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> Jensen Healey & Jensen GT Tech > Electrical & Instruments > Converting Stock tach electronic ignition |
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> Jensen Healey & Jensen GT Tech > Electrical & Instruments > Converting Stock tach electronic ignition |
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| Converting Stock tach electronic ignition | Rate Topic |
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| Posted: 12-14-2024 12:31 am |
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1st Post |
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vnavaret Member 
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Folks: 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 Attachment: IMG_0585[1].JPG (Downloaded 26 times) Last edited on 12-14-2024 12:33 am by vnavaret |
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| Posted: 12-14-2024 02:09 pm |
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2nd Post |
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noomg Member
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Vance, The common problem when going to the Pertronix unit is the tach doesn't work at all. The remedy is to install a ballast resistor from a '73 Dodge van. I believe the wiring details are in the Pertronix paperwork.
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| Posted: 12-15-2024 12:19 am |
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3rd Post |
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Art DeKneef Member
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To further add to the confusion. I replaced the stock distributor and coil with the Pertronix Flamethrower II coil and electronic distributor several years ago. As expected, the tach didn't work. The needle bounced all over the place and never was steady. As I wasn't too bothered by it as I was going to fix it. Well, that didn't happen. I just drove the car and enjoyed it. Early this year, all of a sudden, the tach started working. But now the speedo started to bounce around. What! That doesn't make any sense. There is nothing connected between them. I removed and cleaned the speedo cable but that didn't help. Maybe one of these days I'll try swapping in a different speedo from spares and see if that fixes it. If not, why worry? Nobody drives the speed limit now a days. Art
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| Posted: 12-15-2024 02:24 am |
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4th Post |
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vnavaret Member
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Art DeKneef wrote: I just drove the car and enjoyed it. Early this year, all of a sudden, the tach started working. But now the speedo started to bounce around. What! That doesn't make any sense. There is nothing connected between them. I removed and cleaned the speedo cable but that didn't help. Maybe one of these days I'll try swapping in a different speedo from spares and see if that fixes it.Art: The speedo is cable driven, and more than likely is that the lubrication in the cable has dried up, so the drive binds up and then suddenly releases. Pop the cable off, and remove the wire from the sheath. Squirt a little gear oil in there and reinsert the wire. If you wait too long, the cable will seize and the wire will break, necessitating a new cable. Vance
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| Posted: 12-19-2024 01:02 am |
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5th Post |
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Art DeKneef Member
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I did that with the speedo cable. Cleaned it all up, lubricated it, tested it with a drill and everything seemed to work great. Until I put it back in the car. It's a project "for one of these days" that I'll eventually look at again. Working on the second JH restoration now.
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| Posted: 12-20-2024 05:58 pm |
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6th Post |
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vnavaret Member
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Art DeKneef wrote: I did that with the speedo cable. Cleaned it all up, lubricated it, tested it with a drill and everything seemed to work great. Until I put it back in the car. It's a project "for one of these days" that I'll eventually look at again. Second Restoration? Awesome! =:-o As to the failure to fix the speedo, I will share my experience with a TR6 speedo problem, which you may find illuminating. My speedo jumped around, but not continuously. Instead, it was very rhythmic - A steady reading for a few seconds, then a sudden bounce of the needle, then several seconds of a correct reading. The interval between bounces varied according to speed of the car. I opened the speedo and discovered that the plastic gear that drove the odometer had cracked. When the crack rotated around to increment the odometer, the speedometer would bounce. The force required to drive the speedometer would increase sharply when the cracked gear would try to increment the odometer, causing the cable to twist and then untwist as it overcame the increased turning resistance. The solution was to have a speedometer repair service rebuild the gauge. It was $90 to have it overhauled and calibrated. This was several years ago, so expect to pay more now. An alternative would have been a used speedometer (which may itself have problems) and reset the odometer to the correct mileage before installation. The mileage can be reset using a dental pick, proceeding from the highest decimal place to the lowest. Did this on my TR7 Sprint when I ditched the 85 MPH speedo in favor of a 140 MPH unit. Cheers, Vance
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| Posted: 12-21-2024 02:29 pm |
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7th Post |
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DonBurns Member
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I went with option "C" which was modern units from SpeedHut. They have customization to closely match the old gauge faces. The speedo is GPS, so no cable. When I converted to W58 transmission made it simple. Downside is cost. I think speedo was about $300 and tach about $100.
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