Lotus Elan

Coil / Tacho Query

PostPost by: KevJ+2 » Wed Jun 24, 2015 1:46 pm

Hello All,
Finally getting there with the rewire, but I seem to have an issue with the tacho to coil wiring.
I have a Spyida converted tacho (rvc) and as advised, I've joined the one wire direct to power and the other to the negative on the coil. The coil when energised to positive gives 12 volts on both terminals + & - is this normal, I can't remember? With ignition on, the tacho stays at zero, but when I switch off, it jumps all the way round. I hate to think this is creating a short. I have no earth connection at the coil.
All thoughts welcome,
Thanks,
Kev.
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PostPost by: billwill » Wed Jun 24, 2015 2:39 pm

With the contact breaker open on the distributor there will indeed be 12v on both sides of the primary of the coil. When the contacts close (or equivalent on electronic ignition) the negative end is pulled down to zero volts.

This is a picture of the original type tacho connection.
Image


More discussion on this matter here:
lotus-electrical-f38/rev-counter-works-with-electronic-sparks-t18275.html#p147217
Bill Williams

36/6725 S3 Coupe OGU108E Yellow over Black.
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PostPost by: bob_rich » Wed Jun 24, 2015 3:21 pm

Hi

If you have a RVC conversion then it is not current triggered. Current triggering is shown in Bills drawing above and was the standard method used by Lotus on most elans and +2's with RVI tachos

If you have a normal coil with points arrangement then with the points open there will be +12V on both ends of the coil. You need to make a connection to the contact breaker ( -ve end of coil) and this will be a high voltage trigger as described in the first bullet point of section b in the Spyia instructions . I have attached a link to these instructions

Hope this helps best of luck

http://www.spiyda.com/magento/media/cat ... %20MK2.pdf

Bob
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PostPost by: KevJ+2 » Wed Jun 24, 2015 3:55 pm

Bill and Bob, thanks for your replies.
I have turned the distributor round quite a bit but still get over 12 volts on either terminal of the coil so seems permanent. It's a new Lucas coil with a new TT electronic distributor. Are there any further checks you can think of?
Kev.
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PostPost by: bob_rich » Wed Jun 24, 2015 4:09 pm

Hi Again

Not familiar with your electronic distributor. when electronics system replace the points many will not give out a signal to the ignition coil unless running at a minimum speed ie at least cranking so moving the dizzy may not result in any output. Really need to see what it says about your dizzy in the manual. Cant seem to find it on the web to comment further info o who makes it and what type would help

hope this helps

Bob
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PostPost by: KevJ+2 » Wed Jun 24, 2015 4:15 pm

Thanks Bob, unfortunately I haven't got the car running yet so all this testing is just that.
I suppose my main worry is - have I got a dodgy coil that has 12 volts across +&- All the time.
Trying to get the tacho, dizzy and coil to behave together is proving tricky.
Kev.
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PostPost by: bob_rich » Wed Jun 24, 2015 9:39 pm

Hi Again

To make a useful suggestion it would be necessary to know the exact details of both the coil and the Dizzy.
manufactures name and part numbers should help. Some electronics system especially those magnetically triggered cannot easily be set up statically like you can with points.

cheers

Bob
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PostPost by: KevJ+2 » Fri Jun 26, 2015 11:50 am

The following was emailed to me, which I think is a good explanation, and probable cause -

The problem could be interference from something on the car, it will probably be a relay somewhere.
When a relay or fuel pump solenoid or similar is disconnected from power, the magnetic field collapsing generates a negative voltage.
Modern relays and fuel pumps have a diode built in to short this negative pulse to ground
This problem only occurs on some installations so we do not as a matter of course cover it in the instructions
A positive voltage is normal on both sides of the coil until you start the engine, the ignition system will ground the negative terminal to charge the coil, then when the ignition system disconnects the negative terminal from ground, the spark will fire.
The better ignition systems will leave the coil negative disconnected when the engine is not running so as not to overheat the coil
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