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| Joined: | 03-14-2005 |
| Location: | Atlanta, Georgia USA |
| Posts: | 406 |
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Such are the challenges in adapting 40 y/o instruments to work with modern technology. What you are describing is the same phenomenon with MSD ignitions. Check elsewhere for that thread.
The JH tach is an analog device designed to measure the amount of current going through the coil (i.e. higher RPMs = more current). Along with catalytic converts, and big bumpers, GM introduced their HEI system in 1975 forever changing tachs from analog to digital triggering. I doubt the tech guys on the phone were even alive when analog was king.
The short answer is that there are adapter devices such as the MSD 8920 that will make the tach register by emulating this analog signal. In my experience, the 8920 + Smiths Tach combination was not an accurate one.
In the MSD thread there is mention of a company in CA that will retrofit the Smiths tach to a modern triggering mechanism (ala Autometer, VDO, etc.) which will allow you to retain the stock look and achieve better accuracy.
After all my experimentation and frustration, I found it easier to install an aftermarket tach and focus on other areas of the car.
Judson
Last edited on 08-22-2007 02:53 pm by Judson Manning