Lotus Elan

Oh My God...now its reading double!

PostPost by: jonnyconcrete » Tue Aug 03, 2010 9:13 pm

Only used coax because of its inherent impedence and screening properties...

Today's developments...

Normal wire, but fitted a 0-10k potentiometer in line...guess what...I can now trim the RPM sensitivity from 0 ohms and the needle dances all about reading approx double, up to about 2k when it reads between 5-600rpm all the time. My next step is to get a much more sensitive trimming pot 0-1k and I reckon I'll be able to trim it spot on.
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PostPost by: billwill » Wed Aug 04, 2010 12:42 am

Eeek, I dunno what you have done there.

In normal wiring, the white wire that is looped through the loop on the tacho is actually the power to the primary of the ignition coil For reasons best known to whoever designed the original loom the normal feed from the ignition switch goes almost all the way to the coil and is then split and the white wire goes back to the tacho and then forward again to the coil. [Using coax would have no significant screening effect, unless you had TWO wires inside a shield cover which was earthed].

Putting a 10K pot in that wire would prevent enough current going to the coil to enable a spark. The fact that your engine is running with a resistor in line means that it is theoretically impossible that your wiring is in the right places.

Have you tried what I suggested several messages ago? Reduce the sensitivity of the tacho by reducing the number of turns around the sensing loop on the tacho from one whole turn to just a bit of a turn, i.e. just pass the white wire through the sensing loop without going through twice!

Another thing you can try is reversing the magnetic field through the loop. The easiest way to do this is probably by swapping over the two ends of the white wire in the engine compartment near the coil.
Bill Williams

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PostPost by: simonknee » Wed Aug 04, 2010 2:25 pm

Hey Jonny,

Are you SURE you have an RVI tacho with the current loop.
If so then running a coax flying lead and using potentiometers is not part of the cure.

Earlier you stated that you found the missing cable and attached it to the +ve of the coil. If this was truly the cable that is required to make an RVI tach work then the car would not have run at all until you connected it!

Sounds like perhaps you (and perhaps Bill) have a loom that is designed for an RVC but is being used for an RVI. Which is fine but only if you do it right. If you use an RVC loom then you will be able to run the car without the tach and you'll find a spare cable that would normally feed the input to the RVC tach from the -ve side of the coil.

You need to ensure that your RVI tach has power and earth. Then make sure that of the two (usually white) cables, that run through the sense loop, one comes from the ignition switch and the other goes to coil +ve. That's it. Unplug one of these and the engine will die as the coil has no feed anymore.

Be really sure that you have a proper earth and power to the tach. When one ancillary item makes another fail that is incorrect wiring - usually behind the instruments.

Hope some of this helps
Simon
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'69 +2 50/1370 (stolen '00)
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PostPost by: bob_rich » Wed Aug 04, 2010 8:16 pm

Hi

I am pretty sure you are not using a standard RV1 tacho. In an earlier post you said you had electronic ignition. I guess this was fitted by the previous owner. They may have had the tacho modified as well to voltage pick up to work with the breakerless ignition which is why a lead to the coil CB (or -ve) terminal and tweaking it with a pot is getting you close. Like the other posts said make sure the +12V feed to the tacho and the grounding are solid connections. I don't think you really need the pot and it is masking something else.

hope this helps

best of luck

Bob
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PostPost by: jonnyconcrete » Wed Aug 04, 2010 8:31 pm

Yep, lets put this one to bed.

As I mentioned earlier...its a H&H Coil that doesn't work with the normal supply, which as you quite rightly point out is the power for the conventional coil.

Now I have the H&H coil and distributor wired (fused) directly to the ignition switch with their own independent supply. The tacho lead also connected to the coil...everything is working fine!

Defo RVI tacho...reading your comments I can probably disconnect the independent supply to the coil, but leave connected to the dizzy...I'll try that tomorrow...but all I have done tonight to remedy, is take the feed to the coil and distributor and fused it to the back of the ignition switch, to supply 12v when the ignition is on. Before I had it on a terminal block with other feeds.

As of right now, all is good!
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PostPost by: billwill » Wed Aug 04, 2010 9:37 pm

Ah, OK.

If it is a voltage sensing tacho now, what you could try is using a three terminal resistor (a potentiometer, connect one end of the track to the single-wire from the coil, the other end of the track to earth and connect the slider to the voltage sensing input on the tacho.

You should then be able to vary the sensitivity by rotating the control of the potentiometer.
Bill Williams

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