ausmini :: View topic - Tacho Question

The old RVI tachometers use the current spike from the coil to the dizzy to count the RPM your engine is doing, the RVC measures the voltage spike that is found in the same line instead.

Early tachs have problems with electronic ignition modules as well as the old germanium transistors, but I have one fitted (with an electronic ignition module as well no less) so it is on a case by case basis depending on the condition of the tach.

RVCs have much less trouble, and are generally younger.

Another benefit of the RVC is that it only needs one wire to the cabin, where as the RVI requires a wire to the cabin form the coil, and one back to the dizzy.

Either will work on a Mini, with electronic ignition or without.

Which type was fitted to three clock dashes on the 1970s Aussie made clubman front Minis and LS - RVI or RVC?

The old germanium in the transistors "grow" with age as well, which slowly upsets the balance of the circuit. Eventually it shorts the transistor out.

Mick wrote:

The old germanium in the transistors "grow" with age as well, which slowly upsets the balance of the circuit. Eventually it shorts the transistor out.

I'm pretty sure this is what happened to mine before I had it converted. Another reason to choose the RVC type.

Thanks for the replies....I am still a little confused (not hard). :oops:

I think I understand the difference between the two types of Tacho...one senses current peaks the other voltage peaks. The problem I am having is regarding the 29D Ignition system on my car.... I just want to be sure that if I go and spend a heap of cash on a tacho it will match my ignition and work as accurately as possible and I won't be up for conversion costs.

Tim

Author:	Mini4T [ Sun Oct 11, 2009 5:25 am ]
Post subject:	Tacho Question
I have a 1971 Mini K which has the original 29D Ignition Dizzy on it. I am looking at buying a Smiths Tacho to fit to it and am a little confused on what type I should get. The searching I have done reveals lots of info regarding RVC and RVI tachos and I haven't been able to work out which one I need. Does anyone have a definitive answer which type is best for my ignition system? Thanks Tim

Author:	GT mowog [ Sun Oct 11, 2009 7:59 am ]
Post subject:
Sorry I don't have to hand info on the RVC, (I think this one picks up directly from the points), however the RVI will work and I'm fairly sure that the RVC will also work.

Author:	Mick [ Sun Oct 11, 2009 8:21 am ]
Post subject:
The old RVI tachometers use the current spike from the coil to the dizzy to count the RPM your engine is doing, the RVC measures the voltage spike that is found in the same line instead. Early tachs have problems with electronic ignition modules as well as the old germanium transistors, but I have one fitted (with an electronic ignition module as well no less) so it is on a case by case basis depending on the condition of the tach. RVCs have much less trouble, and are generally younger. Another benefit of the RVC is that it only needs one wire to the cabin, where as the RVI requires a wire to the cabin form the coil, and one back to the dizzy.

Author:	Flute [ Sun Oct 11, 2009 8:42 am ]
Post subject:
You can get them converted too. I have an old Smiths tacho RVI type and put a 123 electronic ignition in so got the Tacho converted and it can now run on either type of dizzy and seems to be more accurate.

Author:	sgc [ Sun Oct 11, 2009 10:36 am ]
Post subject:
Either will work on a Mini, with electronic ignition or without. I have a 4" Smith's RVI tach in my car which worked fine with my 123 dizzy (for a while, it ended up packing it in). I then had it converted by Howard Instruments in Melbourne with electronic guts and it's now working fine. Having been there and done it, I'd not buy another RVI tach. They're less accurate and more difficult to wire in. Get an RVC - it's wired exactly the same way as every other aftermarket tach.

ausmini https://www.ausmini.com/forums/

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Author:	GT mowog [ Sun Oct 11, 2009 11:33 am ]
Post subject:
Mick wrote: The old RVI tachometers use the current spike from the coil to the dizzy to count the RPM your engine is doing, the RVC measures the voltage spike that is found in the same line instead. Early tachs have problems with electronic ignition modules as well as the old germanium transistors, but I have one fitted (with an electronic ignition module as well no less) so it is on a case by case basis depending on the condition of the tach. RVCs have much less trouble, and are generally younger. Another benefit of the RVC is that it only needs one wire to the cabin, where as the RVI requires a wire to the cabin form the coil, and one back to the dizzy. RVI's and RVC's are both still available new. The RVI's are also a handy diagnstic tool if you've got trouble with your points and/or capacitor and/or coil. Either may or may not work with electronic ignition depending on the type, but the original starter of this thread is running standard ignition. I've only ever just 'broken into' the wire off the back of the ignition switch to wire them in, so no wires to run from under the bonnet! No dirty hands.

Author:	Mick [ Sun Oct 11, 2009 11:42 am ]
Post subject:
sgc wrote: Either will work on a Mini, with electronic ignition or without. No, you can get caught out. The electronic ignitions create different waveforms to standard points, sometimes harmonics will upset the sensitive pickup in the tacho and give crazy readings. I've had these problems before. The old germanium in the transistors "grow" with age as well, which slowly upsets the balance of the circuit. Eventually it shorts the transistor out.

Author:	slide [ Sun Oct 11, 2009 11:52 am ]
Post subject:
Which type was fitted to three clock dashes on the 1970s Aussie made clubman front Minis and LS - RVI or RVC?

Author:	Mick [ Sun Oct 11, 2009 1:33 pm ]
Post subject:
I think it was a voltage pickup? A single purple wire going to a spike on the back of the tacho??

Author:	Drexxle [ Sun Oct 11, 2009 1:35 pm ]
Post subject:
i love your little footer pic Mick.

Author:	GT mowog [ Sun Oct 11, 2009 1:36 pm ]
Post subject:
slide wrote: Which type was fitted to three clock dashes on the 1970s Aussie made clubman front Minis and LS - RVI or RVC? RVI

Author:	Mini4T [ Sun Oct 11, 2009 3:09 pm ]
Post subject:
Thanks for the replies....I am still a little confused (not hard). I think I understand the difference between the two types of Tacho...one senses current peaks the other voltage peaks. The problem I am having is regarding the 29D Ignition system on my car.... I just want to be sure that if I go and spend a heap of cash on a tacho it will match my ignition and work as accurately as possible and I won't be up for conversion costs. Tim

Author:	sgc [ Sun Oct 11, 2009 4:40 pm ]
Post subject:
Mick wrote: The old germanium in the transistors "grow" with age as well, which slowly upsets the balance of the circuit. Eventually it shorts the transistor out. I'm pretty sure this is what happened to mine before I had it converted. Another reason to choose the RVC type.

Author:	Mick [ Sun Oct 11, 2009 5:29 pm ]
Post subject:
sgc wrote: Mick wrote: The old germanium in the transistors "grow" with age as well, which slowly upsets the balance of the circuit. Eventually it shorts the transistor out. I'm pretty sure this is what happened to mine before I had it converted. Another reason to choose the RVC type. Its common enough, the germanium grows crystal like tentacles which push though the substrate inside the transistor to short it out...then no worky worky. That's why they were ditched in favor if more modern materials. Usually you can find a replacement transistor directly...but the problem with the old tachos is the same germanium transistors work at a different voltage level. Its only a small amount we are talking about, but the problem is enough to throw out the entire internal circuit if you were to try and switch for a modern type. They also burn out more readily.

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Author:	GT mowog [ Sun Oct 11, 2009 5:37 pm ]
Post subject:
Mini4T wrote: Thanks for the replies....I am still a little confused (not hard). I think I understand the difference between the two types of Tacho...one senses current peaks the other voltage peaks. The problem I am having is regarding the 29D Ignition system on my car.... I just want to be sure that if I go and spend a heap of cash on a tacho it will match my ignition and work as accurately as possible and I won't be up for conversion costs. Tim Tim, I feel for you. I only post here in the hope of parting some knowledge and experince to assist you in your plight....The feed-back has been interesting but has gotten away from your original question. For your intended purposes, either Tacho will work satisfactorily.