Topic: How to test coils?

[Mark K]

'73 CB750K3, coils removed from bike. Is there a way to test them with a multimeter? I did search the forum but 'coil test' returned so many hits. Can someone knowledgeable point me in the right direction?

Thanks in advance!
Mark K

[TwoTired]

If you unscrew the plug caps you can then use the resistance function to measure the secondary of each coil, expecting somewhere around 15k ohms. If your meter doesn’t have auto ranging, then select the 20k scale.

You should also measure each plug cap.  5k, 7.5k, or 10k will all work. They should all be the same, though.

Then Using the lowest scale, probe black and yellow, and black and blue wires looking for something near 4.7 ohms.  Subtract whatever reading you get from placing the probe tips together to eliminate meter lead resistance.

[newday777]

At 2:20 starts the coil testing

https://howtomotorcyclerepair.com/testcoils/

[Mark K]

Thank you both!

[Deltarider]

This kind of testing is just checking for continuity and measuring resistance. The ultimate test is in practice, I'm afraid.

[newday777]

This kind of testing is just checking for continuity and measuring resistance. The ultimate test is in practice, I'm afraid.

Practice? A more in depth reply would be a better response.

[Deltarider]

Maybe 'in practice' is not a one on one translation for the Dutch 'in de praktijk', which (here) means as much as operating for some time, either riding or on the bench in a simulated mode. For the latter you need costly equipment and not many have that. I hope you will except my apologies. Have a nice day!

[bryanj]

Honda used to make dealers have a test cabinet that could do dwell, capacitors, strobe and a coil tester.
Having said that in too many years i have never seen a bad Honda coil, leads broke but they can be fixed

[TwoTired]

Yes the resistance tests are first level tests to see if there is hope of function.  A functional test would be better.  But, few have access to equipment or a set up to do that off the bike.  If the parts pass the resistance test. An on the bike functional test is the next step for most of us, seeing the plug make spark.  Of course, then there is points condenser and wiring to consider which comprises the rest of the ignition circuit, all of which must work in order to make the coils work.  Before google took over YouTube I made a functional bench test video of coils for our bikes. I don’t know if it survived the corporate take over.  And their EULA was too abhorrent for me to support it further.

[Mark K]

My project did not run when I bought October, 2020 and have not idea how long the bike sat, but considering the rust and water damaged parts, it sat outside in the weather for awhile. I'm still cleaning parts removed during the tear down, my goal here is to see if the coils are functional. If they are, I will reinstall and continue testing the other components mentioned by TwoTired The engine is still completely disassembled, so it'll be awhile longer.

[Mark K]

If you unscrew the plug caps you can then use the resistance function to measure the secondary of each coil, expecting somewhere around 15k ohms. If your meter doesn’t have auto ranging, then select the 20k scale.

You should also measure each plug cap.  5k, 7.5k, or 10k will all work. They should all be the same, though.

Then Using the lowest scale, probe black and yellow, and black and blue wires looking for something near 4.7 ohms.  Subtract whatever reading you get from placing the probe tips together to eliminate meter lead resistance.

I tested my coils on the bench. Unscrewed old plug caps and measured resistance on each secondary coil with multimeter set to 20K (no auto ranging on my meter).

Resistance measurements between spark plug leads; numbers are from existing labels on lead wires.
1 & 4 - 14.5 solid
2 & 3 - 20.5 - 22.0 fluctuating

Also measured old spark plug caps.
1 - no reading, cap insulator is broken
2 - 9.3
3 - 10.4
4 - no reading

I have new NGK plug caps, measured numerous times as #2 is lower than the rest, numbers are what I 'assigned' each new cap.
1 - 5.26
2 - 4.78
3 - 5.43
4 - 5.25

Measured the black/yellow and black/blue, using 200, lowest resistance scale. Meter resistance was .6 ohms
Black / Yellow wires = 5.1 - .6 = 4.5 ohms
Black / Blue wires    = 5.1 - .6 = 4.5 ohms

Would anyone care to interpret these numbers for me? I know the old caps are no good and will be dropped in the trash as I have new caps.

Do my coils pass the resistance test?

Is the 4.78 reading on new cap #2 a cause for concern?

[TwoTired]

I expect your parts will work fine.  The coil with 20k fluctuating.  I might trim the leads back 1/8 to 1/4 inch to get a consistent reading.   Or, if your leads aren’t too stiff wiggle them around a bit while you test to see if your intermittent readings gets better or worse.  Do a visual exam on those leads, too.  Cracked insulation is a fail condition.

[Mark K]

Coils installed on bike. Cleaned and installed breaker points assy, they are TEC. Adjusted points to .014 and set the timing as per the service manual, setting 1 & 4 first and then 2 &3. I had a hard time getting 2 & 3 set, as I wasn't getting continuity initially.

Hooked up jumper wires and 12v to test for spark at plugs. Cylinder 1 & 4 both have spark at the plug. Cylinder 2 & 3 have no spark.

Is there a way to definitively test the coil to make sure it's bad before I order a new set? Any and all advice is truly appreciated!

Thanks!

[BenelliSEI]

Swap the 1-4 coil with the wiring for 2-3. If the 2-3 plugs now fire, your problem will be at the points or condenser for 2-3. If not the coil is junk?

[Mark K]

Thanks! I'll try that.

[Mark K]

Swap the 1-4 coil with the wiring for 2-3. If the 2-3 plugs now fire, your problem will be at the points or condenser for 2-3. If not the coil is junk?

Swapped the coil per your suggestion, no spark after the switch. Ordered new coils. Thanks for your help!

[Stev-o]

Hey Mark...what coils did you order?

Many guys have claimed to have bad coils but turned out they were not. I personally have never experienced a bad one in the many SOHC bikes I have had, they are nearly bullet proof. 

[70CB750]

Hey Mark...what coils did you order?

Many guys have claimed to have bad coils but turned out they were not. I personally have never experienced a bad one in the many SOHC bikes I have had, they are nearly bullet proof.

I had HEI coil fail on me in my CJ7 - but it would work fine when cold and fail later down the road.  After half an hour it would start right up.  I wonder if checking the resistance would show anything wrong.  Probably not. 

[CB750R]

I replaced my coils, not because they failed but because I was getting zapped through the plug wires, and while you can do that splice job on the stock coils the Dynas are easier to maintain a good electrical system with.

[Mark K]

Hey Mark...what coils did you order?

This is what I ordered from Vintage CB750. I was originally going to order just the coils, but my brackets and hardware are very rusty and wasn't able to get the screws out without possibly damaging something.

[Deltarider]

[...] the Dynas are easier to maintain a good electrical system with.

Wait till after some years. Rubber caps have become stone hard. Also with Dyna coils you'll have a chance of intermittent loose connections. They look robust, but are not. For more: http://forums.sohc4.net/index.php/topic,191207.msg2225412.html#msg2225412
The best IMO are the OEM ones. Just be careful with the HT leads, like when you pull the caps and they'll go on indefinitely.

[Deltarider]

I had HEI coil fail on me in my CJ7 - but it would work fine when cold and fail later down the road.  After half an hour it would start right up.

Typical for a bad coil, if it isn't the condenser.