Lucas Lord of Darkness rears his ugly head

[tdafforn]

Hi All,
Had a good look at the tacho last night and it is indeed the sensing coil that is toast...
It has welded itself to the tacho coil, but should be repairable..
One question however, I am confused how this fits into the ignition circuit?
Does the sensing coil make up part of the coil supply circuit from the ignition?
The loom had two attachments to the +ve side of the coil which I now assume were what was left of the balast resister system.
the system now has a 12V coil..
Cheers
Tim
PS. There is also a potentiometer in the tacho is this for adjustment?

[rgh0]

The current from the 12V supply flows through the tacho sensing coil to the positive teminal of the ignition coil.

The ignition coil draws current in pulses driven by the distributor connection to the negative ignition coil terminal for each ignition spark required.

The tacho senses each current pulse as it passes through the tacho and converts that to a voltage signal proportional to the number of pulses per minute. This drives the tacho needle to the right position on the tacho dial.

When you had a ballast resistor in the system the current flowed through the ballst resistor before the ignition coil dropping the voltage to 6Volts at the ignition coil.

Rohan

[tdafforn]

So I have cleaned out the carbonised wires from the sensor coil and replaced with a new wire..
Question is, does the gauge need a seperate 12v/0v feed to function. There are two other wire connections one on the case, which I am assuming is for 0V and a second wire. does this need to be attached to a 12v feed?
Cheers
tim

[rgh0]

Yes

The gauge has 12 volt supply and earth connection to power the electonics that sense the current pulses and drive the indicator needle. This what your 2 connections should be

Rohan

[tdafforn]

Hi all,
Jury rigged the tacho using a feed for the sensor coil that is also used for the aldon ignitor electonic ignition power supply and it worked!!
then wired it into the dash with a new supply from the ignition (replacing the old charred supply wire) and now it doesn't work...
Any suggestions?
Tim

[stuartgb100]

Tim,
I think you'll find that you need a clean supply to get it working smoothly.
From memory, I ran the feed from the switched side of the solenoid.
HTH.

Regards,
Stuart.

[stuartgb100]

Tim,

Double checked when back at home. Seems I finally settled on a clean supply drawn from the fusebox.

Cannot recall why I didn't go via the solenoid, since that would be a more convenient route, but there must have been a reason.

Regards,
Stuart.

[rgh0]

Tim

The Tacho has 4 wires connected to it, if you ignore the warning lights and illimination lights

1. 12 volt power supply
2. Earth
3. 12 volt supply into tacho sensor coil for ignition
4. 12 volt supply out of tacho sensor for ignition, runs to coil postive

The Aldon ignitor 12 V power supply needs to come from a separate supply point to any of the above otherwise you may get the tacho being unstable

If all of the above is right and the tacho electronics are working then you should be OK

Rohan

[martinp]

john.p has got it right.

Re repairing the tacho - you might find it hard to pick up the right quality of wire, a heavier gauge (thicker) can be used provided all the turns can be fitted into the space available. Its the number of turns that is important here.
A source for a foot or so maybe found by unwrapping the torroids in the power supply of a scrap PC. Check it is at least as thick. Don't scratch the enamel getting it out.

I encountered an unusual, related problem which was eventually identified because the ignition key felt "hot" when withdrawn. Probably due to age or mis-treatment in a similar fault such as above, the contact made in the ignition switch was slightly high in resistance. This caused it to warm up when carrying current.
After 20 mins or so of "normal" operation the engine would cut out as the switch went so "high resistance" that sparks failed.
Attempting a restart caused the engine to fire up (because of the second circuit to the coil provided by the wire to the solenoid)
It cut out again when the key was released.
The initial conclusion was an ignition coil breakdown with heat and later (when that had been replaced) a fuel system failure of some sort. This was endorsed by the fact that after half an hour or so of "cooling", the ignition switch became normal again and allowed correct running for about 20 min.
A real pain!!!