Thread: Lucas ignition amplifier replacement by Bosch 024

Originally Posted by superquag

Yes, Please Explain !

- Like the precise Technical Explanation !

A digital multimeter will read static resistance. When you move the rotor past the pickup you generate small voltages which add to or reduce the resistance. The meter screen will show this in random resistance readings until you stop moving. Now if you just used a moving coil meter you would see the needle swing a bit, but the voltage pulse is too quick for a meaningful value. To do the measurement properly you need an oscilloscope but most people don't have one of those available. All you need to know is that the coil has resistance, and movement is producing an effect.

Satisfied now?

My mistake, cryptic comment not precise enough...

Just enjoyed use of the adjective, 'haywire'... - Picturesque and perfectly descriptive

Bit concerned over the extra earth wire.... This needs explaining... and if it goes from coil + to earth.... its shrting out the IGN feed to coil etc. !

I knew there was a use for digital meters ...

Very satisfied.

The positive point on the coil came - as part of the kit - with a cable running from it to its harness, which I thought was earth but now see it was connected to a ballast. It had been a long day...

I'm charging the battery now, then out to remove that earth and test coil resistance.

Coil gives a reading of 0.8 resistance with the red pin on the coil + and the black on the - connector. It gives 5.4k when red it on + and black pin is in centre pole and in this setup when ignition is switched on flicking the rotor button across the pickup makes the reading go, well haywire!

Since I had the rotor lined up I tried testing the spark from the coil again, using a sparkplug connected it to the end of the high voltage cable and nothing.

Is my coil screwed?

As I'm not going anywhere right now, also installed old coil, connected it to the new amp and a spark plug. Wasn't sure if it would work given the current setup (smaller air gap, timing advanced to about 8BTDC, new amp).

Gave it a try regardless, and no spark when flicking the rotor button, checked it was receiving 12VDC to each pickup too.

Originally Posted by simon_m

Coil gives a reading of 0.8 resistance with the red pin on the coil + and the black on the - connector. It gives 5.4k when red it on + and black pin is in centre pole and in this setup when ignition is switched on flicking the rotor button across the pickup makes the reading go, well haywire!

Since I had the rotor lined up I tried testing the spark from the coil again, using a sparkplug connected it to the end of the high voltage cable and nothing.

Is my coil screwed?

Ooh I hope your meter survived, if you were measuring the coil output and the amplifier was firing the coil you would have been trying to push 15,000 volts through it. I meant for you to test the pickup on the dissy! At least it shows the amplifier is firing the coil. Unless its insulation has broken down it should be working.

checked it was receiving 12VDC to each pickup too.

??

a ballast

Probably the suppression capacitor.

Now I tested the rig before I sent it, and I suggest you retest the coil by inserting a paper clip or similar into the coil output post and arranging a 5-10mm gap to the metal chassis or bracket. You may have a dud coil lead for some reason.

Ok, just tried that with a large safety pin in the coil outlet and no spark.

Time to go back to basics...

Testing coils 101.

The theory is, supply the coil with 12v onto the (+) terminal,

Take a short wire and connect one end to the (-) terminal.

PUt a half-unbent paper clip into the coil HT (centre) post and bend the end over to within 5mm of the the bodywork. This is your spark-gap.

Take the short wire and touch it to earth.... every time you lift it OFF, you should get a spark.

If this works, then replace the paper clip with your HT lead (in case this is kaput as Bee suggested) and try again. This will tell you if your coil is OK, followed by "is the HT lead ok"

The above assumes you have the coil mounted as normal, in the car, which will supply you with an appropriate source of (12v) power....

If you're REALLY in doubt, try touching the HT lead.... - but I don't advise this when Little Children or Ladies are within earshot!