Club 80-90 Forums

CJH wrote:

what2do wrote:With the temporary live feed to the coil in place, I have measured between 15 and 1 - OV

OK, I don't understand that - if pin 1 shows continuity to earth, and pin 15 (rather than the disconnected cable from the alternator) measures 12V with respect to a (different) good earth, then I can't see why you don't get 12V between pin 15 and pin 1. This seems to be a contradiction.

With the temporary live, I left all original connectors in place and found a 3rd spade to put the red probe of the meter on to test. Did the sme test with all of the connectors removed from the coil, same thing!! It may sound daft but should I put a lead between the Earth on the body and the earth on the engine, not for any particular reason other than I'm baffled?!?!

what2do wrote:Did the sme test with all of the connectors removed from the coil, same thing!!

I'm getting muddled. What do you mean by same test, same thing? Did you measure between the removed wires, or between the (now bare) coil terminals?

Do a voltage test between the female spade at the end of your temporary alternator wire and the female spade at the end of the green wire that goes to pin 1, but remove both wires from the coil, so they're not connected to anything. That should measure 12V. If it's not, you've got a bad earth wire or a bad supply. But I don't see how it could be anything other than 12V, since you've already said the alternator wire measures 12V to a good earth, and the green wire has continuity to earth.

Right, a bit of progress. Whilst in the engine bay, my neighbour strolled passed and he works in servicing aircraft (or something along those lines) and so I pulled him in to look over what I was doing because I knew it had to be my methods that were at fault.

Anyway, he quickly saw that I was testing the coil incorrectly as I had one probe on the feed side (with the temporary live) and one probe on the other side of the coil. He got me to put the black probe ob the body earth point and we got 12V - hurrah. Then went on to measure the 3 pin plug going to the dizzy, got 11.5 V (why? Isn't it meant to be 5V?) put a wire in the middle terminal, scratched it against the earth point and a very strong looking spark was visible/audible from the King lead which was held by the earth point.

It's a start.

now try starting with some fuel down carb.

itchyfeet wrote:now try starting with some fuel down carb.

Really? I thought the above result meant that the hall sender was fubarred.

Although, I see where you're coming from. Will try it later.

So, how hot is it on the island? Not too hot for cycling I hope.

what2do wrote: Anyway, he quickly saw that I was testing the coil incorrectly as I had one probe on the feed side (with the temporary live) and one probe on the other side of the coil.

That's my fault - bu11shiting with confidence! So can someone with better electrical knowledge than me explain why, if Pin 15 is at battery voltage and Pin 1 is showing continuity to ground, there's no voltage difference between Pin 1 and Pin 15? Is it because the resistance between Pin 1 and Pin 15 (the primary resistance, ~1 Ohm) is much less than the resistance of Pin 1 to ground through the ignition control unit?

the coil will be the same voltage both ends unless it's connected to ground by the ignition module because no current is flowing
even a high resistance will show the same V both sides if no current flows

V across R is

V=IR
no I means no V

multimeters can lie to you because they show continuity does not mean a current can flow

what2do wrote:
So, how hot is it on the island? Not too hot for cycling I hope.

Very hot ..saw 10 police bikes decend on the local stadium where the Spanish football team are practicing this morning.

itchyfeet wrote:the coil will be the same voltage both ends unless it's connected to ground by the ignition module

Understood, but Pin 1 is apparently showing continuity to ground. What is the default state of the ignition module? I assumed it was normally closed (Pin 1 connected to ground), but opens the circuit in response to the hall sensor in order to collapse the field.

itchyfeet wrote:multimeters can lie to you because they show continuity does not mean a current can flow

Ah yes, so maybe the default condition of the ignition module is to be open circuit then, but via components that allow a very small current.

it's a semiconductor device not a switch

even a broken wire can show low resistance with the tiny current a multimeter uses but try passing a high current and it can't

itchyfeet wrote:it's a semiconductor device not a switch

If the job of this semiconductor device is to turn the coil on and off then that's a subtle distinction if I may say. And as I understand it, it turns the coil off every time a spark is needed, so it would still seem sensible that its default state is 'on', i.e. pin 1 connected to earth. But I agree, a semiconductor switch in the 'off' position could appear to be 'on' if measured with a digital multimeter. That reminds me - I really should have taken my chance to pick up a second hand analogue meter when I saw it at an autojumble last weekend.

itchyfeet wrote:even a broken wire can show low resistance with the tiny current a multimeter uses but try passing a high current and it can't

That's what I said.

if you want to measure continuity for a high current circuit use a light bulb not a multimeter