29 replies to this topic

[andyt]

Can anyone tell me what the difference is between RVI and RCV smiths rev counters?

 

Andy.

[jpw1275]

Rvi is for points ignition and rvc is for electronic ignition

Cheers james

[jt19]

can rvc work ok with points?

[KernowCooper]

Yes but I have loads of smiths RVCs here in the shop which all suffer from needle bounce (damping not that good when new) , most are 40yrs old and the electrical components are past there best, you can get them redone by Speedy cables but its about £100 with p/p

[dklawson]

Warning:  Technobabble to follow.

RVI tachs are often called impulse tachs.  They are wired in series with current flowing through the ignition coil.  They do not actually "touch" the ignition system.  A loop of wire (either inside the case or literally looped on the back) inductively couples current pulses flowing through the ignition system wiring to the "counting circuit" inside the tach.  (Failure of an RVI tach means the gauge may not work but your engine will still run).

RVC tachs are a later, more modern design.  They use a single sense wire connection to the "low side" of the ignition coil.  They count the voltage swings between 0V and 12V each time points open and close.  (Failure of an RVC tach MAY result in an engine that won't run if the gauge experiences an internal short).

 

RVI tachs expect the ignition coil current pulse to be on and off for prescribed percentages of time.  Because electronic ignitions don't turn on and off the same way points do, the RVI tachs are typically unhappy and refuse to work when wired with electronic ignitions.  RVC types (because they effectively count voltage pulses) work happily with points and most electronic ignitions.

 

As Kernow said, most of these instruments are quite old now (both RVI and RVC). There are components for the RVI that are simply no longer available.  While modern parts can be made to work as replacements, full re-calibration is required to have a reading anywhere near accurate.  Speedy Cables and other places are your friend for making those old gauge work again.

 

Babble Over:

 

For the DIY person there IS an answer that is available now from Spiyda Design (link below)

https://www.spiyda.c...sion-board.html

The link is to their conversion board that replaces the old RVI electronics with a modern voltage sensing board (an RVI to RVC type converter).  However, that same conversion board could also be used to replace a failed RVC board in a later tach.  The conversion board has been getting good reviews on several forums and is priced right for a DIY person.  There are also online instructions and YouTube videos for how to calibrate the converted tach at home.

[Dan]

Rvi is for points ignition and rvc is for electronic ignition
Cheers james

Well, no. RVI tachos are current / impulse sensing, RVC tachos are contact (meaning points but fine for electronic) sensing. RVI units have the ignition system power supply cable physically running through an inductive pickup at the rear of the instrument and sense the tiny impulses generated by this pickup as the coil charges and discharges from the cable. The needle is being driven round by an amplifier running off this tiny induced voltage. RVC tachos are connected directly into the low tension side of the ignition and use / count the impulses through the LT circuit.

Electronic ignition systems, depending on the exact type, don't pull enough current for the RVI inductive pickup to generate enough power to push the needle. Or more accurately the current the electronic system uses doesn't fluctuate enough as the sparks are fired. The electronic system is using some power all the time to run while points physically switch the whole ignition on and off at each spark so the pulses are less noticeable with electronic systems. So RVI units are not compatible with most electronic systems.

RVC types are only looking at the power to the coil itself so they are more finely tuned to what is actually going on. RVC tachos will work with just about any ignition system.

Edited by Dan, 08 June 2014 - 10:36 PM.

[Keith Grendahl]

I am in the rare group who's RVI works with electronic ignition. I always assumed it was accurate but since fitting a 3.1 diff I've doubted its accuracy. Today I routed my timing light into the dash and went for a drive. Tachometer reads high with the delta increasing at higher revs.

Tach Timing light
2000 1880
3000 2770
4000 3660
5000 4550

I can live with it but can this be calibrated?

Keith

[jpw1275]

Oops me bad.....:0(.... I always that was correct

[Vipernoir]

My Smiths RVI rev counters work "fine" with 123 ignitions, but the one in Betty does appear to be about 1000 rpm slow at 5000 rpm as it reads 4000.  I haven't checked Tubbs yet.

It's a sliding scale as well as the engine speeds up, and explains the anomalies between rev counter, speedo and GPS.

[spiyda]

I am in the rare group who's RVI works with electronic ignition. I always assumed it was accurate but since fitting a 3.1 diff I've doubted its accuracy. Today I routed my timing light into the dash and went for a drive. Tachometer reads high with the delta increasing at higher revs.

Tach Timing light
2000 1880
3000 2770
4000 3660
5000 4550

I can live with it but can this be calibrated?

Keith

 

 

Keith,

 

The fact that your RVI works on electronic ignition is more down to the ignition system than the tacho !

Generally, simpler electronic ignitions have limited decoupling and draw bigger pulses from the 12V supply

 

As for adjustment, there are two main RVI boards, one with one transistor and one with two transistors.

They both work by sensing the input pulse via a transformer and using this to fire off a fixed length pulse to the movement.

This is known as an astable multivibrator.

The movement integrates this output so that the more often the pulses, the further the needle moves..

Both can be calibrated a small amount by adjusting the on board variable resistor.

The Variable resistor alters the width of the pulse that the circuit sends to the movement.

Some (but not all) tachos have a small hole in the case to adjust the variable resistor.

If no hole, you will have to open it up.

If the capacitors on board have gone tired, you may run out of adjustment.

 

Chris  (-=Spiyda=-)

www.spiyda.com

[TiMtImInYtImTiM]

 

Electronic ignition systems, depending on the exact type, don't pull enough current for the RVI inductive pickup to generate enough power to push the needle. Or more accurately the current the electronic system uses doesn't fluctuate enough as the sparks are fired. The electronic system is using some power all the time to run while points physically switch the whole ignition on and off at each spark so the pulses are less noticeable with electronic systems. So RVI units are not compatible with most electronic systems.

 

 

I started this thread to try and understand how to fit my Smiths tacho

http://www.theminifo...ths-tachometer/

 

I got mine working by running the -12V wire to the earth on the coil and the + wire to the positive side of the coil. It works but doesn't seems to go over 2500 RPM. Could this be to do with the lack of power issue you describe? Is there a way around this or do I need a new gauge?

Edited by TiMtImInYtImTiM, 16 September 2016 - 09:14 PM.

[tiger99]

There is no earth on the coil. The -12V terminal should go to earth on the dashboard/bulkhead area.

[1963MKI]

My rvi tech came out of an Mgb and sat in a box on my shelf for 40 years. I put it in my mini and it worked for about 10 minutes. Now, nothing. I’m stumped, it has 12v supply. I find it hard to believe it would have laid idle all that time and have broken within 5 minutes

[Spider]

My rvi tech came out of an Mgb and sat in a box on my shelf for 40 years. I put it in my mini and it worked for about 10 minutes. Now, nothing. I’m stumped, it has 12v supply. I find it hard to believe it would have laid idle all that time and have broken within 5 minutes

 

Ham-fisted,,,,,,

 

More seriously,,,,,,, it might well just be a dry solder joint or two.

[sonscar]

Just think:40 years ago it may only have had ten minutes to live.Seriously any capacitors in it could have failed as older ones are prone to doing,free to take a look,Enjoy faulting it,Steve..