Lotus Elan

Unsteady tach 69 S4 dhc

PostPost by: billwill » Thu Jul 08, 2010 4:05 pm

Apparently it's written on the dial somewhere.
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PostPost by: saildrive2001 » Thu Jul 08, 2010 4:06 pm

Ignore my last post, I see that the answer was given earlier, sorry.
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PostPost by: simonknee » Thu Jul 08, 2010 8:36 pm

rgh0 wrote:While it may be a rule of thumb it is not universally true. My orginal current sensing tachs works well with electronic ignition on both my Elan and Plus 2. You may fix a problem by changing the tach type but the problems are typically caused by 1 of 2 reasons

So did mine but once it decided to bounce that was that. I replaced caps in the tach and powered the electronic ignition from the battery (on a relay controlled by the ignition switch) and still it would bounce around. Personally I think you can fool them for a while but once they get wind that you have electronic ignition they throw the toys out the pram :D.

Put a 'scope on the -ve of the coil and you'll see what tachometers have to put up with so it is no wonder they get crotchety.
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PostPost by: rgh0 » Fri Jul 09, 2010 11:46 am

Yes I dont understand why the current sensing tachs work with some electronic ignition installation and not with others. All I know is that everyone I have seen if you wire the supply for the electronic ignition from a stable positive source and not the coil supply and fix the capacitors the problems go away. I am sure there are other potential problems with other components on the tach circuit board or in the electronic ignition that could give the tach problems that I have not yet come across.

I have also seen the reverse where a friend replaced his current sensing tach with a voltage sensing unit because he wanted a 10000 rpm tach for his race engine ( he uses 9000rpm routinely in his twink :D :D ) and it would not read reliably. It took a while to figure out that he was getting RFI voltage spikes in the voltage sensing wire to the tach from his HV ignition wires which ran close by.

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PostPost by: stugilmour » Fri Jul 09, 2010 2:44 pm

In the "Service Notes" section of my Plus 2 Parts List book it says the following under Tachometer:

The fitting of a high output coil will sometimes affect the working of this instrument. The characteristic in this case is that the needle will stick at around 3000 RPM. If this is the case, disconnect the 'loop' and run the 'loop' wire straight through the 'bridge' without taking a turn around the 'bridge'. This will effect a cure.

My car's RVI tach worked fine on electronic ignition with no loop. During re-wiring I have taken Rohan's advice and run a separate power feed to the electronics. I am not installing the loop and will try it out. When working car had the electronic points deal that fits inside the stock dizzy. New setup will be a Pertronix replacement dizzy.

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