Lotus Elan

Ignition ballast voltage drop question

PostPost by: dcmarsh » Thu Feb 18, 2021 4:20 pm

Been researching ignition ballast for a bit, since it isn’t shown in the Elan +2s Federal wiring diagram, although there is an update addendum, but without a wiring diagram. I’m stumped by a voltage drop which makes no sense to me. While running, the 1.5 ohm ballast is designed to cut down the voltage down to my 1.5 ohm coil from 14 v down to more like 8. For starting, the ballast is bypassed to allow full voltage to the coil.

The bypass is working fine, and the supply side of the ballast shows 14v, but only when the other end of the ballast is disconnected from the coil. When it is connected, it shows 11v at the supply end and shows 5v, which is too low.

These voltage measurements are rounded, not exact, and are all static, not while cranking.

Why would the supply side end drop?? The measurement is not taken on the resistor, but on the supply wire connection. Here is a simplified diagram I’ve attempted.
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PostPost by: 111Robin » Thu Feb 18, 2021 4:52 pm

Have a lookk at pages 19-21 of the attached Lucas document, this might help with your diagnostic testing.
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PostPost by: Craven » Thu Feb 18, 2021 5:13 pm

Ensure your Negative probe is connected to good chassis connection.
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PostPost by: Bud English » Thu Feb 18, 2021 5:45 pm

There are a couple of problems with your diagram. In the top figure, a voltage drop at the supply end means that there is another resistance upstream from the ballast resistor. That narrows it down to the contacts in the switch, the connections, or the wire. That's born out by the voltage at the ballast resistor going up to 14V with no current passing through the switch circuit.

The only other problem is that you do show a voltage drop at the coil end of the resistor with the coil disconnected. There shouldn't be any voltage drop without current flow. Since you show the same voltage drop (6V) in both diagrams, the current would have to be the same in both cases and that can't be happening. Double check that voltage. It should be 14V as well.
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PostPost by: billwill » Fri Feb 19, 2021 12:01 am

The most likely cause is that the contacts in your ignition switch are mucky or that the earthing points, especially the fat lead from the battery to the 'chassis' are oxidised.

Clean the battery and 'chassis' connections, with a wire brush unbolt the fat wire from the chassis and clean off any crud or paint from both the wire lead and the chassis to expose the steel of the chassis ir the zinc plating if it is galvanised. Apply Petroleum jelly (Vaseline) to the connecting surfaces when you bolt it back together.

If you think you are capable, take out the ignition switch, dismantly it and clean the contacts, otherwise buy a new ignition switch, they are still available.

~~~~

The Ballast resistor is intended to drop about 3 volts, roughly the same voltage drop that would occur in the wiring when the starter motor is in use. The starter current is so high that voltage drops of about 3 volts are inevitable, so:

The way the ignition system gets around this is to use an ignition coil DESIGNED TO WORK AT ABOUT 9 VOLTS. If you connected it without the ballast resistor the 9volt coil would be fed 12 to 14 volts which would probably eventually destroy it. Hence the supplied voltage at the coil is dropped by the ballast resistor to about 9 volts when the starter motor is NOT running.

When the starter is running the coil is fed directly from the main connector which due to the starter motor current is now down to about 9 volts. So the coil is working at its normal voltage and giving nice fat sparks to start your engine.
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PostPost by: dcmarsh » Fri Feb 19, 2021 12:49 am

Let me clarify a bit.

The circled 11v is 14v as a bare wire when not connected, and also connected to the ballast when the other end is not connected per the second drawing. That rules out a dirty connection, bad ground, and bad ignition switch doesn’t it? The ignition switch is new, but the point is there is a solid 14v at the source side of the ballast resistor, when the coil is not connected to the other end. From every other diagram I’ve seen, the source should stay at 14v while running, and the resistor drops it to 8v, as designed. But it’s dropping to 11, meaning the coil only gets 5, which is way under spec.

Thanks, 111Robin for the great debugging chart. What it does is confirm I’m correct in believing voltage at the input side of the ballast (V2 in the diagram) should ALWAYS be battery voltage. There isn’t an explanation as to why that would be the case only when the coil is disconnected from the ballast.

Weird. But I have no electrical engineering background so it’s probably some basic concept I’m missing.
Last edited by dcmarsh on Fri Feb 19, 2021 12:32 pm, edited 1 time in total.
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PostPost by: Bud English » Fri Feb 19, 2021 6:03 am

dcmarsh wrote:Let me clarify a bit.

The circled 11v is 14v as a bare wire when not connected, and also connected to the ballast when the other end is not connected per the second drawing. That rules out a dirty connection, bad ground, and bad ignition switch doesn’t it? Actually it doesn't. With the coil end of the resistor disconnected there is no current flow in any point in the circuit. You will read 14V no mater where you measure it, even at the coil end of the resistor. If your meter has an ohms function, measure for any resistance from the switch terminal to the switch end of the resistor. Any resistance will cause a voltage drop when there is current through the circuit.

The ignition switch is new, but the point is there is a solid 14v at the source side of the ballast resistor, when the coil is not connected to the other end. Again, this is what you will read at any point in the open circuit.

From every other diagram I’ve seen, the source should stay at 14v while running, and the resistor drops it to 8v, as designed. But it’s dropping to 11, meaning the coil only gets 5, which is way under spec. If it's dropping to 11, it's because there is a resistance in the circuit on the source side of that point.


Note that the coil primary goes through the center fuse box, the tach, and the totally unreliable anti theft switch in the glove compartment before going to your ballast resistor. It's not a straight shot and corrosion at any of the connections between your new ignition switch and the ballast resistor could be your problem.
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PostPost by: Andy8421 » Fri Feb 19, 2021 7:20 am

Just to add to Bud's excellent post, for a voltage drop to occur, you need both a resistance and a current.

No current, or no resistance, then no voltage drop. So even a badly corroded, high resistance connection will show zero voltage drop when there is no current flowing through it. Slightly more info than you need, but Mr Ohm came up with a formula for this in the early 1800s. Applied to your case:

Voltage drop = current through the cable x resistance of the cable

So no current, no drop.

Applying a bit of schoolboy maths to your problem, you have a rogue 0.8 ohm resistance somewhere between your ignition key and the ballast resistor. Searching for this by measuring the resistance will be tricky, low-ohm measurements are unreliable with a hobby multimeter. I would recommend working your way along the circuit from the ignition key with the coil connected, measuring the voltage, the fault will be just upstream of where the voltage drops.

As Bud rightly points out, the most likely culprit is the hopeless anti theft switch. If you still have it in circuit, irrespective of whether it turns out to be at fault this time around, I would consider removing it.

Good luck.
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PostPost by: 2cams70 » Fri Feb 19, 2021 9:29 am

Andy8421 is correct. The other way to isolate the problem area is to run a temporary jumper wire from the main supply side of the ignition switch direct to the ballast input side. If voltage at the coil returns to an acceptable figure it's likely to be the ignition switch. Similarly connect the jumper wire to the output side of the kill switch and see what effect that has. Also if there's any spade or bullet terminals in circuit they can cause problems through corrosion or loss of spring tension over time. With spade terminals for instance I often nip the female side slightly with a pair of flat wide nose (non serrated type) pliers to restore the tension and improve circuit continuity.
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PostPost by: dcmarsh » Fri Feb 19, 2021 12:21 pm

Wow, this is incredibly helpful! Yes, I do still have the notorious anti theft switch and other original connections so I will check that out and try some things suggested. I do have a Power Prob 3 which can provide battery voltage anywhere I want so I’ll try supplying the power end of the ballast resistor with that. I’m assuming I should see 14v in that case instead of 11v, and that would prove my problem is somewhere in the spaghetti elsewhere.

I knew I should have taken some kind of electrical engineering class in my youth!

Thanks everyone. I’ll post an update.
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PostPost by: 2cams70 » Fri Feb 19, 2021 12:37 pm

dcmarsh wrote:I do have a Power Prob 3 which can provide battery voltage anywhere I want so I’ll try supplying the power end of the ballast resistor with that


You'd be better off just measuring the voltage at different points of the circuit as per Andy8421's suggestion or running a jumper wire than using that tool and trying to inject voltage
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PostPost by: billwill » Fri Feb 19, 2021 4:25 pm

dcmarsh wrote:Let me clarify a bit.

The circled 11v is 14v as a bare wire when not connected, and also connected to the ballast when the other end is not connected per the second drawing. That rules out a dirty connection, bad ground, and bad ignition switch doesn’t it? The ignition switch is new, but the point is there is a solid 14v at the source side of the ballast resistor, when the coil is not connected to the other end. From every other diagram I’ve seen, the source should stay at 14v while running, and the resistor drops it to 8v, as designed. But it’s dropping to 11, meaning the coil only gets 5, which is way under spec.

Thanks, 111Robin for the great debugging chart. What it does is confirm I’m correct in believing voltage at the input side of the ballast (V2 in the diagram) should ALWAYS be battery voltage. There isn’t an explanation as to why that would be the case only when the coil is disconnected from the ballast.

Weird. But I have no electrical engineering background so it’s probably some basic concept I’m missing.



No it DOESN'T rule out dirty contacts or bad earth, those are exactly the symptoms expected from such a problem. If the ignition switch is new then you probably have bad/dirty connectors feeding it. The other thing that fits the symptoms is that your car battery is old and failing and near the end of its useful life.

Your lack of understanding of things electrical is somewhat obvious. This is really basic OHM's law stuff.

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PostPost by: dcmarsh » Fri Feb 19, 2021 5:31 pm

Ok, will try that next.

Spent the morning trying to get the anti theft switch out. Manual says the glove box removal requires removing the dashboard, which I’m trying to avoid. I drilled out the rivets to remove the switch and discovered that the wires connected to it have approximately 2 nano-mm of play, and access to it from underneath can be done with needle nose pliers but due to the aforementioned play, I moved to a more drastic action out of frustration. I dremeled a hole around the switch to pull it into the glovebox, thinking there might be more play in the wires. Wrong, sparky. But I was able to at least see which was the white/purple wire to the horn and disconnect it. I then ran a jumper across the terminals and remeasured the voltage at the ballast but no change.

Will now continue with measuring voltage at various points and the other suggestions.

I should mention that I’m trying to root cause my hard starting issue. Have to crank for 5 minutes (30 seconds at a time) for it to fire, and then die. Repeat. Once going, it runs fine. Thought it was a bad fuel pump so replaced that and am getting fuel to the carbs. Rebuilt the carbs and found one leaky float. Replaced the starter just for grins. All my voltage measurements are with the battery hooked up to a battery maintainer. Have tested the relatively new battery with a load tester and it’s fine. Now on to electrics so tested and replaced the coil with the correct 1.5 ohm and replaced the ballast with a 1.5 ohm. Discovered that the coil is not getting the correct voltage so that is what I’m pursuing now.

Thanks so much to you all for burning brain cells to provide much needed guidance on this!

And BillWill I’m only 10 years behind you but my engineering career has been in software, and that knowledge is completely useless in this case!

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PostPost by: dcmarsh » Fri Feb 19, 2021 11:12 pm

BDDE9E9D-4858-43F5-BC03-8C0ED8B10B3A.jpeg and
Blue arrow showing where ground braid is attached to coil base bracket but bolted through fiberglass into the wheel well
Update.

12.8v at the battery, 11.8 at one wire to anti theft switch, 11.2 at the other, and 10.2 at the ballast source. So all the education is helping. I have connection analysis to do.

Another dumb question tho. Does the coil outside case need to be grounded? There is a large ground braid going from the engine oil pump to the coil mounting bracket bolt, which is mounted in fiberglass (blue arrow). The wiring diagram doesn’t show the outside connected to ground so wondering why that is there. The original coil and bracket were not painted though, and the replacement is so if it needs to be grounded, I have paint to remove. Seems a huge ground braid would be overkill for that, if needed at all

However, there is a ground bolt connected to the front right chassis upright with a couple of ground wires, (green arrow) and I wonder if that is where the ground braid should be going. It would be a stretch though.
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Green arrow showing bolt connected to frame upright
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PostPost by: ericbushby » Fri Feb 19, 2021 11:23 pm

The coil case is probably earthed for radio interference suppression. The windings of the coil are powerful electromagnets.
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