Lucas electronic ignition - no sparks

[Gridgway]

I am slowly learning  about the wiring in my 84 BDR.  I'm trying to get to a point where I can see if the engine will run.  But I have no sparks.  It's got a Lucas ignition amp and a disti with electronic ignition (no idea of the make).  Pic below.

I'm simplifying the problem (I have removed what I think are the tacho wires from the coil and the rev limiter), but would like to check how the wiring should be if anyone has knowledge.

The ignition amp has the red and the blue wire from the distributor.  Does that just make and break a connection like points would do?  Or with electronic ignition does it work differently?

Then in the pic two wires in a sleeve that got to + and - on the coil.

Then there is a separate 12v feed to the coil.  So two connections between the ign amp and dizzy, two from the amp to the coil and a 12v feed to the coil.  Is that wiring correct?

Primary coil resistance is about 1.2 ohms.

I can crank the motor and see no sparks from a plug which is why I think there's a fault.

Also with my static timing light bulb the light seems always on.

So it seems to me that either the electronic gubbins in the distributor is not doing its thing or the ignition amp is not doing its thing.

Or there's something else wrong.

As usual, all thoughts about what to try are very welcome!

 

[Gridgway]

And as I was posting, I was thinking, what would the first reply be?  The obvious question about checking earths.  For goodness sake, what kind of amateur do people take me for?  No of course I'd not checked them.  Now they all cleaned and checked.  As it happens as I was doing it, I remembered problems that people had had with master switches.  I measured mine and the resistance varied between about 8 and 50 ohms, moving about and depending on how I turned the key.

So I have taken that out of the equation.  Shame really as that's a great way of quickly isolated the battery when working on electrics.  I'll get a new on I think, unless they are clean-uppable or similarly mendable.

[Jonathan Kay]

Are the markings on that amplifier AB14 and 48039A?

Jonathan

[Gridgway]

Definitely AB14, I'll check the other number

[Gridgway]

So on with some more googling and I found a very good article, that shows in turn how to test the coil, the ignition amp and the distributor signalling.  Here it is.

I have tried the "manual" way of energising the coil and I didn't get a spark.  So that means the coil is not working.  I have a spare coil from my other crossflow before I upgraded to electronic ignition.  I might just swap them and see what happens.

I don't have a way of measuring the AC from the distributor.  But if the coil change works, I won't need to!

I do enjoy trouble-shooting! 

[Roger King]

This is a constant energy system which means that the coil is a special part with low resistance so that it can charge very quickly. I've never tried running a standard type coil with this system but I expect there will be issues.

[Gridgway]

Thanks Roger, that's a good point.  My plan was to use a spare coil to try and get a manual spark.  Then take it from there.  At the moment, I can't get anything to work!

The resistance of the LT winding is around 1.2 ohms.  So the current flowing is about 10A which I have measured.

HT resistance is about 5k ohms.

Is 1.2 ohms about right for a low resistance LT circuit?

[jbcollier]

That particular Lucas electronic unit takes a standard AC Delco module commonly used in 70s GM HEI distributors.  Test the inputs.  If they are good, change the module.

The inputs are 12v switched, tach feed and the two to the distributor.  The two to the distributor can be checked using the AC voltage setting on a multimeter.  You should get roughly 2vac when cranking.  The pick up output in these systems is go/no-go.  If it works, it works.  If it doesn't work, there's no signal at all.

The most common failure point is the module inside the box.  The main reason for failure is the break down of heat transfer from the module to the box and the module overheats.  This happens inevitably over time.  Clean the box's mounting surface carefully and use the paste supplied with new module to ensure long life.

[Englishmaninwales]

May or may not apply to your set up, but I cooked an old Lumenition ignition system on my Elan using the wrong voltage rated coil (no ballast resistor) as a temporary replacement.

[Gridgway]

#8 thanks for the info.  I watched a youtube vid of the taking apart of a Lucas box, so understand that bit.

Sadly my multimeter doesn't have an AC volts setting.  Maybe I have some diodes left over in a box somewhere from projects in the distant past so I could make a rectifier bridge??!!  Or a half wave one if I can only find one diode!  Or maybe just get a new meter!

I am slightly surprised that the distributor produces that much voltage on cranking.  I just assumed it would be much lower.  Millivolts rather than volts.

I will sneal away onto the garage this evening to see if I can make any progress.

[Gridgway]

#9 good point/  The system I have is ballast resistor free.  I'll be taking care with the change of coil though!

[Gridgway]

had a bit of time in the garage.  swapped the spare coil in, but still no sparks, suggesting the coil may not be the fault.

Also checked the resistance from the case of the ign amp to earth.  It was about 8 ohms. so not earthed very well.  I rectified that so the case is now properly earthed.

So I thought I would try and generate a spark from just 12v and the coil and the king lead.  Connect the -ve on the coil to the earth.  Locate the king lead end about 2mm away from the block.  Switch 12v to the coil on and off.  Thought I would get a spark (as the article I linked to above described).  Neither coil did that so I must be doing something wrong.  Also tried a different king lead I had.  No different.

Time for a cup of tea and a think!

[Ian Macquarie]

The usual way a coil is wired is constant 12v supply (possibly through a ballast resistor) then it is earthed to allow a current to flow in the primary windings. When the earth is disconnected (points open) a high voltage is generated in the secondary winding which gives the spark from king lead to earth.

[Gridgway]

#13 that's what I understood.  In my test I tried +ve to the coil and interrupting the -ve to earth and vice versa.  Neither worked to make a spark.

One question is that when the -ve is interrupted, this also disconnects the HT negative connection because of how the coils are wired.  How does the spark happen without the -ve side of the HT circuit being connected?

[Ian Macquarie]

How does the spark happen without the -ve side of the HT circuit being connected?

I was hoping someone else would answer! I think the earth side of the circuit is the block and the path to earth is across the spark plug gap. The windings and iron core in the coil make it an impedance circuit so you can't think about it in the same way as a steady resistive DC circuit. However I never could really get my head around the details of mathematics of what happens.

If we are lucky someone who really understands this in detail may chip in with a better more authoritive explanation.

Just a thought - there is a good earth between the battery negative and the block?

[Gridgway]

thanks Ian.  It is strange.  In other transformers the +ve and -ve of each winding have to be connected to their relevant components to make a circuit.  That's why I don't understand how it works with one side of the HT winding isolated unless the magnitude of the voltage changes it.  An yes the earthed body of the spark plug provides the potential difference to ionise the air in the gap and make a spark, I just don't know how!

Re earths I have been on an earth hunt!  So far I have removed the master switch from the neg circuit which was the highest resistance (variable between 8 and 50 ohms), cleaned the connections form the battery to the body, cleaned the connections from the body to the block and cleaned the earth path from the Lucas box to the body (another 8 ohms).

Having a bit of a head scratch at the moment waiting for my AC tester to arrive so I can test the reluctor in the dizzie.

Part of me wants to get it working with the "period" parts, but I might just give in and get a new electronic ignition - Lumenition with the hall effect in the dizzie and suitable coil or similar.  But that's all a chunky cost.

[jbcollier]

Duh, sorry, asleep at the wheel again.  Here's a diagram that shows how a Delco 4 pin amplifier is wired:

 

[Gridgway]

#17 thanks, it's very straightforward!

Still unsure how the HT circuit makes a circuit!

[Gridgway]

Quick update, not much progress, but I did try a test by replacing the coil with an led.  It stays off when cranking showing that there is no action from the ignition module.  Next test is to see if the dizzy is producing any ac now my new multi meter has arrived.

I've also worked out that your can't do static timing with this system as it requires an ac voltage to be produced for the ignition module to sync to.  Rusty won't happen unless the dizzy is turning and it'll need a number of pulses to sync to.

On the verge of knowing the culprit...

[jbcollier]

I have actually never seen a failed pick-up or coil.  I've heard of failed coils but not pick-ups.  Modules?  Changed lots and lots of those.

[Gridgway]

#20 thanks for the info.

I tried out my new multimeter that has AC volts.  With the engine cranking, it doesn't register.  It looks like it should be able to see down to 0.1v AC.  But who knows!  On looking further and squinting under the carbs I could see what looks like damage to the pickup wires.  So I bit the bullet and took the dizzy out (after taking the carbs off of course).  The wires have indeed been squished.  Looks like somehow they got caught between the distributor and the cap somehow.  Picture below.  In my experience wires are pretty hardy so unlikely to be broken, but I can't tell.

I can't see a way of actually checking the continuity of the wiring without cutting the outer to see and I don't really want to do that.  Is there a way to check them as they are?

Now the dizzy is out, I can spin it faster with my drill.  I'm soon going to be changing bits!  As I can't find the magnetic pickup on the internet,so  I'll try changing the module in the AB14 (as that seems to be what fails).  I much prefer to find the faulty bit and change it rather than changing out parts speculatively 

[John Vine]

If you can't get at the ends of the cables, you could insert very thin needles into the insulation each side of the "squish" and measure continuity via those. 

JV

[Gridgway]

Thanks JV, I was wondering about that approach.

However, I have finally made some progress.  Spinning the distributor using my electric drill I was able to reliably measure 0.1v AC - the smallest my new meter will read.  So that's enough evidence for me to decide it must be the HEI module and source one of those.  I'll leave it all apart so I can do the same test with the module connected and see if it works.

Now just to source the right module and some computer chip paste (haven't changed a CPU in many an age so the supply is low!).

Next update will be a while as next weekend is Castle Combe.  As ever, thanks all for all the enthusiastic and helpful help!

[Gridgway]

Quick update.  I got the new HEI ignition module from ebay and after some fettling fitted it to the AB14 module.

I still have the dizzie out so I can spin it with my electric drill.  Fitted it back in the car to be the easiest way to test it.  I've got an LED that I can out in place of the coil, didn't flash.  Then I attached the coil trying to generate a spark from the king lead, nothing.

So need to have think.  The new HEI module from ebay could be duff, or the issue lies somewhere else.

Only four elements.  The AB14 (HEI module, capacitor, zener diode) distributor, coil and king lead.  How hard can it be?

Getting close to buying a new electronic ignition!

I have tried a different coil and a different king lead.  I think I know the distributor generates AC volts.  Two unknowns are really the AB14 and my test method.

[SM25T]

Stupid question, but I assume the battery is still healthy and all cables connected ... you seem to have replaced almost everything else !