Ignition ballast voltage drop question

[2cams70]

Since the tach is current sensing and therefore directly in series with the ignition circuit I'd suggest running a jumper wire across the tach to temporarily bypass it and thereby eliminate it as a cause of the voltage drop

[billwill]

I can't work out what you actually mean in this sentence: "Once applying power again, with the ignition in running position (red light on tach), the voltage at the ballast source was 9.8 instead of 9.75. "

Voltages don't mean much in this kind of situation unless you describe BOTH ends of your voltmeter connection, because with high starter currents you cannot assume that the ground (chassis) voltages are all the same.

And what was the 0.3 mentioned in another sentence, if it was volts (again describe both ends) that would be OK but if it was OHMS, that would be a disaster at 250 amps. as it would drop an impossible 75 volts. I really doubt that a multimeter could accurately read 0.3 ohms anyway as the resistance of the test leads and connections to the meter are likely to be much higher than 0.3 ohms.

I will do another version of my diagram above an pput an identification alphabetic letter on key points and give some indication of what I would expect the voltages at those points to be in the two cases of (a) distributor contacts open, (b) distributor contacts closed.

[billwill]

Since the tach is current sensing and therefore directly in series with the ignition circuit I'd suggest running a jumper wire across the tach to temporarily bypass it and thereby eliminate it as a cause of the voltage drop

On the tachos that have the coli inside, the ignition coil connector (white lead) has one female connector and one male connector, so they can be pulled out of the tacho and plugged into each other to bypass the tacho.

[billwill]

OK,here is the revised image with various key points identified by Letters of the alphabet.

Basic ignition and starter circuit for classic cars with significant places identified with letters.

I have added the correction that the bottom ends of the coil in the ignition coil container are connected together, which in turn reminded me that the circuit would need the capacitor across the contact breaker for that 3-connection coil to work. I haven't bothered to include the tachometer; the wire from the ignition switch, to point H loops through a sensor in the tachometer.

First measure the Battery Voltage (A->B), you should use pointed probes on your multi-meter and press them into the lead alloy surfaces of the battery terminals. a 12volt battery will measure at about 12.9 volts when it's fully charged and about 11.4 volts when it is fully discharged. Let's assume for the rest of the discussion that it is 12,6 volts.

For the ground points C or E, you need to make good connections by poking a sharp-pointed probe through any paint and dig it into the metal of the chassis or engine block or you should fasten/crimp a short wire to a connector washer and screw it to a cleaned area of the chassis or engine, such as one of the bolts that holds the cam-cover on.

A multi-meter on the Voltage setting will consume very little current, so we can assume that it will have a non-measurable effect on measurements.Because the meter takes such a low current there is no significant effect in using a long wire as one of the meter probes, so that you can, for instance, take voltage measurements in the engine compartment to a ground point in the boot/trunk i.e. Letter E.

I'm only going to describe some of the measurements at this time as it will take a while to work out the theoretical values.

With all the circuits OFF except with the ignition switch in the engine-running position, use a spanner on the front pulley to turn the engine until the contact breaker points are open. (you may need to take the cap off the distributor, to see the contact breaker). Make sure the radio is OFF and all lights are off, so doors need to be closed so that cabin courtesy lights are off. Do not leave the ignition switch in the Run position for more than a minute or two with the contact breaker closed as the constant current might burn out your ignition coil.

So with those conditions set up: Contact-breaker open, ign switch in RUN, all other electric gear off all of the points D, H,I,J and G, should show the battery voltage (12.6 volts), relative to the engine metal E.

G is the strange one, (that is the extra contact on the starter solenoid which will provide full battery voltage to the ignition coils when the solenoid is closed). It isn't closed at this time but it will be carrying the battery voltage due to a BACK-FEED through the ballast resistor. The voltage there at G at that time is un-required but harmless.

If any of D, H,I,J and G are not showing the full battery voltage then some piece of electrical equipment is still turned on, or there is a circuit 'leak' somewhere.

To be continued.

[billwill]

Oh drat, I just realised that the Alternator will still be connected, which will influence some of those voltages.

Measure them and let us know the results.

[2cams70]

I think we are making it way more complicated than it needs to be. It’s essentially just a one wire circuit from battery to ignition coil and you just need to check at points along that one wire circuit. Think of it as a water pipe. What’s upstream affects what’s downstream. You’ve already determined it’s ok just after the ignition switch so you need to check downstream from there. The tach is one item that’s downstream in series circuit from there so it needs to be eliminated as a cause.

[billwill]

The task is to get the OP, (a self-professed non-understander of car electrics), to do the measurements properly, your message presumes too much knowledge on his part.

[Slowtus]

The task is to get the OP, (a self-professed non-understander of car electrics), to do the measurements properly, your message presumes too much knowledge on his part.

With the greatest respect, I would say that your posts will overwhelm him.

Again, if he does as I suggested (twice) the issue will be resolved - at least to the point of knowing which component or wire etc is at fault.

Components.
Connections.
Wires.

Check the above in that order, substitute components if possible.
Bypass existing connections
Substitute (using temporary wires) for originals, including ground(s).

[dcmarsh]

I’m loving all this great detailed information. Hardly overwhelming - it’s incredibly helpful. I have advanced degrees, just not in this field, so you guys taking the time to post this is much appreciated. Saying things twice doesn’t hurt either. Makes one laugh at oneself.

In my semi retirement I’ve been doing a lot of disaster recovery volunteer work in the US and overseas, and been able to use my home and commercial AC wiring experience to help a lot of folks. Just basic stuff, as I’m sure this is for you guys, but it’s appreciated. This feels like it’s my turn to be on the receiving end of a different kind of electrical help!

Armed with the latest posted info, its back at it today. Will post updates, with relevant letters of the alphabet. Grin.

Thanks to all again.

Dave

[billwill]

I didn't read it all in detail, but this webpage:
https://www.howacarworks.com/ignition-s ... on-circuit
seems to describe the fault finding well with nice artistic pictures.


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AS usual in such descriptions the sentence: The coil in a ballast-resistor system is rated at 7-8 volts instead of 12 volts. When the starter motor is operating, the coil is fed with 12 volts from the starter. is wrong; it fails to mention that the battery output voltage is reduced under heavy current conditions, by the internal resistance of the battery.

It should read something like: The coil in a ballast-resistor system is rated at 7-8 volts instead of 12 volts. When the starter motor is operating, the coil is fed with the full voltage of the battery, (which is reduced by a few volts from its nominal 12 volts by the high starter current).


~~~~

In case you didn't know, the names Capacitor and Condenser are just two names for the same device. Electronic engineers tend to call them Capacitors and Motor engineers tend to call them Condensers.

[billwill]

Oh drat, I just realised that the Alternator will still be connected, which will influence some of those voltages.

Measure them and let us know the results.

~~~~~~~~~~~~~~~~~~


On overnight thinking I realised that an Alternator is ALWAYS in circuit, there is no way to switch it off other than pulling its plug. However it doesn't matter much because the diodes in the Alternator output circuit prevent current flowing from the battery back into the alternator, so the only consumption with the IGN switch on, is the small flow though the IGN light bulb into the alternator field control circuit which is inside the alternator case.

Hopefully this will not seriously affect the suggested measurements above but it can be stopped if necessary, by reaching behind the daskhboard, pulling out the IGN light from the tacho and taking out the IGN light-bulb.

[dcmarsh]

Ok thanks.

Interesting side observation. All multimeters are not the same!. With a common battery connection, I'm seeing a pretty big difference. I guess since I'm looking at relative voltages, the absolute doesn't matter, as long as I stick with the same device, true?

The yellow one is from Taiwan from 10-15 years ago. The orange one is from China bought recently, because I needed amp measurements for something I was working on in the house.

[billwill]

Multi-meters (well all meters) need to be tested and calibrated too.

[Slowtus]

Multi-meters (well all meters) need to be tested and calibrated too.

Must find my Avo Model 8 and see how it compares with all these digital models.

[mark030358]

Multi-meters (well all meters) need to be tested and calibrated too.

Must find my Avo Model 8 and see how it compares with all these digital models.

Proper meter that