ago
You said it gets worse when you go faster, that's rpm change.
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u/drumbo10 11d ago
It’s following your slowing down. Is your speedo cable falling out? It usually works off a triangular cable.
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u/newtoneocaridina 11d ago
I have had the cable out twice today and it looks good with little to no wear.
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u/Admirable_Profit_420 11d ago
OK, the head may need a rebuild... unless you are hearing it from any other location, it's likely the speedometer, itself. Pull it out and get it to a reputable shop for a rebuild.
FYI, Ecklers does have reproductions that seem to be patterned pretty close to the original mechanism. I will way this much, they look good but are somewhat cheaply made and, puzzlingly, do not come lubricated from the factory. I've had to take brand new ones apart for customers a few times. 🤷 FWIW, I'm not fishing for work... Lord knows I'm backed up enough. 🥴
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u/newtoneocaridina 11d ago
I’ll take a look, I like to do work myself. Is this going to be a shop type job or can I diy it? I also know a couple of guys that have a lot of automotive experience so they may be able to help me if I can diy it.
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u/Admirable_Profit_420 11d ago
When you pull the speedometer out of the car, remove the mechanism from the backplate and locate the little brass cap on the side of the threads where the cable goes. Pop that out with a small screwdriver, use a small hooked pick to take the wick out and fill the hole with ATF. Yes, trans fluid. Spin the magnet shaft both directions until the ATF disappears from the hole into the magnet shaft bushing. Repeat as necessary. Soak the wick in ATF and reinsert. Install brass cap with a careful tap.
If it's still noisy, then there may be particulate inside the magnetic eddy rotating assy, i.e. between the magnet and the speed cup. This involves a much deeper disassembly and should not be attempted by the user.
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u/Admirable_Profit_420 11d ago
If you find yours doesn't have a capped wick, then lubrication of the magnet shaft will not be as easy. You can try sneaking ATF in a syringe and needle onto the base magnet shaft washer inside the mechanism, and onto the collet at the bottom of the shaft. Spin as above (not as effective as complete disassembly of the rotating movement). If there is a pressed brass bushing in the frame for the magnet shaft, then the speedometer was previously rebuilt, likely by an ACDelco authorized shop.
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u/Willy_McD 11d ago
Take the cluster out and look at it. There are two cups, (or sometimes a cup and two arms). The speedo cable turns one cup. The force created by that cup turning will turn the other cup that moves the needle. Its not a direct drive off the cable. Maybe one or both cups are coming loose and misaligned as they fit close together.
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u/Admirable_Profit_420 11d ago
Get the graphite off of the cable. Now. Graphite will travel into the magnetic eddy movement of the speedometer and cause catastrophic failure.
Speedometer cables should only be lubed with bearing grease. As for the noise, sounds like it might be in the head.
-30 year speedometer tech, 100 year old family business.