Is my coil any good?

[Justin71]

What is the best way to tell if your coil is shot?

Recently had a problem where I had a heater hose leak, and it sprayed down the distributor coil really well. After that I noticed sparking from the coil to the heater hose clamp, and rough running when it would spark. Shortly after I found the coil wire came out, and i replaced wires. It's been running rough since. Could I have a bad coil? How can I tell?

Jay

 

[c9zx]

What is the best way to tell if your coil is shot?

Recently had a problem where I had a heater hose leak, and it sprayed down the distributor coil really well. After that I noticed sparking from the coil to the heater hose clamp, and rough running when it would spark. Shortly after I found the coil wire came out, and i replaced wires. It's been running rough since. Could I have a bad coil? How can I tell?

Jay

Spray down the inside of the cap, distributor, and coil female post with silicon spray to displace the coolant. See if that helps. Chuck

 

[Don C]

You'll need a multimeter to accurately check it. The stock coil should measure around 1.40 ohms across the two side terminals (with everything disconnected from them and around 8,000 ohms between the center post and either side terminals. Performance coils may be more in the 0.7 ohms and 11,000 ohms ranges. The coil should draw around 2.5 amps with the engine running, and around 4.5 amps with the engine stopped and the points closed or the negative coil terminal grounded (with the ingition switch turned on).

 

[Wolverine]

Both displacing the water and ohm meter are good bits of advice.

However, you do need to know the specs of the coil you have, and that can be hard to find unless there is a model number on it somewhere.

However, for $30 you can get a replacement coil and hook it up in 5 minutes. And if it doesn't work, you have a spare coil.

You can also MEASURE the primary and secondary impedance of the new coil and securely store this information away. That way, you can properly test your coil in the future....if you can find the information you stored.

 

[caspianwendell]

Both displacing the water and ohm meter are good bits of advice.

However, you do need to know the specs of the coil you have, and that can be hard to find unless there is a model number on it somewhere.

However, for $30 you can get a replacement coil and hook it up in 5 minutes. And if it doesn't work, you have a spare coil.

You can also MEASURE the primary and secondary impedance of the new coil and securely store this information away. That way, you can properly test your coil in the future....if you can find the information you stored.

This is true...coils can vary a lot in resistance depending on brand and they are abundantly available

 

[Justin71]

Thanks. If $30 will surely solve the problem, that's an easy call. And I really appreciate the tech advise above about the various readings... I can take those on the new coil per the advice, and I'll have a good benchmark for future performance. Great advice guys.. glad to have this forum for help!

Now on to the obvious question.... I would imagine I can spend as much as I want on a coil right? I am not looking to race this thing, but I do want to buy a quality replacement. Do I need to think about which coil to get, or will just anything do?

 

[Wolverine]

Do I need to think about which coil to get, or will just anything do?

Yes, you do need to think about the coil.

1. What type of points: breaker or electronic?

2. If electronic, what type?

3. Do you have a tach? If so, is it working/accurate?

BREAKER POINT IGNITION

If it is a breaker point ignition, then you only need a stock NON INTERNAL RESISTOR coil.

The primary resistance of the coil OUT OF THE BOX (measured from the 2 posts, not the center plug) should be 1.5 ohms.

You should also check the resistance of the wire that terminates at the + terminal on the coil. Put an ohm meter probe on the end of this + wire and put another probe on the "I" terminal of the starter solenoid on the fender well by the battery. Turn your ignition key to on, and this should measure 1.5 ohms as well. Turn your ignition key off. If you get a different reading than 1.5, say X, then you should get a coil with a primary resistance of X.

The reason for the resistance in this wire is to assure the voltage across the breaker points does not exceed 6 volts. If the voltage at the breaker points exceeds 6s volts, the points will arc, your car will misfire, and you will probably burn out your coil....ask me how I know.

If you have an electronic ignition, Pertronix, Mallory, HEI, etc., then the coil choice is going to depend on which ignition you have, if you have a tach, and if you want your tach to run or be accurate. Again, ask me how I know. Post this info and we'll see if we can help.

Be as detailed as possible.

 

[Justin71]

Wow. Great info. I'll warn you I should be considered a "newbie" in this area, BUT, I am not afraid, and I love learning about this stuff. In any case, if I ask some "not so swift" questions, you'll know it is nothing other than obvious ignorance.

So far, I know this:

I do not have electronic ignition. It's points and condenser.

I measured the resistance across the 2 terminals. While the center post WAS hooked up, the reading was ~2.0 ohms. hmm.

I measured from the center post to either post (while unhooked from the distributor center post), and I got 9300 ohms. Both are identical at 9300 ohms. hmm.

Now, when I measured the resistance of the "+" terminal on the coil, to the "I" post on the solenoid (other choice was "S" - which always measures "open loop") it measured 0.7 ohms (with the ignition key OFF). When I turn the ignition key ON - I measure 0.00 Ohms. Exactly ZERO ohms (not "open loop"). So that is odd, given what you suggested above.

The condition of things inside the dist cap visually looks good. I will say I only acquired this car in March, and I am still discovering some of the things the PO did with it, and it's real "condition". The car had been sitting for 4 yrs (no driving), and was driven lightly for the 11 years prior to that, having been thru a mild restore approx 15yrs ago. That is about all I know of the history, but most of the tech work was done by shops - best I can tell.

Let me know if this data tells me anyhting, cause right now, I am still scratching my head....

Thanks so much for the help!

Jay


Here are some pics of the distributor and the point/condenser. Looks pretty clean in there....

 

[Wolverine]

Wow. Great info. I'll warn you I should be considered a "newbie" in this area, BUT, I am not afraid, and I love learning about this stuff.

I'll warn you, I am not always right, but I am not afraid of giving bad info. I love it when my ideas blow up people's stuff...okay, not really. I do try to give nothing but accurate information and if I make an error I do go back to my post to correct it.

So far, I know this:

I do not have electronic ignition. It's points and condenser.

I measured the resistance across the 2 terminals. While the center post WAS hooked up, the reading was ~2.0 ohms. hmm.

At least you know how to use a VOM.

I measured from the center post to either post (while unhooked from the distributor center post), and I got 9300 ohms. Both are identical at 9300 ohms. hmm.

Now, when I measured the resistance of the "+" terminal on the coil, to the "I" post on the solenoid (other choice was "S" - which always measures "open loop") it measured 0.7 ohms (with the ignition key OFF). When I turn the ignition key ON - I measure 0.00 Ohms. Exactly ZERO ohms (not "open loop"). So that is odd, given what you suggested above.

The condition of things inside the dist cap visually looks good. I will say I only acquired this car in March, and I am still discovering some of the things the PO did with it, and it's real "condition". The car had been sitting for 4 yrs (no driving), and was driven lightly for the 11 years prior to that, having been thru a mild restore approx 15yrs ago. That is about all I know of the history, but most of the tech work was done by shops - best I can tell.

Let me know if this data tells me anyhting, cause right now, I am still scratching my head....

Thanks so much for the help!

Jay


Here are some pics of the distributor and the point/condenser. Looks pretty clean in there....

 

[Wolverine]

Wow. Great info. I'll warn you I should be considered a "newbie" in this area, BUT, I am not afraid, and I love learning about this stuff.

I'll warn you, I am not always right, but I am not afraid of giving bad info. I love it when my ideas blow up other people's stuff...okay, not really. I do try to give nothing but accurate information and if I make an error I do go back to my post to correct it.

So far, I know this:

I do not have electronic ignition. It's points and condenser.

I measured the resistance across the 2 terminals. While the center post WAS hooked up, the reading was ~2.0 ohms. hmm.

2.0 Ohms is a little high, for reasons explained below...in too much detail.

At least you know how to use a VOM. I imagine people buy the wrong coil all the time, as well as I imagine auto parts stores provide the wrong coil all of the time.

I measured from the center post to either post (while unhooked from the distributor center post), and I got 9300 ohms. Both are identical at 9300 ohms. hmm.

I don't think the ohms being the same is the issue. The issue is what is the spec of the coil to begin with. Also, to be clear, measuring ohms is only one characteristic of the coil. Another characteristic is the integrity of the coil housing. If the housing is bad, even a TINY TINY TINY electrical leak will cause you problems.

Now, when I measured the resistance of the "+" terminal on the coil, to the "I" post on the solenoid (other choice was "S" - which always measures "open loop") it measured 0.7 ohms (with the ignition key OFF). When I turn the ignition key ON - I measure 0.00 Ohms. Exactly ZERO ohms (not "open loop"). So that is odd, given what you suggested above.

Let me restate the measurement you need to make:

0. Label the wire attached to COIL +, and the wire attached to Solenoid I.

1. Remove the wire from the + of the COIL, and attach a test lead to the wire terminal.

2. Remove the wire from the I of the solenoid and attach a test lead to the wire terminal.

3. Ohms at ignition key in off position should be OPEN.

I would expect ohms at ignition key in IGNITION ON position should be 1.5.

The PRIMARY resistance in the coil should match the measurement you get in 3 above(only in FORDS)....for the following reason.

1. You may have heard the saying, water/electricity will always follow the path of least resistance. For water, one measure of resistance is PIPE DIAMETER, for electricity it is OHMS. The higher the ohms, the more resistance. The numbers we are concerned with are

A: Resistance of the wire from the solenoid, through the ignition key, through the resistance(tungsten) wire, out the firewall to the coil.

B: Primary resistance of the coil(+ & - terminals).

2. IDEALLY, when the car was shipped from the factory A=B. As a result, A gets 1/2 (A/(A+B))the power of the circuit and B gets 1/2(B/A+B) the power of the circuit. What is the value of the power: 12 Volts. As a result, B, the coil(and the breaker points) EFFECTIVELY see 6 Volts. This may have been previously said, but the breaker points need to be at 6 volts otherwise they arc and your car runs crappy(ask me how I know).

3. Right now, if your coil is indeed 2 ohms, and your [i Sol to + Coil wire] is .7 ohms, the coil is only EFFECTIVELY getting:

A/(A+B)

.7/(.7+2) =.7/2.7= 7/27= about 1/4 of the power

1/4 of 12 = (1/4)*12=3 Volts

or effectively 1/2 the power(3 volts/6 volts) what you should be getting.

Conclusion: Regardless of what you measure, the coil resistance is high. If you look at Summit Racing site, they usually provide the primary resistance of all the coils they carry.

A breaker point coil needs to run at 6 VOLTS, so there has to be a resistor WIRE(Ford) or a built in coil resistor(CHEVY) to effectively reduce the voltage at the breaker points.

Also, FORD tachs need to run at an effective voltage of 6 Volts. So if you go to an electronic/HEI breaker point system, you may have tach issues(which I am going to eliminate in the next few weeks).

Do you have a TACH?

Keep us updated.

Let me know if this data tells me anyhting, cause right now, I am still scratching my head....

 

[midlife]

"Now, when I measured the resistance of the "+" terminal on the coil, to the "I" post on the solenoid (other choice was "S" - which always measures "open loop") it measured 0.7 ohms (with the ignition key OFF). When I turn the ignition key ON - I measure 0.00 Ohms. Exactly ZERO ohms (not "open loop"). So that is odd, given what you suggested above."

If I take what you said at face value, the only thing different between the two readings is that the key is OFF and RUN. The resistance shouldn't change, but the voltage may. I wonder if you were reading voltage and not resistance. If you were truly measuring resistance, I don't see why it would change. The wire at the I terminal is joined at the firewall plug with the + wire at the coil. From there, the nichrome (not tungsten) wire goes to the ignition switch. The purpose of the I terminal is to provide 12V to the coil only while the starter motor is cranking; otherwise, it is a blind circuit (no current flowing through it). Repeating: the only time any current flows through the I terminal is when the key is in CRANK position.

If you were measuring voltages (inadvertently), when the key is off, some really unusual voltages can be read, but they are stray voltages. A 0.07V between two points in the circuit is not unusual with no voltage applied. When voltage is applied to the circuit, however, the differences between these two points should (ideally) be zero, but typically 0.001 volt differences may appear due to minor resistances built up at connectors.

In any case, your measurements don't make sense to me. Wolverine has many excellent points (no pun intended!)...my one quibble is that the points system is ideally designed to operate at 6V, but anything from 6 to 9V at the coil does not affect point operation or life.

 

[Tnfastbk]

Here you go

 

[Justin71]

OK guys. I am crawling along here. spending more time thinking about this than actually out at the car. But, the last 2 msgs really helped me understand stuff, and I got better measurements.

OK, well I actually took the coil off the car (maybe because it made me *feel* like I was actually doing something as much as anything else), but I did find that I could not get a very solid reading on the primary resistance where I had taken it last night. I could get it to fluctuate by moving the test leads around. So, I tried something novel and sanded the contacts on the coil. Guess what? EXACTLY 1.5 ohms solid. So, maybe the coil is in OK shape..

Now, previously - I did not understand the measurement I needed to make (prob due to not reading carefully enough), but *tonight* I measured the resistance of the "harness from the I term at the solenoid through the ignition and the resistance wire to the wire that hooks on the + term at the coil". Last night i measured from the coil post to the I solenoid post.. NOT right. Tonight- I put the leads on the connector of the wires that go to those places. Sure enough, with the ignition key OFF, I got open-loop, and with it ON?..... 0.3 ohms. Hmm. That should be a higher reading through the "resistance" wire right? Note: this is with the wires disconnected from the posts (which I did not do last night).

So, back to the equation:

with A=0.3, and B=1.5:

A/(A+B)*12

0.3/(0.3+1.5)*12

(0.3/1.8)*12

= 2 volts????

I think this means my "resistance wire" is not producing enough resistance. This COULD be the case, because I see that the PO (or his minion) has spliced the wire near where it connects to the coil. Lots of lovely electrical tape after the original wire exits the harness... (GRR)

It's just an uneducated guess at this point. so what do you think? At least I got better data tonight, and this feels like progress. (and thank GOD I did not use my VOM to read volts when I wanted ohms!)

And IF my theory is correct... what is the solution? do I go get a new resistance wire? splice in a resistor? I need that reading (with the key ON to be approx 1.5 ohms right? to give me 6 volts in the distributor?)

Thanks guys- love the help. It's very educational!

Jay

 

[Wolverine]

"Now, when I measured the resistance of the "+" terminal on the coil, to the "I" post on the solenoid (other choice was "S" - which always measures "open loop") it measured 0.7 ohms (with the ignition key OFF). When I turn the ignition key ON - I measure 0.00 Ohms. Exactly ZERO ohms (not "open loop"). So that is odd, given what you suggested above."

I think there is a misunderstanding where to take the readings from.

You first need to REMOVE the wires from the posts on the solenoid and coil.

You indicate you are measuring at the posts on the solenoid and coil, which is not correct.

Maybe we can skype tomorrow to get this straightened out.

james

 

[Justin71]

No, maybe I didn't explain it well. I DID remove the wires from the posts, and I put the VOM leads to the wires (the second try) where I had put them on the posts (first try, also without removing the wires). When I tested it this way, I got 0.3 ohms..

 

[midlife]

If your two probes were on the connectors that are now disconnected from their posts of the I switch on the starter solenoid and the coil + post, you should measure 0 ohms (+/- .5 ohms due to measurement error), regardless of the position of the key in the ignition. The resistor wire runs to the plug inside the engine compartment, but from there one wire goes to the starter solenoid and the other to the coil. That test of resistance does not measure the resistor wire.

To accurately measure the resistance wire, you'll need to disconnect the battery from the starter solenoid and measure from that battery post on the starter solenoid to the wire that goes to the coil + terminal with the key in the RUN position. You should read 1.5 ohms (+/- 0.3 ohms).

The resistor wire changes resistance when the car is running, choking the current as it heats up and changes resistance. The test for proper resistance is only when the car is not running.

 

[Wolverine]

What was I thinking???? :s :s :s

Midlife is correct. You have no idea how bad I feel....I really do know better. I have attached a diagram and wiring schematic.

FordWiringDiagram_BreakerPointWithTach.pdf

To summarize:

1. [i on Solenoid] to [+ on coil] = 0 Ohms

2. With Battery Red Cable Removed(to measure while car is not running)

and ignition key in RUN POSITION:

[solenoid Battery Post Furthest from Battery] to [Coil + Wire] = 1.5 ohms.

My thanks to midlife...but in all fairness, I did have a disclosure regarding blowing up in my first post. Didn't think anyone would have to take it seriously.