Ign coil temp (how hot)

billfish

billfish

Senior Member
I seem to have another bad coil on my stock 51 8ba pos. ground Victoria. Engine seems to miss at cruising speed after a short run time, not at idle. A new Napa 6V Echlin (no resistor required) coil #904 corrected the problem for a while but it has returned. How hot should the coil get, it seems hot to the touch after a short run. It is properly connected with ign wire to - and +wire to dist.
 
More information-- with engine running
from - post on coil to +post on batt 5.2 volts

from + post on coil to - post on batt 3 volts
 
That isn't a correct NAPA number, nothing shows with that. What I see for 6v flatheads on NAPA's site is either IC7, or IC7SB. Doesn't say anything about internal or external resistor.
 
6V coil should have a resistance of 1.3 to 1.6 OHMs. Some times coils are marked incorrectly and following battery ground does not work. Snyders sells a tester that plugs into the dissy and coil and indicates correct or incorrect wiring regardless of coil markings. I have had NAPA coils that were marked incorrectly.
 
Here is a picture--must be an old piece
 

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how do I measure the resistance. engine runs smooth, no miss for the first 10 to 15 minutes, as it heats up and I'm cruising at a steady speed in any gear I can feel it missing. This has happened before and I could correct it by changing condenser or coil. I think the new Napa coil is an IC7. I have a box with that number on it so I must have tried that coil also. But I was just wondering if the hot to the touch coil is an indication of what the problem is.
 
billfish said:
More information-- with engine running
from - post on coil to +post on batt 5.2 volts

from + post on coil to - post on batt 3 volts
Click to expand...

Numbers are numbers :

A battery voltage at coil of 5.2 volts is too low. Battery voltage should be 7.2 volts on a charging 6 volt system.
So our voltage is either low ( regulator issue /generator) or we have a voltage drop of 2.2 volts from the battery to the coil.
Lets worry about this first , get a voltage reading directly at the battery, if its low we need to fix that issue. If battery voltage running is 7.2 volts then we need to fix the voltage drop from the battery to the coil. Is there another resistor we are missing somewhere , like under the dash.

Then we can troubleshoot the coil....:eek:
 
Bubba thanks for your input--volts with engine running at batt is 6.3 and at coil neg terminal to batt pos terminal is 5.4
 
readings on coil in car and spare coil

both the same

coil + to - terminals --1.5 ohms on both coils

coil + or - to center post on coil== 9,900 ohms on both

I think i'm doing this right, but it looks like my battery reading at 6.4 with engine running is to low should be 7.2
 
Something to understand with generators is they do not charge at engine idle like an alternator does. All voltage readings should be taken with the engine revved up to around 2,000 RPM. Voltage of a charged 6 volt battery at rest or engine idle should be approx. 6.3 volts. With engine revved up the voltage across the battery terminals and at other places in the cars circuit should be at least 7.2 volts and preferably 7.5 volts. When taking these voltage readings it requires some little time for the voltage to rise up to the set reading, say around 2 or 3 minutes, especially if you are testing after a cold start. Regards, Kevin.
 
So at 2,000 rpm I have 7.2 volts at the coil. I guess I'm back to square one, is the coil getting to hot for some reason or is this normal. It's more than warm, I would call it hot but not enough to burn your hand?

Do the resistance readings I posted above look normal? Appreciate the help.
 
With your readings, you have about 4.8 amps at the coil. That is too much. You want it down around 3 amps. You need more resistance in the system.
 
billfish said:
So at 2,000 rpm I have 7.2 volts at the coil. I guess I'm back to square one, is the coil getting to hot for some reason or is this normal. It's more than warm, I would call it hot but not enough to burn your hand?

Do the resistance readings I posted above look normal? Appreciate the help.
Click to expand...

7.2 at the coil is to high if the coil doesn't have a built in resistor. Try
different resistors to get the voltage down to 4.0 volts or enough resistance
until it runs a little ruff then lower it a little. The coil will run cooler when
you get the voltage down. G.M.
 
The ideal coil primary winding current draw should be around 2.5 to 3 amps which will give the distributer contact points a fairly long working life. A 4 amp or more current draw would be too high. To achieve this 3 amp current draw with approx. 7+ volts powering your system requires a specific resistance of the coil you choose to use and plus if required an external resistor in series with the coil primary winding. With 7 + volts the resistance in the coil primary circuit needs to be approx. 3 ohms either made up of the coil alone or the coil plus a resistor. Your two coils you tested were 1.5 ohms each across the primary. You could use either one of these coils but need to add a 1.5 ohm resistor in the coil supply. A good final test is to fit an accurate (preferably analogue) TEST AMMETER (not a dash ammeter) in series with the coil battery supply and run the engine from idle to speed and hopefully read an approx. 3 amp current draw. You can mess around with coils, resistors, and voltage readings till the cows come home BUT in the final analysis the primary coil windings need a 3 amp current draw and no more! The reason your coil is getting very hot is that the current in amps drawn by it is too high. Regards, Kevin.
 
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