Weird Speedo issue
Thread Starter
Racer
Joined: Jul 2005
Posts: 420
Likes: 0
From: Yerington NV
Weird Speedo issue
My speedo was rebuilt to 160mph (I forget what it goes up to, but way over 85mph) about a year or so ago, and I am using orig speedo cable and just recently starting having issue where it works up to about 45mph and levels off there!
Bad cable, speedo? I was thinking about putting a drill on the tranny end of cable and see if I can get it up over 45mph ..... would eliminate cable I guess .....
Any other ideas?
Bad cable, speedo? I was thinking about putting a drill on the tranny end of cable and see if I can get it up over 45mph ..... would eliminate cable I guess .....
Any other ideas?
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
The drill will give you an idea but the problem might disappear with the drill and reappear when you reattach to the trans. I see most of the problems in the cable at the trans where it bends almost 90 degrees.
I should ask do you have cruise ?
I should ask do you have cruise ?
Racer
Joined: Mar 2001
Posts: 272
Likes: 3
From: Marion Ia
If the gear in the transmission is worn it will act like this. The oil in the trans gives enough friction until it gets up to speed and then it slips. Mine would work until 25 mph. Then it would stay at 25, or drop to zero. When you let the car sit for a minute or so it would work up to 25 again.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Most likely a rounded-off [square] drive hole in the tranny speedo drive gear shaft (as mentioned above). Under low speed loads, it works OK; under heavier loads at elevated speed, the cable slips in the drive slot.
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Race Director
Joined: May 2006
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From: Dayton, Ohio
Former Vendor
Joined: Aug 2006
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
It sure sounds to me like you might have a problem in the transducer. More times than not on speedometer issues with the 1978-1982 cars the transducer is the problem.
GM re-designed the speedo head in 1978 and refined it even more when they got away from the oil impregnated bushing for the first worm and magnet. The plastic pop in design bushings seem to wear and last a lot longer.
If you know the RPM's of the drill you are going to test with, here is a chart taken right off the face of our speedometer calibration machine.
500 RPM's = 30 mph
750 RPM's = 45 mph
1000 RPM's = 60 mph
1500 RPM's = 90 mph
and so on. This holds true for every speedometer GM produced 1953-1982.
Willcox
GM re-designed the speedo head in 1978 and refined it even more when they got away from the oil impregnated bushing for the first worm and magnet. The plastic pop in design bushings seem to wear and last a lot longer.
If you know the RPM's of the drill you are going to test with, here is a chart taken right off the face of our speedometer calibration machine.
500 RPM's = 30 mph
750 RPM's = 45 mph
1000 RPM's = 60 mph
1500 RPM's = 90 mph
and so on. This holds true for every speedometer GM produced 1953-1982.
Willcox