S2 Tachometer reads 15% high after conversion

I installed a 123 Distributor in my 1969 S2 and recently had the tach converted so it will read with both points and electronic ignition. It works well on both systems but reads a constant 15% high on both. I contacted the person who did the conversion but he hasn’t returned any of my emails and doesn’t appear to be in business anymore.

Is it simple to adjust the electronic tach or should I send it to another vendor?

Thanks Andy S2 FHC

Andy, high compared to what?

The tach is reading high compared to the true rpm of the engine; which I have verified with 2 shop quality timing lights with built in tachs and the rpm shown on the gauge as compared to the actual rpm value vs speed shown in the Owners Manual with stock tires and wheels.

It’s a constant 15% above the true rpm of the engine at all engine speeds.


Hi Andy…the tach is just reading the rpm of the engine…nothing to do with speed,tires,wheels…if its reading incorrect against a known calibrated tach then you need yours recalibrated by a shop…Steve

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For what it’s worth, the original tach does have a calibration trim pot accessible through a hole in the back. Not sure if your modified one still has this function.

No, rpm of the engine is directly related to the speed of the vehicle being a manual gearbox with a known final drive ratio and a known ratio of the rear end. The numbers will be the same and correct to the drivers manual as long as the tires are the same diameter.

Hi Larry…how does the tach give a reading then when the car is stationary

When you’re disengaged from the driveline, either clutch in or in neutral, then that doesn’t apply. But in gear and driving, you have engine speed driving gears in both the tranny and rear end and those relationships are fixed and directly correlated.

Best regards,
Larry Trom

I’m assuming your timing light is triggered by an inductive clamp over one of the spark plug leads. That should certainly lead to a valid rpm value, assuming everything with the light is in working order. The rpm based on the speed of the car, to me, introduces too many variables to be conclusive. Regardless, you say your tach reading is the same with points or the 123 distributor. Especially the reading with points should be the same as the timing light value, except you get a make/break on the points for every spark plug vs on the timing light you are getting a signal for only 1 spark plug i.e. they differ in rate by a factor of 6. Of course, the timing light should take this into consideration. Typically you have the “tell” the timing light if the engine is 4, 6, or 8 cylinder. None of which explains a 15% difference. Now if you had said “as compared to the tach generator” I’d say there’s your problem but not the case. As another check, you might want to borrow an automotive multimeter from someone but it uses the same signal (the coil) as your tach so again is not conclusive. The same problem could be affecting both the tach and the meter. I’d say the timing light is your best reference. I’d have to say the tach needs to go back for a calibration check/adjustment.

I think some are confusing the workings of a tachometer and speedometer. A tachometer is engine speed only, and COMPLETELY unrelated to transmission, rear end, tires, etc. etc…

(It could even be sitting in a test rig by itself and still function correctly )

The drive train elements of transmission, rear end, tires, only come into consideration with a speedometer, which measures the speed of the car, and not engine speed


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Interesting that Jaguar tend to disagree with this as they give road speed at certain engine revs in their workshop manuals, are they confused?
I don’t think so.
They take into consideration the tyres they recommend for the car and work out what the owner should expect to see with regards to MPH/RPM relation.

No Jaguar do not disagree…the charts printed in the operation manual are as Jaguar state at the bottom of the page theoretical figures showing the relationship between roadspeed, gear, diff ratio, mph & rpm…what this thread started as was a tach not reading correct…the tach just reads pulses produced by the engine…the last thing you need to do to solve this problem is drive the car…you just need to start the engine and measure the rpm with a known good tach…this is basic stuff…tyres,diff,gear ratio have nothing to do with what the taco reads…try it…get in your car start the engine…rev a bit and watch the tach needle move…are some of you guys really saying that a tach reading incorrect could be a problem with diff, tyres, gear

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I believe we’re placing the cart before the horse.
If the engine is locked at a constant rpm the gearbox ,differential and wheels will always rotate at a fixed rpm based on the ratio of the selected gear and differential.
The SPEEDOMETER is calibrated to display the vehicles speed based on those ratios and a given tire OD.
What Andy is saying ( I believe) is that the calibration of the TACHOMETER is incorrect and is constantly displaying 15% more RPM’s than what the engine is actually turning.


Yes Marco…exactly…its just a tach fault thats reading the rpm of the engine…why anyone starts mentioning diff, tyres, gears etc is on the wrong track

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The point is that, when driving using a manual gearbox, the speed and engine rpm have a fixed relationship that can be determined using basic arithmetic OR through measurement over a fixed distance. So, x mph at y rpm is something one can determine. As has been pointed out, examples are given in the manuals.

So, displayed speed vs displayed revs CAN be a reasonable cross-check.

That said, the 123ignition app (assuming the wireless distributor is installed) provides the RPM and a GPS - based speed. Clearly comparing the app to the tach would be a better approach.

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Robin, no I don’t think the factory is confused, but I do think you are. Again, the OP is NOT talking about road speed, but you are.

He is only talking about engine speed.

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Hi Mike…you dont have to explain the relationship between the diff,gears and speedo…I know that the speedo is calibrated to the given diff/tyres…but to fix a tach fault none of this has anything to do with it

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In the OP’s second post, he does reference road speed as a method he was using to check the accuracy

Sorry Andy,
But I think you need to send the Tachometer off to get it calibrated. I’ve used MoMa and Nissonger in the past and have had good results from both but tend to use MoMa as they tend to be a little cheaper and quicker.

Yes and no. My tach in a “different” car is right on, the speedo, also electric, not so much.
So, comparison only works to a degree!!

Here I think the reference to the relation of road speed to engine speed is for comparative purposes only.

Imprecise. Speedo error in one comparison.
And the math version, working backwards by gear ratio and tire diameter is also imprecise.
Tires of the same size as on the sidewall vary in diameter. Nominaly somewhere in the 28" to 30" area. Bit "rolling circumference is the “true” factor.

Agree, the strobe number ids the most reliable.

There are devise that are dedicated to determining the rpm of a revolving device for diagnostic reasons. I’m a bit fuzzy there.

Tachs are adjusted by inserting a resistor of a specific value. Here, the conversion folks got it not quite right. Find the right value and fix???


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