Topic: Need help/advice: How to check spark plug cap resistance

[Mooshie]

I am wanting to check my spark plug caps and if they are still giving good resistance.  I have a CB550F and am assuming the caps are 10K.  I want to use my multi-meter and my understanding is that I can just the ohms on the meter and the probes to get the resistance. 
Here are my questions:
1) On my cheapy Harbor Freight multimeter (which works pretty well for so cheap) I only have 200/2000/20K/200K/2000K as ranges.  So what do I set it to if I believe my caps are 10K? I would just assume go with the 20K but maybe I need to use a less cheap meter that actually has more range choices?
2) Once I figure that out where do  place the meter probes?  One set of instructions I got was to place one probe on the little screw tip inside the wire end of the cap, and the other probe to the spring socket where the sparkplug snaps into the cap.  That sounds so simple but has me confounded.  SO one inside the cap and one on the sparkplug? Both probes in the cap? (there just isn't enough room inside that cap)....
3) My standard disclaimer I am a newbie AND a girl just learning all of this.  I have a basic understanding of things and have done a bit of work on this bike myself but I learn visually so any assistance will be greatly appreciated.

Thank you

[jerry h]

Hi Mooshie,
20k setting on the ohm meter is correct.  You would have to un-screw the plug cap from the end of the spark plug wire, then put a meter lead in each side of the plug cap.  Doesnt matter which side you put the positive or negative lead in the cap.  When you put the plug caps back on the plug wires, you may want to clip off a tiny bit (say 1/4") of the plug wire, so you have a good surface to screw the cap into.

[Stev-o]

Hi Mooshie....I applaud your desire to learn and work on your bike.
And always love your disclaimer - "I'm a girl"!

See HM's thread  (and you may want to upgrade your cheapie HF meter?)

http://forums.sohc4.net/index.php?topic=67096.0

[Deltarider]

and you may want to upgrade your cheapie HF meter?)

Not at all necessary. I know several workshops that work with very cheap multimeters.

If the plugcaps are still connected to their wires, why don't you start the easy way. Insert one probe inside one plugcap and the other inside the second plugcap that is on the same coil (that will be cyl 1 + 4 and cyl 2 +3 on the other coil). You'll probably need to select 200K for this. Report back to us what you read.

If you have the plugcaps seperated already from their wires, you can measure them as mentioned by Jerry (select 20K). With the plugcaps detached, you may also measure the coils by inserting the probes in the wire (where the plugcaps were attached before you seperated them). Please report back what you read. It's easy.

[Mooshie]

I love you guys.  Thanks.  I do not have the plug caps unscrewed from the wires.  I will try using wires for 1+4 and 2+3 and see what happens (that seems simplest).  I kind of like my cheapry multimeter Steve then I can blame that for anything I can't do    Stupid HF multimeter! 

[TwoTired]

Yes, start easy.  Insert your meter probes into the plug caps of the 1.4 coil.  (The cheap meter is fine for this.)
Resistance is additive, and you will be measuring two plug caps and the coil secondary resistance.
Assuming 10 K caps x 2 = 20K plus the ~15k coil, resistance should yield about ~35K, so you would use the 200K resistance scale on your meter to read this value.
If you measure something in that region, call it good to go.
If you have 5K plug caps then the expected measurement would be ~25K total resistance plug cap to plug cap spark plug connection.

I wouldn't trim any off the end of the lead wires unless the plug caps won't screw on tightly.  (If you decide to take the plug caps off and measure caps individually from input to output.)

Cheers,

[Mooshie]

okay I put the probes on 1+4 and 2+3 using 200K ohms setting
1+4= 36.2
2+3= 34.8

So given that I have no idea what that means....  good? bad? indifferent?  Just read TwoTireds post and it would seem that it is fine...should be around 35...correct?

[lucky]

Just use the ohms x 1000 scale.

[Deltarider]

okay I put the probes on 1+4 and 2+3 using 200K ohms setting
1+4= 36.2
2+3= 34.8

Well done, you´ve measured 36,2 Kohms (which is the same as 36200 Ohms) and 34,8 Kohms (which is the same as 34800 Ohms). That leads me to the conclusion you have 10Kohms plug caps and that one or two (the ones connected that read the 36,2 KOhms) might have become a bit older but TT will guide you further.

[Stev-o]

BTW, is the bike (dash?) running fine? Or is there an issue that made you want to check the cap resistance?

[Mooshie]

Yes it is Dash.  I put on a new points plate and he has been running nicely. I even learned how to set gap and timing.   But then I began to have some electrical issues.  Chased it to the ignition switch and just put that on yesterday.  All seems well today.  Thought I would check out the spark plug caps because they appear to be original and figured it would be something nice to learn how to do.  Next I'm going to pull the plugs and see how lovely they look.  After all of this is said and done I'm going to check out the #3 carb which enjoys intermittently spitting out gas from the overflow tube.  Imagine all the time I would have if I wasn't constantly fussing over Dash...

[70CB750]

There is no better spent time.

I bet dash feels rather spoiled by you

[TwoTired]

okay I put the probes on 1+4 and 2+3 using 200K ohms setting
1+4= 36.2
2+3= 34.8

So given that I have no idea what that means....  good? bad? indifferent?  Just read TwoTireds post and it would seem that it is fine...should be around 35...correct?

The resistance values all have a 10% tolerance.  I'd say those coils and caps are serviceable with no need for further attention.  Move on to next task. (Whatever the bike needs elsewhere.) 

Cheers,

[Mooshie]

Thank you Twotired, Steve, and Deltarider for the suggestions on how to do this properly.  Now to stuff I actually know how to do and I will be ready for the 3rd Relay (will there be a 3rd relay?)
One last question (it never ends) when measuring the resistance this way what would an unacceptable tolerance be?  One that would tell me it is time to replace the caps?  I was a girl scout and we like to be prepared too!

[TwoTired]

10% of 10K is 1K

10% of 15K  is 1.5K  ( Measured a couple 550 coils I had in the parts bin of 15.1K and 14.9K)

10% of 35K is 3.5K.   So, if I had readings above 38.5K or lower than 31.5K for the assembly, then I would take the plug caps off the wires and measure the components individually to find what is out of tolerance.  That would tell me what to replace.  (But, I don't know where you'd get a new 10K plug cap.   And, 5k caps will increase coil heating (unquantified), so a change to resistor plugs (an added 5K) would probably be a safer path for coil longevity. imo.

Usually, when the resistors break down, they increase in value and even go open which is infinite resistance.  However, since it is just an extra spark gap in the circuit, you still get spark until the gap gets too large.  You will get misfires and or hard starting at low system voltages when this occurs.

Cheers,

[Mooshie]

Thank you TwoTired, very easy to understand. Now I know how to do one more thing for my beloved Dash.

[bollingball]

Thought I would check out the spark plug caps because they appear to be original and figured it would be something nice to learn how to do.

Mooshie  If they are stock and the out side looks good then I think you can take them apart and change the resistor. TT or HondaMan will know for sure. It may not be worth it. Just a thought I read about  here a long time ago. I just went with the NGK 5K caps. If nothing else take them apart to see how they work.
Ken