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C3 Speedometer Failures................HEI Caused?

Mar 9, 2009
I installed an HEI in my big block '71 back in January 2010 and by January 2012 my original speedometer was reading 33% slow; 40 mph at an actual 60 mph. As the speedometer's trip meter wasn't working I bought a new speedometer and two years later it was also reading 33% slow. I thought it was really odd both of my speedometers failed the same way in only two years and wondered if my HEI could be causing the failures because of the powerful electromagnetic field generated by the top mounted coil with no shielding covering it. As I happened to have a compass I started my engine and held the compass near the face of my speedometer and the needle went wild. So I modified a used plastic HEI shield off a 1982 C3 and installed it; making sure it was grounded with the braided copper ground wire. With the shielding in place my compass read normally which indicated the shield was working.

I had always read the distributor shielding was for blocking the electromagnetic field to prevent radio static in the C3's because of their fiberglass firewalls. But I'm wondering if that powerful field can also slowly kill speedometer magnets. Speedometer shops expose speedometers to powerful electromagnetic fields of different intensities to calibrate the speedometer magnet's strength so it seemed logical that the HEI's powerful field could also kill the magnet over time. My '71 is a daily driver and as I live in the mountains I have to run up a 10 mile long 6% grade coming home; meaning my engine is under considerable power coming up the steep hill.

I managed to re-calibrate my failed speedometer by intentionally killing its magnet then epoxied two tiny Radio Shack permanent magnets onto the killed magnet. Then I calibrated the speedometer to read correctly at all road speeds up to 100 mph. But I'm still wondering if it was my HEI that caused both of my speedometers to fail in the same way in only two years of daily use.
the only reason my speedo had quit was a disconnected or broken cable.
All Corvettes built in '75 or later had HEI. Many of them have lost their shielding. Never heard of the speedo issue.

Have you ever heard the radio noise produced when running the old solid-core spark plug wires and standard non-resistor spark plugs? The "buzzing" in the speakers when you put the radio between channels? The more the throttle is opened (a higher load) the louder the buzzing got. That buzzing is the electromagnetic field from the ignition system and the field will kill non-permanent magnets. As I had to pull a 10 mile 6% grade coming home every day the electromagnetic field was VERY strong for the 15 minutes it took to pull the hill; something that normal cars seldom do.

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