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Hopelessly Addicted
Join Date: Jan 2003
Location: Central PA
Posts: 314
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erratic speedometer
I've searched this forum for erratic speedometer behavior, but didn't find anything like this.
 The one on my '81 SC works without fluctuating, as well as the odometer, up to 18 mph or so, then both quit at the same time. Anyone else experience this? Thank you.
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'81 SC Targa '86 944 '49 Ford F-3 '13 Yamaha FZ-1 (Adrenaline Machine) 
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06-24-2017, 11:23 AM
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Registered
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hi i do not know why 18mph but have a look at the engine compartment plug on left.possibly loose connections...
Ivan here https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSdcSk6RepfpdUhKnU4FVqEE7WhSys2f LdbmH4eNZZ6dKz4g8ygww |
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06-24-2017, 12:20 PM
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Registered User
Join Date: Jun 2014
Location: Scotland, UK
Posts: 63
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Mine used to work perfectly up until about 60mph then start misbehaving. A new speed sensor sorted that out - it was brittle and falling apart with lots of white powdery corrosion behind it.
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06-24-2017, 12:35 PM
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Hopelessly Addicted
Join Date: Jan 2003
Location: Central PA
Posts: 314
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Thank You
I'll check both.
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'81 SC Targa '86 944 '49 Ford F-3 '13 Yamaha FZ-1 (Adrenaline Machine)
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06-24-2017, 03:16 PM
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Hopelessly Addicted
Join Date: Jan 2003
Location: Central PA
Posts: 314
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Update
I ordered a new sensor from Pelican since it was 36 years old anyway and installed it, after I checked all connections.No corrosion anywhere, and now indicates mileage and speed up to 20 mph, then needle drops to 0 and odometer quits. Send speedo unit to Palo Alto?
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'81 SC Targa '86 944 '49 Ford F-3 '13 Yamaha FZ-1 (Adrenaline Machine)
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07-02-2017, 02:43 PM
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Before sending the speedo of, jack up the rear of the car to check you are getting 8 pulse per wheel revolution. Connect a VOM at the connector in the tunnel to verify that the reed switch is switching 8 times per wheel revolution. If you are getting 8 pulses, then there is a problem with the speedometer's pull up circuit that converts the reed switch action to 12 vdc pulses. If you are not getting 8 pulses, then one or more magnets on the disk inside the transmission have dislodged causing the pulse rate to be unstable.
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-Dennis 1977 930 Slant, MS3 EFI, Carrera intake, Twin plug, Powerhaus headers, Magnaflow muffler, Garretson intercooler, GTX3071R |
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07-03-2017, 08:49 AM
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Hopelessly Addicted
Join Date: Jan 2003
Location: Central PA
Posts: 314
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Thank you
Got 8 pulses
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'81 SC Targa '86 944 '49 Ford F-3 '13 Yamaha FZ-1 (Adrenaline Machine)
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07-03-2017, 04:47 PM
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Eng-o-neer
Join Date: Dec 2011
Location: Los Angeles
Posts: 3,108
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My money is on a loose connection in the transmission tunnel by the shift coupler:
Speed sensor, bouncy gauge, sketchy wiring... |
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07-04-2017, 05:35 AM
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Hopelessly Addicted
Join Date: Jan 2003
Location: Central PA
Posts: 314
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Update
I checked all connections, then sent my speedometer to a VDO authorized repair shop here in PA.
They called today, but I couldn't answer the call at that time. The voicemail informed me that what I sent them could not be repaired, but I could buy from them a reconditioned unit for $675, including shipping. I imagine they will ship my "irreparable" unit back. I'll call them tomorrow to get more details. (core, condition, warranty, etc.) Does this sound reasonable? Thank you for your responses. 
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'81 SC Targa '86 944 '49 Ford F-3 '13 Yamaha FZ-1 (Adrenaline Machine)
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07-18-2017, 03:15 PM
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