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| Joined: | 04-07-2022 |
| Location: | Oregon USA |
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As many of you know, the stock tachometer is not happy when the ignition is converted to an electronic one. The tach typically will read low, but may do just about anything except report the correct engine speed.
This is because the stock ignition power is routed THROUGH the tachometer before reaching the coil. Electronic ignitions pass more current that the stock breaker points do, and the tach reads the average. When the current amount changes, so does the tach reading.
Many have converted to a more common voltage/frequency style of tach to accommodate the upgraded ignition, but this requires wiring changes as well as a tach change. Messy.
I found an interesting article on the web that stated current sensing tachs can be converted to accommodate electronic ignitions. It was not specific to Smiths tachs, but a little investigation revealed that the Smiths tach is of the same design. See the attached photo of the tach internals.
As you can see, the ignition circuit is comprised of a heavy white wire that passes through a toroidal transformer. In my tachometer, there are two turns of this wire through the toroid, which acts as the primary for the current sensing transformer.
The article states the tach will happily coexist with electronic ignition if the number of turns through the toroid is reduced to a single turn. This can be done by GENTLY unclipping one end of the white wire, passing it through the toroid, and reclipping it in the original position. There are very fine wires present, so care must be exercised while doing this. No wiring harness changes are needed, and no aftermarket modifications to the tach are needed.
CAVEAT: I have not personally done this, but am passing along what I have read if someone wants to give it a go and report back here. I will also mention that there is a calibration adjustment available through a hole in the back of the gauge, with a slotted screw visible through the hole. It may be possible to trim out any error from a conversion by adjusting this calibration screw. You may wish to try this first before opening the tach to remove a turn from the transformer.
Tech article: Smiths Tachometers
Vance
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Last edited on 12-14-2024 12:33 am by vnavaret