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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
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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
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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
    35.7 KB · Views: 27

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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