voltage to the coil

[ammachine390]

it doesn't look like you have bypassed the resistor.

He wouldn't get 12 volts at the coil with the car running, as posted, if the resistor wire was still in the circuit. He would get 6 volts.

Checked again.  This time it showed 8.3v in run, 12 when running.

Yes, but if the resistor has been bypassed, then he should also have 12 with key on engine off, not 8.3, thats very low.

I get much lower voltage going from the battery + to the coil +.  I don't remember what it was exactly, but I think 2 or 3.

Do you mean 2 or 3 ohms. That's really high, even with the resistor it should only 1.35 ohms in the stock wiring.

[jim]

Very low voltage from the battery positive to the coil positive.  I'll check again and pay more attention to the exact voltage, but it may be a day or two.

I appreciate all the comments and opinions about this.  I've learned some things, even if I am confused about some, also.

[ammachine390]

Ok, so I guess your doing a voltage drop test then. Right? Your putting one probe of your voltmeter to the positive battery post and the other to the positive of the ignition coil? If thats what you are doing, then, that's a voltage drop test. If your getting 2 to 3 volts, that means that in that wiring between the battery and the ignition coil, you are losing 2 to 3 of your 12 volts. You definitely have resistance in your wiring. 

Do you have an ohmmeter? I would check the resistance, putting the probes in the same place as with the voltage drop test, and see how much resistance you have. If its stock, it should be no higher 1.35, if you have bypassed the resistor, it should be close to 0.

[jim]

Yes, I did the voltage drop test.  My multimeter will read ohms.  I'll do that, but it will be a day or 2 before I can do it.

[Jurjen]

That is no problem for the ignition module, the Amps will go down.
Higher Ohms of the total system means that it will take longer to "charge" the coil before each spark. Not a problem at revs, because there is enough time between each spark to charge the coil.
Could be a problem at higher revs.

[Canoe]

Checked again.  This time it showed 8.3v in run, 12 when running.

Yes, but if the resistor has been bypassed, then he should also have 12 with key on engine off, not 8.3, thats very low.

Except with the engine off, and the key on, in addition to the coil, there's other electrical loads on the battery that can pull the voltage down; the combined load is in series with the internal resistance of the battery. You can't assume that "engine off, key on" means you're starting with 12 VDC. With "engine off, key on", you have to measure what you see at the coil and at the battery.

For example, with a stock alternator, engine running at idle, what voltage do you see at the battery with the headlight/highbeams on versus off... Measure that again with the engine off...

[ammachine390]

Except with the engine off, and the key on, in addition to the coil, there's other electrical loads on the battery that can pull the voltage down; the combined load is in series with the internal resistance of the battery. You can't assume that "engine off, key on" means you're starting with 12 VDC. With "engine off, key on", you have to measure what you see at the coil and at the battery.

For example, with a stock alternator, engine running at idle, what voltage do you see at the battery with the headlight/highbeams on versus off... Measure that again with the engine off...

True, but it shouldn't make much of a difference. These cars do not have a ton of electronics, so just turning the key to the on position is not too much of a load, if you keep your accessories off. I do not think I've ever seen a battery drain below 9 volts just by turning the key to the on position though.

But I guess what I should have said is, when the coil reads 8.3 Volts with key to on, what does the battery voltage read? They should be close, if the resistor has been bypassed.

[jim]

Engine off, key on:

solenoid to coil - 8.3 - 8.9v

battery to coil - 2 or 3v
will check again to get actual reading

[ammachine390]

You said you have 8.3 volts to the coil when the key is on, engine off. Battery should have 12 volts.

You have NOT bypassed your resistor.

Your coil has 3 ohms primary resistance. The resistor wire has 1.35 ohms of resistance. This is series circuit, total resistance is 4.35 ohms. Using the voltage drop formula: (1.35/4.35)x12=3.724 volts dropped across the resistor. Meaning the coil is only receiving 8.275 volts. (12-3.724=8.275). You have confirmed that you only have 8.3 volts at the coil, therefore, you have not bypassed the resistor.

Therefore: Since the flamethrower is a 40,000 V coil, when applied 12 volts. You are only getting 27,586.2069 V at the maximum with the way you have it currently set up.

[jim]

The next question is whether I should bypass the resistor.  Rebuilt motor, Motorcraft carb, and it starts and runs so very well I'm hesitant to begin cutting wires and messing with it.  What improvement am I likely to get if I bypass it?

[ammachine390]

It might help it run even smoother. Possibly better mpg. What are you plugs gapped at currently? Would help the engine run better at higher rpms as the coil would be able to saturate better. You have nothing to lose by bypassing. It can't make anything worse.

[Canoe]

You'll be getting somewhat better than the 27.5K volts calculated, as when running the alternator should be providing around 13.8 VDC (that's how the battery gets recharged). But you won't be getting the full benefit of your TFI's 40K volt capability.

But, bypassing the ballast resistor with the stock Ignition Module with a TFI: the belief was that this was a recipe for burning out the IM. At least people started having them fail. This is one of the reasons for upgrading to the GM-HEI IM. It can take the voltage and current, so you get the full benefit of your TFI, but this IM does need a proper heat sink (mount to inner fender is fine). Hanging in the air off wires is not fine, and it will heat up: the cheap ones will burn out, the good ones have a protection circuit that will turn themselves off til they cool down.

Bonus is that the dwell is improved, but I don't remember the details in what way it is improved. This is why people upgraded to the GM-HEI even with the stock coil.

Remember that as you'll be getting much more spark voltage, if you didn't increase the spark plug gap before, you'll want to do that now. You don't have to adjust your carb.


Check out this thread on the GM-HEI. Specific post has more on ballast resistor.
http://forums.amceaglenest.com/index.php?topic=31827.msg296317#msg296317

[ammachine390]

Ok, assuming 13.8 V while running, the max coil voltage is still only 31,733.33 V. That's still less than 80% of its full potential.

You CAN bypass the resistor with the Flamethrower coil because it has a primary resistance of 3.0 ohms. The ignition module can't handle more than 5 amps. So with the flamethrower coil and no resistor, total resistance is 3 ohms, and the current through the module is only 4 amps.

You cannot bypass the resistor with a STOCK coil.

[jim]

The plugs were gapped to 45 when the bigger cap and adapter, 8mm wires, and hotter coil were installed.

[Jurjen]

Thanks.  I'll check it out.  I found the specs on my coil.  It's 40,000v, 3.0 ohm.

It looks like Jim has the 3 Ohm coil, so he can by-pass the resistor wire.

The resistor wire goes to a prong on the starter relay (red with green text "resistor").
It is together with a yellow wire. This yellow wire is the positive wire going to the coil.
Cut this yellow wire at the spade and splice this wire with the yellow going to the Ignition Module. This yellow wire is coming directly from the ignition key "run" position.