Tach and speedo inaccuracy. How to calibrate.

[Johnfm]

Since getting the plus 2 running, I have had a chance to give it a run out to check tacho and speedo.

It is a 4 speed on standard wheels with standard tyres.

Satnav showed 54mph at an indicated 4000 rpm. Speedo showed 70mph.

At 5000 rom indicated, satnav shows 66mph. Speedo at around 90.

So, both tach and speedo reading way over I think.

Is there any DIY adjustability at all or is the only option to 'send them off'?

Also, shouldn't the speedo have a little dial to reset the trip meter? Mine doesn't.

[William2]

There is a possibility that your car has the wrong nylon gear fitted to the gearbox that drives the speedo cable. They are matched to the diff fitted to a particular car.

[stugilmour]

Posted a bunch of links here that might help

lotus-electrical-f38/speedometer-rebuild-t29691.html

I had the same issue. Took the tach apart and exposed the adjustment screw. Got it to work for a while, but eventually went for full repair/conversion by a specialist. The components in the tach appeared to be well past their best before date and the tach was unstable with temperature change. Overall advice is either fix it or send it off, not worth trying to adjust, but it is possible.

If you want to take on the tach conversion yourself, this thread looked very promising.

elan-mods-f31/rvi-tachometer-conversion-t26919.html

Trip Odometer is set with a remote knob. Cable attach's to the back of speedo similar to the speedo cable. Knob locates on dash bracket by the door light switch. Is this missing from your car?

Check odometer is reading correctly first using a test section on the highway, mile posts, etc. You can see turns per mile (TPM) of speedo head in small print on speedo face. TPM can be changed by a specialist if required. I needed to do this after changing transmission and diff ratio.

If your odometer is basically correct now, the speedo head needs repair to read speed right; I just sent it off as it is very delicate and needs a test bench to fix correctly. There are pages on the net that discuss home repair and calibration with a pistol drill, but i didn't bother.

William raises an excellent point regarding transmission gear. Just check the odometer reading to see if you are close first. If the odometer is right the transmission gear is right. I think speedo will typically read high speed as the internals weaken, but it has been a while since I read up on the topic.

Use David's excellent spreadsheet to calculate things for your exact transmission, diff ratio, tire size, etc. I adjusted the tach to GPS speed readings at calculated revs for my setup. This worked very well, but instrument would not stay adjusted.

Can provide more detail if required, but ended up sending both instruments to a specialist anyway.

HTH

[ricarbo]

The tacho has a potentiometer, so can be fiddled with to get the right reading. The speedo, being mechanical, has no easy adjustment. If you suspect you have the wrong speedo, check the number shown on the dial, as a first thing. If it's the right speedo, it's possible you have the wrong plastic gear for the speedo cable in the gearbox and these can be changed. It depends on which diff gears you have.
Accurately calibrating the tacho is easy, providing you have the right set up equipment, which is not difficult to make, but has to be calibrated itself and for this you need a frequency meter or an oscilloscope. Alternatively you can with care use the AC house mains through a transfomer, but can then only calibrate the tacho at one low speed, which is not much use if you are interested in not exceeding the engine's max rev limit. I understand you can use your computer to generate square waves at any frequency using freely downloaded programs. you would then use the audio output of the computer to drive the tacho through a simple amplifier. I haven't tried this, as I have the other apparatus. There is quite a lot to getting a tacho right, and in my experience you can only achieve full accuracy at two speeds, but if you choose those two to be the correct tickover speed and the maximum speed, that's ok.
regards
Richard

[stugilmour]

John, to clarify my reply and Richard's excellent adjustment technique write-up.

The adjustment pot on my tach required disassembly to access. The back case has to be removed to see the pot. I drilled the back case to provide adjustment access with the tach re-assembled. I think this is covered in one of the links. The tach can then be adjusted in place (hanging out of the dash hole), but it is very awkward and the adjustment range is extremely sensitive. I adjusted to known correct GPS speed readings as I don't have the test equipment and knowledge Richard mentions.

I see from some other threads you have been struggling with the tach reading a bit with electronic ignition. I got my tach adjusted and working OK with my Pertronix setup, but it was not stable. From other posts I understand this is typical as the electronic bits degrade over forty years of use. If you are thinking you will end up sending it off for repair anyway, I would advise conversion and/or professional repair at the outset. Note this is consistent with Rohan's advice; a properly working tach will work with the electronic setup, but it is unlikely that a forty year old unit is going to work without electronic service. For me, I was more worried about buggering the the thing up.

HTH & best of luck getting it right

[AHM]

Check that you have the correct speedo/drive gear by comparing the distance travelled on the satnav with the odometer which is a direct drive.

Indicated speed is achieved by magnetism and is adjusted by changing the proximity of the magnetic bits.

[Johnfm]

Cheers all.

Re : electronic ignition, revs are rock steady now. Which is great.

I will have a read of the links and take a view.

[fatboyoz]

John,
Not a direct comparison, as I have an S4. Standard 4 speed, 3.77 diff' and 165/70R13 tyres.
At 60mph speedo' indication (checked within 2 mph on GPS) the engine is turned at 3400rpm. This works out at exactly 70mph at 4000rpm. Perhaps you need a second (different) GPS opinion?
Here's a calculator to play with:
http://www.roversd1.nl/sd1web/wheelcalc.html
Regards,
Colin.

Since getting the plus 2 running, I have had a chance to give it a run out to check tacho and speedo.

It is a 4 speed on standard wheels with standard tyres.

Satnav showed 54mph at an indicated 4000 rpm. Speedo showed 70mph.

At 5000 rom indicated, satnav shows 66mph. Speedo at around 90.

So, both tach and speedo reading way over I think.

Is there any DIY adjustability at all or is the only option to 'send them off'?

Also, shouldn't the speedo have a little dial to reset the trip meter? Mine doesn't.

[Johnfm]

I think the GPS is the only accurate data point - it is within 2mph in my 'modern' daily driver on the same stretch of road, so I am happy that the speedo and tacho are reading way over.

What I find perplexing is that the speedo is mechanical - so it isn't like the two dials are both connected - one is gettig a signal from teh ignition and the other from a gear driven in the gearbox!

Damn frustrating!

[Johnfm]

Check that you have the correct speedo/drive gear by comparing the distance travelled on the satnav with the odometer which is a direct drive.

Indicated speed is achieved by magnetism and is adjusted by changing the proximity of the magnetic bits.

OOoh - good idea. Will do that tonight re: odo distance travelled.

[Quart Meg Miles]

As both tach and speedo seem to be in agreement are you sure you haven't got some crazy CWP ratio at the back, like a 4.8:1. I haven't checked your cars history so it's a long shot but fairly easy to check.

[MickG]

Hi John,
I tend to agree with Meg on this one.
The speedo and the rev counter are roughly in line with the speeds expected at the given revs. Using the data in the manual you can calculate what these speed should be. From this it would appear that the gearbox/speedo drive wheel is correct.
You must assume that the sat nav is also correct and that an increase of 1000 rpm gives you an increase in speed of 12 MPH.
The standard rear axel ratios given in the manual range from 16.5 up to 18.5 MPH depending on ratio and tyres etc.
So as Meg says you may have some other ratio installed.
Easy to check, jack up car mark prop shaft flange (easiest bit to see) rotate the rear wheel one rotation and count the number of rotations of the prop. This will give you an approximate ratio.
What you intend to do about it is another matter. Changing the gearbox drive wheel would correct the MPH reading but not the rev counter which would read the same but for lower indicated speeds ie 4000rpm would equal 54 MPH
Hope that make sense.

Regards MickG

[RogerFrench]

For very little money I bought a Tach / Dwell meter, something like this:
http://www.ebay.co.uk/itm/VINTAGE-KAL-EQUIP-MODEL-4015-TACHOMETER-DWELL-METER-DIAGNOSTIC-TOOLS-TESTER-/291013988088?pt=UK_Car_Parts_Vehicles_Automobila_ET&hash=item43c1c90ef8

Extremely useful for setting points, and also gives a check on your own instrument.

[Johnfm]

As both tach and speedo seem to be in agreement are you sure you haven't got some crazy CWP ratio at the back, like a 4.8:1. I haven't checked your cars history so it's a long shot but fairly easy to check.

at 54mph, speedo was reading 70. We know that is wrong.

The only way tach could be right is if gearing/diff is at 13.5mph/1000 revs. Unlikely, as the acceleration in 1st would be awesome and off the scale!!

I think a whacky diff ratio would mean the tach is correct and it is just the speedo that is wrong. If that was the case, the engine is happily revving well beyond the indicated redline, as if I run it in first until the engine is 'excited', it is well into the range of impending crank doom!

I expect that it is just 46+ year old instruments out of whack. I will mark the prop and count thr revs as mentioned.

[William2]

As a point of interest I have sent my S4 tacho to Speedy Cables at Swansea for repair, calibration and refurbishment. Approx ?75 which I thought was very reasonable.