Ignition ballast voltage drop question

[Bud English]

Another quick place to check voltage is at the fuse box. The fuse boxes aren't in a great location, although if they do get damp the headers will soon have them dry.

I don't remember off hand which of the four fuses in the box is the one you need, but for each fuse there are the two push on crimped terminals and the fuses themselves that clip in. All are subject to corrosion.

I keep saying center fuse box because that's what the wiring diagram shows. The illustration "ELAN+2 'S' ELECTRICAL COMPONENTS AND CONNECTIONS" also shows a white wire on the second fuse in the center box and it's a white wire that runs to the anti theft switch (unfortunately by way of the + side of the oil pressure gauge, the heated rear screen switch and a four way crimp or bullet connector, if the wiring diagram is to be believed). By the time you have this sorted you'll be the master of the ignition/coil circuit!

[2cams70]

You haven’t given us the voltage directly across the ignition switch. Voltage at input (not at the battery itself) and voltage at output to the ballast feed. Are you measuring the voltages with everything connected up (including coil) and ignition switch in “run” position? Alternatively you can measure the voltage directly across the switch to get a direct reading of the voltage drop across it.

[Billmack]

Hook up the system so the voltage drops then check voltage at each connection all the way back to the battery. Could be something as dumb as a weak battery. Another seat of the pants method would be to touch various connections till you find one that's warm and there's the problem

[billwill]

Here is the basic logical wiring diagram of an ignition and starter-motor circuit of a classic car with a ballast resistor circuit.

Ignition and starter motor circuit, with ballast.

Right-button click on the picture and choose "Open Link in a New tab" or equivalent to see the high definition version of a picture.

[Slowtus]

Hook up the system so the voltage drops then check voltage at each connection all the way back to the battery. Could be something as dumb as a weak battery. Another seat of the pants method would be to touch various connections till you find one that's warm and there's the problem

"Have to crank for 5 minutes (30 seconds at a time) for it to fire,"

A weak battery won't allow that.

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).

[Billmack]

Good point about the battery. Rest of words still stand

[dcmarsh]

Ok, it’s now warmed up enough in the garage for me to check voltage at each connection.

Battery: 12.88
Ignition switch (new) #1: 12.53
Ignition switch (new) #2: 12.56
Ignition switch (new) #3: 12.45
Center fuse box: 12.43
Wire at anti theft switch (removed and wires now joined): 11.44
Source end of ballast: 9.75
Output end of .8ohm ballast (new): 6.75
Coil +: 6.75

Have removed and thoroughly wire brushed and cleaned the starter solenoid (with the red starter button) in the engine compartment. Ditto the new ignition switch connections. Treated with Deoxy. Voltage at the cable source side of the solenoid is the same as battery voltage.

So I’m losing a volt somewhere between the ignition switch and the anti theft switch, which means behind the dashboard somewhere I think. And another almost 2 volts between the anti theft and the ballast, which snakes via the tach according to the diagram, also behind the dash.

Since removing the dashboard and rewiring the car is beyond me, this leads to the dreaded “owner electrical hack”, in this case running a new wire to feed the coil directly from the center fuse box, with no ballast, and a 3 ohm coil, since it would be bypassing the ignition switch. Or possibly a new wire from the start lug on the ignition switch to the ballast, and another from the run lug to the coil. That I could do by using existing access through the firewall via the reverse light grommet.

Any other ideas?!

I’m considering drawing up a simplified diagram with these voltages if the above info isn’t enough.

[billwill]

It seems you still have not cleaned & checked the most obvious candidate for your problem i.e the strap/cable that goes from the battery to earth on the 'chassis'.

I gave you the relevant diagram above.

Are those figures with the contact breaker closed or is it open?

[dcmarsh]

You’re right! I don’t get why all the other voltages would be ok though, but I will do that today.

I’m embarrassed to admit that in an earlier life while going through college I worked as an electricians apprentice for a few years. But in my defense, that was all 120/240v AC. Voltage was always constant, regardless of load. A switch positive leg measured 120v, regardless if the light was turned on. If the neutral connection was weak, it either measured 120v or 0, not 80. So that was the logic I was using to debug this. Clearly DC is very different!

And thank you for the diagram!

[billwill]

A starter motor can take as much as 250 amps on load (while cranking) it would only need a resistance of 0.012 ohms in series to 'lose' 3 volts.

The Internal resistance of a car battery might be as much as 0.02 ohms, which means it 'loses' 5 volts at 250 amps. Hence the whole ballast resistor caboodle.

Most ohm meters cannot show such low resistances and connections have to be really clean and tight to get really low connector resistance.

[Craven]

Quote:-
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.

It is a simple matter to run the ignition system completely independent of any connection to the cars electrical system.
Remove the + connection from the coil. Connect in it’s place your Power Probe 3 set 9volts. Negative connection to the engine block. Start the car.

[dcmarsh]

Boot ground before cleaning

Boot ground before cleaning

Thanks for the idea, Craven!

BillWill, I just redid the ground connection in the trunk, er, boot. New washers, wire brush, Deoxit. Measured resistance between the battery and the ground connection to make sure nothing wrong with the cable. Then for grins, I checked for resistance between the negative post of the battery and ground point in the engine compartment. Measured 0 at the ground connection braid between the engine and the front turret. (I moved it there instead of where it was, on the fiberglass under the painted coil bracket, which made no sense to me. ). Measured 0 at other points I could see, but from the battery negative post to the negative coil post was .3, on my auto scale multimeter.

This was all with the positive battery cable disconnected. (Knife switch)

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.

The crank is at TDC and the rotor is pointing to the #1 lead. It doesn’t seem to make any difference if the coil lead to the distrib is plugged in or not. The spark plug leads are off though, since I’m also debugging why spark plugs with exactly the same model number that I just ordered have a different tip projection and thread length than ones that were in the car from 20 years ago that are stamped with the same number. That’s a different thread on FB regarding all the fake auto parts out there I’m discovering. But I digress.

So I’m thinking it’s not a bad ground, but back to bad wires or connections behind the dash, unless there is another concept I’m missing.

[Slowtus]

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).


In case you missed the post above.

[Bud English]

"Since removing the dashboard and rewiring the car is beyond me, this leads to the dreaded “owner electrical hack”, in this case running a new wire to feed the coil directly from the center fuse box, with no ballast, and a 3 ohm coil, since it would be bypassing the ignition switch."

If your hack is for troubleshooting, great. If your hack is a temp fix, you realize that the tach will no longer operate. The wire runs through the tach, not to power it, but so that tach can sense the current draw on the coil primary and count the pulses.

[dcmarsh]

Ah. Wondered about that. The tach bounces all over the place as it is, perhaps another tip? Thanks Bud.