Wayward Rev Counter

[sinkyb17]

To all you electronic experts out there I have a problem which I cannot fathom out. I have an early S2 and having just returned from 300mls in the highlands in Scotland within the last few days , I find that the rev counter is reading wildly high. As I have only had the car a year I contacted the previous owner and he informed me that he has fitted 3 rev counters to the car and they have all gone the same way eventually. On my old S3 there was a small hole in the rear of the unit which is used for calibration against a calibrated unit. On my car this hole is not present and without removing it from the dash I cannot see another hole. I obviously have two questions, are the units prone to going out of spec or is something happening electronically to give me a false reading. My understanding of the unit was that the white wire which is looped through the plastic clip on the back only senses what is being fed to it hence my problem. Help !!!!!


Dave.

[memini55]

Hi Dave,
I am in the process of rebuilding a couple tach for my S2's and have found some good articles on the web. I will try to locate them and post links for you. As to the white wire, it is feed from the ignition switch and then the loop on tach and out to coil and distributor. I have changed the size of the loop as an experiment but no change. I have three tachs, one reads twice value high, another is dead and the final reads about right but will not rev up about 4k rpm's.
The electronics inside the tach reasonably straight forward, but the parts used will go bad over age. The transistors for switching or triggering are germanium and only a couple replacements available and the capacitors can dry out with time and then short out causing complete failure. I have found replacement parts for everything needed to replace every part but the meter movement which in most cases should be fine. I am building a test jig with an old distrbutor and coil so we can calibrate each unit when finished with the rebuild.
The screw slot you talk about is a calibration pot which is on the circut board inside the housing and will be used to setup calibration.
With in the next couple weeks I should have everything worked out as to repair and calibration. The old design has a shunt resistor across the meter which we may try to change with another calibration pot to try to get things a little more accurate. The original pot will get it close but we think we might be able to improve it a bit.
If you wish to stay in touch with our work ahead I have posted my email
[email protected]

Mark

I found the post:

http://www.classictiger.com/techtips/motach.html

http://www.lotus-cortina/electric/convert.htm

[RichardS]

Dave

Have you got electronic ignition? This can make the rev counter wander all over the place and is fixable!

Richard

[rocket]

Richard,i have electronic ignition and my tach wanders,tho playing with an earth at mo is helping..when you say can be fixed would you care to expand?

[elansprint]

If you have electronic ignition this will be the problem the current in the circuit is a lot less than with points & this causes erratic operation as the tacho's are current impulse devices. To work reliably it will need conversion to voltage impulse
Ian

[elansprint]

If you have electronic ignition this will be the problem the current in the circuit is a lot less than with points & this causes erratic operation as the tacho's are current impulse devices. To work reliably it will need conversion to voltage impulse
Ian

[RichardS]

Richard,i have electronic ignition and my tach wanders,tho playing with an earth at mo is helping..when you say can be fixed would you care to expand?

There is a good article "bouncing tachs" on this forum's home page under the Technical Tips section. I sent mine off to Speedy Cables [mentioned in the article] and they have made the necessary modification. My Elan is still off the road so I have yet to see a nice steady rev counter reading but I do remember how it used to bounce all over the place after I fitted Lumenition!

Richard

[rgh0]

The normal problem with Tachs and electronic ignition is people taking the power supply for the electronic ignition module from the 12 v power supply side of the coil. This injects extra current pulses into the coil power supply which are read by the tach resulting in a false high reading.

The current pulses the electronic ignition causes in the coil that are sensed by the standard tach are generally the same as for a points system as it is the current pulse that creates the spark and you cant change it to much or you dont get a spark.

People convert their tachs to voltage sensing from the earth side of the coil which works because it hides the problem of the incorrectly wired electronic ignition module.

Of course changing to a voltage sensing tach may be easier than trying to fix the older style current snesing tachs if the tach is faulty in the first place. But I have always been able to get the orginal tachs to work with electronic ignition systems of various kinds by making sure the power supply to eletronics module was not connected to the coil power supply.

cheers
Rohan