Smith's Tachometer Information and Notes    
9/2008 Revisions ongoing, R. Kwas

Note:  I am looking for:  1.  Specific circuit for the Gen 1 voltage Sensing Module installed in front of Radiator.  2.  Specific info on interaction with Electronic Ignition  modules.  If you can help with info beyond what you see here, please e-mail...thanks!  R


Explanation of Why Sensing Polarity Matters
Notes on Conversation with Mark Olson.
Comparison of Smith's Tachometer Versions as fitted to Sport Volvos. 


Smith's Second Generation Model No. Model No. RVI 3410/00

First things first!  Over the production run of the 1800 model, three different styles of tachometers were fitted...and after 50 years, a vehicle may not have the original it came with...infact, Volvo's own shop literature recommends replacement of the Gen1 units with Gen 2 units if the Gen 1 units ever came in with a failure (Reference:   ) here is info for determining which style a vehicle has:  Tachometer Commercial Sites

Here is my compilation of info on the Smiths supplied Tachometer, as fitted to all Volvo P1800 vehicles.  This info is presented for reference, some is included for completeness and unconfirmed by me...I will always try to attribute information from other sources.  It has been a long time since I've been inside an 1800 Tach, and the last time I had one open, was long before electronic cameras were commonplace.  The next time I have one apart, I intend to confirm details, and that the circuit within is as shown here.  Until then, here is a circuit diagram I have found in my research:   

Circuit Diagram of Generation One and Two Smiths Tachometer as used on the Volvo 1800 ('61 -'69, see below). 
(Attribution shown, and I have recently spoken with Mr. Olson, see below.)


Notes on Function: 

The Tach is a current sensing type, signified by the I in Smith's Part Number RVI 3410/00.  Input (primary of T1) consists of an inductive link (also:  "Impulse Loop") only.  There is no galvanic connection between input (Ignition of vehicle) and internal circuit of Tach.  This gives two distinct advantages:  1.  It assures that no failure of Tach can disable vehicle IGN, and that 2.  ...the same Tach (with some simple reconfiguration) can be used with "positive earth" vehicles (typical configuration of many British vehicles of the era, and this should not to be confused with the polarity of the current in white "sensing wire".  See Sensing Polarity Matters below!). 

T1 Primary!  It may not look like a transformer, but it is...and that bracket (probably of a ferrous metal) holding the two turns of primary wire is actually part of the magnetic core!  Missing here is the little plastic spacer intended to prevent vibrations from chaffing through insulation and making electrical contact with internal conductors.    

A disadvantage of this inductive-only coupling is that the Tach is compatible with only single pulse electronic ignition systems like Pertronics, or '75 240 Volvo Ignition Amplifier electronic ignition conversions.  If an 1800 is upgraded to multiple spark (Multiple Discharge or also "Shower of sparks") electronic ignition systems, the Tach is unlikely to function at might deflect as the T1 does couple in some edges, but indication will be anything but accurate. 

Tachometer wiring excerpt of 123GT diagram, 1800 tach wiring is substantially the same. 
Ignition Coil Primary Current path is shown in Blue,
and only one polarity of White sensing wire will allow correct Tach function! 


Theory of Operation (source:  M. Olson):  

The two transistors together form a monostable multivibrator, or one-shot. Normally, the collector of Q1 is at 6V. An ignition pulse couples through the transformer to trigger a one-shot voltage pulse to 12V on the collector of Q1 for a set amount of time. Every time an ignition pulse is detected through the transformer, the collector of Q1 will pulse from 6V to 12V for a fixed amount of time. While the collector of Q1 is at 12V, the top of the meter is held at 6V by the Zener diode, so current will flow through the meter, causing the needle to deflect. The width of the voltage pulse is determined by the combination of the 0.25uF capacitor C2 and the combination of resistors R3, R4 and pot. R5. The one-shot is triggered by every ignition pulse, so the voltage waveform looks like a series of pulses when the engine is running. Since the pulses are fixed in width and the frequency of the pulses is determined by the engine speed, the ratio of the time the waveform is at 12V vs. 6V goes up with increases in engine speed and down with decreases in engine speed. The way the meter works, the more time the waveform is at 12V, the more the needle is deflected and the less time the waveform is at 12V, the less the needle is deflected. 

Ignition Wire Waveform at Low and High RPM


Age Related Problems:  

C2 looses capacitance as it ages.  Slow failure of this cap. causes symptoms of Tach being erratic, temperature sensitive or just plain dead. C2 problems are the most common.  Replacement is the only remedy.   

The slider on the calibration potentiometer (R4) goes intermittent and/or open after many years.  Associated symptoms are:  ????  To permanently fix this, measure terminals of variable resistor, and replace with soldered-in fixed resistor(s).  Recheck calibration after repair.  [This used to be my recommended repair, but better yet, rather than make this major modification, simply apply a drop of Deoxit D5 to potentiometer to protect the resistive element, allowing one to keep Tach original.  Reference M. Olson Conversation below]  ! 

Electrolytic capacitors such as C3 are also known to open as they age and electrolyte dries up.  I recommend replacing C3 as a matter of course... and apparently Mr. Olson has some experience with other components failing.  [I have spent some quality time on the phone with Mr. Olson who has lots of experience with these Tachs as he operates a service repairing and calibrating Tachs of various manufacturers.  See my Notes on Conversation with Mark Olson. ] 

UPDATE:  Note that this failure mode of C3 opens the interesting option of a non-invasive repair...since this filtering component is on the power node (which is accessible on the outside of Tach)... a perfectly effective repair can be implemented without opening up Tach(!) ...because there is nothing sacred about the physical location of the component...only the electrical location is important! the filter cap can simply be added externally as shown below!  That would also explain the success of "Rainflye's" addition of a remote capacitor to calm down the easily excitable Tach on his '73 1800ES.  See: 


Back of a Generation Two Tach (Smiths model RVI 3410/00),
showing location of hole necessary to access calibration potentiometer,
and a possible location for an external filter capacitor.



Generous Tachometer and other Issues, Explanations and Repairs 1800

Guys;  [Yellow highlights are not part of my original posting, but added here for clarification and emphasis.  Ron]



1800 (Smiths) Tach problems are not uncommon after 50 years...they are caused by one or a combination of the following:

1. Open connection of internal calibration pot slider R4. (Most common, symptom is "generous Tach") or
2. Partly or totally dried out electrolytic capacitors C2, C3. (Undoubtedly the case after 50 years!! Symptoms are varied...from whacky, jumpy readings to totally dead)
3. Failed (Germanium) Transistors Q1, Q2. (Occasionally the case, but symptom then is a totally dead Tach...)

Given this experience, I suggest the first thing you try should be, to (CAREFULLY!! Like on a drill-press where bit won't bite and be drawn into the internals, [instantly] destroying them!! [then you'd be done before you even started!]) drill an access hole to the Cal pot positioned as shown, DROP (not spray) in a few drops of Deoxit D5 on the calpot and (after having taken note of the slider's starting position and marked its angle on the case) work the slider back and forth through its range with a diddle screwdriver to clean and remake its slider connection. After a dozen or so sweeps of the control...which SHOULD be enough to clean and remake the slider connection), return slider setting to the marked position (former, factory cal position), reconnect Tach* in vehicle and try it...if you are lucky, and the electrolytic caps are still working well enough...they are certainly old, but may still be exhibiting enough capacitance to allow the circuit to work normally...and if you're lucky and the movement of the slider has cut through 50 years of surface contamination/corrosion, or you just don't want to insult the swadged (SP?) chrome case to gain access to the internals**, it may work fine for you...if not, internal repairs are required...

* The input (sensing) pick-up loop is confusing to some but shouldn't's quite direct galvanic connection is made to the points node in order to preclude the possibility that a failed Tach could cause a total ignition failure (that would be very BAD, poor design practice, and given the known track record of British automotive electrical equipment of the era, likely defined as unacceptable by Volvo when they specified the instrument)...that is why they specified a "non-galvanically connected sensing link"... because something that is not (directly) connected, can't hurt you! The way Smiths implemented this is by sensing the RPMs with a couple of turns of the points wire through the magnetic core of T1 on the back of the Tach...(the solution is brilliant really!) these turns would be the primary of T1...which would couple the Ignition pulses into the Tach circuit by way of magnetic coupling ONLY... and since the direction of this primary winding determines the polarity of the pulses presented to Q1, the direction of the turns of the sensing coils MOST DEFINITELY MATTERS (I've seen discussion on this)! [See:  Explanation of Why Sensing Polarity Matters below.] 

**If you are qualified and/or confident working on the internal electronics of the Tach, I recommend GENTLY prying open the chrome bezel swadging to allow the Tach to be disassembled, to gain access (there's no other, more gentle way of getting access to the internals!), replacing both electrolytics, freeing up the calpot, then recalibrating the assembly while on the car and verifying it on a calibrated Ignition Analyzer (because unless you have bench test setup which is the equivalent of the non-galvanic sensing system, that's the only way to get it to work!), checking it for proper function.

Finally, after it's working and calibrated, best practice would be to measure and replace the pot after calibration with fixed resistors which will NEVER open again...that's what I'd do and have (successfully) done...

I have given the above detailed info to enable others to perform the work...I do not offer it under Sw-Em services...I am focused on all the other kits and projects...I just can't allocate time for is HIGHLY time-consuming and therefore probably couldn't be offered at an affordable rate...

Good Hunting


Explanation of Why Sensing Polarity Matters:  Input transformer T1 couples pulses from the Ignition coil Primary circuit into the Tach circuit.  In studying the voltage and current waveforms of the Primary circuit in following (generic) oscilloscope waveform below, it is clear that these are quite non-symmetrical...meaning the waveform coupled into the transformer secondary will be also non-symmetrical...and since the Base-Emitter circuit of Q1 is unidirectional and polarity sensitive, only the higher pulse excursion (occurring at time o, Points Opening) provides sufficient input pulse to the Q1 base to trigger one-shot (Ref. Theory of Operation above)...if sensing wire is reversed, excursion of waveform coupled in the required polarity is insufficient to trigger Q1...resulting in pulse not making it through Q1 and no tach function.  Note:  I am confident in this explanation, but it is theoretical only...I haven't verified this in the lab!        

Source:  Review of Ignition System and Troubleshooting Notes


Additional Links: 

Link to Brickboard Thread of subject of tach:

...and a Link from that Thread, to an informative site mentioned:

Links to UK Volvo Forum Thread:   John Twist of University Motors converts a positive earth MG Tach to negative (this video has much good info applicable to the Volvo Tach):  Of note is that the MG version of the Tach is packaged in a bayonet case which can be opened relatively simply (hardened rubber O'Ring not withstanding) to work on the internals.  In the Volvo version, the chrome bezel is permanently swaged-on, needing to be bent back carefully to allow disassembly. 


Tachometer Commercial Sites (I have no interest in these outfits, nor experience with them...this info is included as a reference only, anyone with first-hand experience working with them is invited to relate this to the author.):  This outfit, apparently in Canada, has excellent info on identifying the three models of Smiths Tach used in Volvo 1800s, as well as a good explanation of why they are not compatible with electronic Ignition conversions.  I see no changes or updates to their site in years, so I don't know if they are still active...I have sent an e-mail inquiry (Update:  I have spoken with Roger D operator of the site who says that he is keeping the site up, but is winding the site down.).  Model info from this site:

'61 - '64 (to Chassis No. 125001) First Generation Model No. Model No. RVI 1310/00, identifiable by "4 Cyl Negative Earth" on Tach face, and a separate, remote sensor module for ignition pulses, containing the meter driving circuit, located in front of the vehicle radiator, to keep the sensitive electronics out of engine compartment with its' wildly varying temperatures.
Note:  The remote module is NLA, so many of the first generation Tachometers have been replaced by second generation type, because the form of the gauge part in the dashboard is the same.   Both are still subject of age related problems described here.
'65 - '69 Second Generation Model No. RVI 3410/00, 
identifiable by "4 Cyl" on Tach face.  Ignition pulse sensing and meter driving circuitry is integrated in gauge.
'69 - '73 Third Generation Model No. RVI 3412/00, identifiable by black face.

See also Reference Information:  Comparison of Smith's Tachometer Versions as fitted to Sport Volvos.

Nisonger Instruments in New York.  This outfit specializes in rebuilding British Instruments such as the Smiths supplied Volvo Tach.  A gentleman there informed me on the phone that the current (Mar 2015) cost of an Volvo Tach OE simple rebuild is $225 (note that the result is not compatible with electronic ignition conversions), and a rebuild with conversion to make it additionally compatible with electronic ignitions is $325

Mr Olson's Company:  Accutach Co.  He has been working on Tachs for a long time and in my discussions with him, it was clear that he certainly knows his stuff!


Notes on Conversation with Mark Olson (Mar 2015), on the 1800 Tachometer .   Mr. Olson was very forthcoming with information, and although he runs a Tach servicing/rebuilding operation, he specifically mentioned that since the Tachs as fitted to 1800 are permanently crimped shut, he does not like working on them because of the mechanical insult they must first be subjected to before gaining access to the internals... 

Specific notes: 

C2 always needs replacement, but with 0.25uF being a somewhat unusual value, 0.22 or 0.27uf Mylar type are OK to use.  Indeed, some variations in the installed component values of C2 and R3 even occur from the factory, but when replaced with these values, the Tach circuit still operates as designed and within a calibrateable range. 

The Thermister (T) crumbles into nothingness, resembling rust, all on its own, resulting in loss of temperature compensation.  [I doubt therefore it was a quality component to begin with.  I have asked him to supply more detailed info as to resistance value and temperature coefficient so that I can possibly cross-reference other components.] 

Germanium transistors Q1, Q2 do degrade, but they cannot be replaced with less sensitive silicon replacements.  

He does not agree with me that the calibration pot is a major source of problems, and should be replaced.  He feels the number one source of problems is the C2.  [I will defer to his expertise and experience on this, but I have made him aware of the benefit of a protective drop or two of CAIG's Deoxit D5 into the calibration pot.]

Placeholder for info on Internal his Permanent Magnets findings.  Result is somewhat indecisive!...see below!


Excerpt of e-mail Thread with M. Olson:  


Neither of the magneto repair guys had a gauss meter.  They simply felt the magnetic force against a piece of steel before  and after recharging.  I have found a gauss meter for $50 and am about to try to order one so I can get actual data.

I can only speculate as to the cause of the generosity of the pre-repaired Tiger tach.  The tach is a one-shot whose time constant is determined by the main timing cap and the calibration pot and the series resistor.  The only thing that could increase the one-shot pulse would be if the capacitance increased as the main timing cap aged.  The calibration was not a straight line, but an arc with the old cap, so it appears to also be frequency related.

Since I do not know the failure mechanisms of those old caps, it is very hard to predict the symptoms.  However, most of the original caps I see are cracked and failing, I typically replace them as a matter of course and it solves a lot of strange problems.  Bottom line is increased capacitance at lower frequencies is my guess for the cause of the generosity of that particular tach.

 The "Balanced Magnetic Moving Coil" gauge you reference is also known as an "Air Core" gauge.  Air core gauges replaced D'Arsonval gauges such as the ones in the Smiths tachs.  Air core gauges have the benefit of not being dependent on permanent magnets.  As air core gauges age, they simply take longer to point to the same angle.  They were in use in automobiles of all kinds until they were replaced by stepper motors (in 2005 in the case of the Ford Mustang.)  Early instrument clusters use microcontrollers to control the air core gauges, where the process or controls the current in the reference coil and the moving coil to point the needle very accurately.  Stepper motors are accurate too, and they use a lot less power.

Probably a lot more than you wanted to know...



Mark Olson
Accutach Co.
5467 Glennan Ct.
San Jose, CA 95129


From: Ron Kwas
Sent: Thursday, April 16, 2015 8:16 AM
To: Mark Olson
Subject: Re: Tiger Tachometer Tech Article
Mark;  Thanks for response...see my comments/questions in Blue.  R

On 4/15/2015 12:56 PM, Mark Olson wrote:

That Canadian Tiger tach has been an interesting adventure.  The tach was completely out of calibration when it arrived.  I had suspected that the input cap was allowing the one-shot to trigger on both edges of the input pulse, rather than just one edge as it should.  But that was not the problem.  Dialing down the calibration pot got it into the normal range, but the tach response to RPM input was a very curved line.  The main timing cap was cracked ad looked bad, so I replaced it, and the tach instantly got linear up to about 4K RPM.  At 4500 RPM it read 4400, at 5000 RPM it read 4700 RPM and at 5500 RPM it read 5K.  It's like trying to work 5 simultaneous equations...

At this point I was suspicious that the permanent magnet was in need of a recharge.

I had located a retired guy who can recharge magneto magnets about 45 minutes South of me, but the guy was pretty cagy about getting together to see if we could recharge my spare magnets.  He kept saying he would call me back to schedule a time and place to meet, but it never happened.  So I did some more internet searching and located a guy almost all the way up in Oakland who rebuilt magnetos.  He was much more helpful, so I drove the hour up to his shop yesterday and he recharged my 4 spare magnets.

After I got home, I loosened the bottom tach board and swapped in one of my recharged magnets.  When I put it back together and tried it, the meter movement acted like it was binding on something.  I tried repositioning the new magnet different times, and finally got it to work again.  It raised the linearity falloff point a bit, so I decided to try a different recharged magnet. ...question is:  To what strength...I would expect there to be levels of magnitude, and getting them "recharged" does not guarantee their strength is as it was when Smiths installed there any way to quantify their strength...can you think of a Gauss measurement fixture? about a "one variable at a time method" perhaps using a "Standard Tach" and installing the magnets (precisely positioned) with a known range of gauge rpm inputs, then reading /graphing the indicated values.

I had the same problem getting it to work again after swapping magnets, and was worried that I may have destroyed the tach.  I finally got it working again, and it raised the linearity falloff point a little more.

As an experiment, I decided to replace the original magnet from the tach to see what would happen.  I was very surprised to see that the linearity was now even higher with the old magnet than it was with the recharged magnets.  Very curious.  But the guy wants me to ship his tach soon so he can hand carry it back to Canada on his business trip, so I had to quit screwing around with it.  It is now linear up to red line.  At 5K RPM it reads 4900 and at 5500 RPM it reads 5200.  I am recommending he accept that, although I could reduce the Thermister linearizing resistor to get it up to the top.  But that compromises thermal compensation. 

So the results are somewhat encouraging, but also a bit confusing.  I suspect I will have to dig out my old electro-magnetics books to get this truly figured out.  But my differential equations are VERY rusty.  :)  I would prefer to stay away from those are certainly the most experience on these instruments at this point.  What would you suggest was the root cause of the "generous tach" at this point...multiple issues?...I would like to include a one-liner explanation for it in my article...age components, aged magnet...? 

I also invite you to see my notes on the VDO supplied "Balanced Magnetic, Moving Coil Indicator"  as used in the Fuel Gauge.  (  Its' design balances the vector interaction of coils at right angles for a supply voltage independent indication...pretty brilliant IMO. 

Stay in touch!  Regards, Ron


Mark Olson
Accutach Co.
5467 Glennan Ct.
San Jose, CA 95129

From: Ron Kwas 
Sent: Wednesday, April 15, 2015 6:33 AM
To: Mark Olson
Subject: Fwd: Re: Tiger Tachometer Tech Article

I am still very interested in cause of the "generous Tiger tach", and would like to include this info in my article...can you shed any light on this yet?




Reference Information:

Comparison of Smith's Tachometer Versions as fitted to Sport Volvos.    

Note:  Although they look very similar, it is not possible to interchange Gen 1 and Gen 2 Tachometers in any way!  The Gen 1 dashboard instrument really only contains a meter movement and no support electronics, whereas the Gen 2 Instrument has sensing and meter driver self-contained.

Generation 1 Model No. Model No. RVI 1310/00

Fitted from 1961-1964
Notable identifying features:  Gauge face,  Remote (Voltage) Sensor

Generation 2 Model No. RVI 3410/00

Fitted from 1964 - 1969
Notable identiying features:  Gauge face, Internal (Current) Sensor

Generation 3 Blackface No.  RVI 3412/00

Fitted from 1969 - 1973 (Injected Cars)
Notable identifying features:  Gauge face, Internal (Current)Sensor

"Smiths, Made in England" on Blue/Green Gauge face. 
Redline caution area starts at 5500RPM.

"4 Cylinder, Negative Earth, Smiths" on Blue/Green Gauge face.
Redline caution area starts at 6000RPM.   

Black face. 

Meter positive at Yellow, from remote Tach Sensor/Driver Module, Negative at Green, Instrument Lighting wiring, plus an intermittent connection in waiting in Blue!

Integrated instrument, with an Ignition Primary Current Sensing provision directly on the back of the gauge. 

Placeholder for Picture...I expect it looks much like the Gen 2 version!

Remote Ignition Voltage Sensing Module located in front of the Radiator (where it's a lot cooler than in the engine compartment!).

Detail of Current Sensing loop (Primary of T1) located on the back of the Instrument.  It's also a lot cooler in the passenger cabin, than in the engine compartment!  The little metal bracket securing wire and plastic holder (missing here) is of ferrous metal (tin), and as such, part of a transformer (no kidding!), and is necessary to complete the magnetic circuit.  Input coupling will not work without the little bracket!  
Again, similar to Gen 2.
Ignition Voltage sensing is remote from Tach Instrument.  Ignition Primary Current Path is not routed by way of sensing circuit.  Excerpt from chassis No. under 10,000 Wiring Diagram.  Ignition sensing is local at Tach Instrument.  Ign Coil Primary Current must be routed to Dashboard Instrument Current Sensing Loop.  Excerpt from chassis No. over 10,000 Wiring Diagram.  Ignition sensing is local at Tach Instrument as with Gen 2.  Ignition Power is supplied from Fuse Panel, not armored cable as in  earlier versions.  


Reference Data for NTE158 Germanium Transistors:  Source:



External material sources are attributed.  Otherwise, this article is Copyright © 2008-2015.  The terms Smiths, CAIG, Accutach Co., and Volvo are used for reference only.  I have no affiliation with any of these companies other than to present my highly opinionated results of the use and care of their products here, for the purpose of helping other owners keep their vehicles on the road, safe and reliable.  The information presented comes from my own experience and opinion, and can be used or not, or ridiculed and laughed at, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles! 

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you donít, youíre just a lazy, scum sucking plagiarist, and The New York Times wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and maybe wise-a** comment. 


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