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1965 BJ8 Tach

John Egan

Freshman Member
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Tach reads fine when car 1st run but after car heats up and shut off for a period, Tach reads 1000 RPB high when restarted and run?
 

RAC68

Darth Vader
Offline
Hi John,

As many have, In the 1990s, I experienced eratic readings on my Smiths tachometer caused by a deteriorating electronic component. If I remember correctly my replacement of the diode did satisfy my issue. However, your description seems to point to a part called a thermistor that adjust the circuit to changes in temperature (see circuit below). The problem with replacing this unit is that I was unable to find the specifications for gaining a replacement so, if it is the cause, hopefully you will be able to gain some information from printing on the unit.

Hopefully this helps,
Ray(64BJ8P1)

motach1.gif


 

Bill Young

Senior Member
Country flag
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John

I have been doing some work to reverse engineer the Smith's RVI tachometer. I have done a couple of rebuilds using new transistors, capacitors and thermistor and have achieved linear indication from 0 to 7000 RPM. The thermistor is not easily identified. Tom Haydon of the Sunbeam Club has done a lot of work to identify this device and we think the 330 ohm 3560k rating is close to the original. I am currently working on a temperature controlled box to see what low and high temperatures do to the calibration. Send me your email address and I will send you a .pdf of the attached .jpg drawing. This is a preliminary drawing as I work through design and bill of materials. I am planning a 3 part article for the Healey Marque magazine starting in January showing repair and calibration.

Bill Young, BJ8, BN2, AN5
sjahc.com
 

Attachments

  • Healey Tachometer.jpg
    Healey Tachometer.jpg
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RAC68

Darth Vader
Offline
Hi Bill,

I really like what you have done and would appreciate seeing a full size rendition.

Thanks and all the best,
Ray(64BJ8P1)
 
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