1982 CM250C CDI Box - Honda Rebel Forum
 
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post #1 of 8 Old 03-24-2016, 02:49 AM Thread Starter
 
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Location: Moscow, ID
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1982 CM250C CDI Box

I still have no spark...

I pulled my CDI unit out. All the youtube videos I have watched seemed to indicate that you can't test a CDI unit. The procedure they all seem to engage in is to test everything else (coils, stater, wiring, grounds, etc.) and if it all checks out then you blame the CDI unit. However, my Haynes manual has a CDI unit testing procedure and it comes with a test table (see the picture). A couple questions:

1. Can I test this unit?

2. How does "infinity" ohms register on a multimeter? I am assuming it's either "1" or "0" but not sure which?

3. If the answer to question 2 is "0," I am seeing a lot of "1s" as I cycle through the testing. Does that indicate the CDI unit is dead?
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post #2 of 8 Old 03-28-2016, 10:53 AM Thread Starter
 
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It seems to me that the test in the Haynes Manual is a continuity test. However even if there is continuity that does not mean the CDI is actually operating properly. But . . . if there is no continuity then you know it's bad.

In other words, if the CDI passes the test you still don't know if it's good but if the CDI fails the test you can be pretty sure the CDI is BAD?
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post #3 of 8 Old 03-28-2016, 04:53 PM
 
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I've not tested a CDI before but I've used plenty of multimeters, so maybe this helps a little:

Most multimeters have settings for resistance (in ohms) and a separate setting for continuity. The continuity tester only measures if the pins are "connected" or "disconnected" and gives no reading of the resistance between the pins. If you use the resistance setting when you have infinite resistance, (when the pins are not in any way connected) then you get a reading that will be something like "---" or "O.L" on the screen. It will be the same reading that you get when you hold the two multimeter probes separate from each other before you begin testing.

If you want to measure something that has a real resistance listed (for example the KS to EXH pins) then you need to use the resistance setting and should see 5 to 100 ohms according to the chart.

For any that are marked as infinite that just means those pins shouldn't have any connection. I doubt that even a broken CDI would cause those pins to be connected. Effectively the manual is saying you don't really need to test those ones. I would start by testing all the connections that are not marked as infinite. If they appear okay then check the infinite ones.

For any pins that are marked as "0" that means a direct connection between the pins (for example E to P-). For those ones you can use either the resistance setting, or the continuity setting. If you use continuity then test out how it works by holding the probes of your meter together and checking that it beeps/buzzes or gives you some reading on the screen. That shows they are connected. Now test the pins using the probes and if they have 0 ohms you should get the same buzzer going off.

If you use the resistance meter to measure 0 ohms, just be careful to know that you will get some reading that is likely not exactly zero. There is already a little resistance in the probe leads and your meter may auto-range to read in milli-ohms. So you're just looking for a small number here.

One last thing to mention - if your multimeter has old batteries then it can start giving some weird readings for resistance and continuity. If you get constantly cycling numbers it could be the batteries in it, so worth checking that.

Honda Rebel 250CC (1985)
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post #4 of 8 Old 03-28-2016, 04:58 PM
 
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And yes you're probably right that a test can show it is broken, but there won't be a good test to show that it is working.

The CDI is a piece of electronics that contains probably thousands of transistors on a chip. What you're measuring with a multimeter when the bike is switched off is just a tiny part of that circuit. There's no guarantee that the rest of it is working.

Honda Rebel 250CC (1985)
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post #5 of 8 Old 03-28-2016, 08:26 PM Thread Starter
 
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Thank you! That is all very helpful. I meant resistance rather than continuity in my second post. Thanks for pointing that out.

I think some of my confusion on the multimeter stems from the fact that it seems as though different multimeters have different readings for infinite resistance. When I set mine to ohms and the two probes are not touching anything my screen reads "1." I am assuming that a reading of "1" means infinite resistance.

I did what you suggested and tested the prongs that are supposed to be connected. some of the connections on the four-prong box were in the specified range but all of the connections between the 3 and 4-prong box were reading "1." Some would have a reading for just an instant and then go to "1." The instantaneous reading I would get would be in excess of the specifications for that connection and then would almost immediately go to and stay at "1."

Anyone know why I would get a reading for an instant and then go to "1?"

Safe to say the CDI is bad?
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post #6 of 8 Old 03-28-2016, 08:37 PM
 
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Yes if your meter reads '1' when not touching anything then that should be the reading for infinite. They are all different.

There's a couple of possible reasons for getting a quick change in a reading. The first one is that when you have any corrosion (even oxidation of the points on the end of the multimeter) you can find it hard to push through that to get a good solid conduction. Pushing harder or even cleaning the surface with some fine sandpaper can help. The second - and I suspect more likely case here - is that there is some capacitance in the circuit. This allows the circuit to conduct for a fraction of a second when you attach the leads, before shutting off the conduction when the capacitor is filled. Bear in mind that when making voltage and current readings the meter doesn't supply any voltage itself, however when measuring resistance the meter does supply a voltage through the circuit. You'll want to use the reading after things settle down, which in this case sounds like infinite.

I'd guess that if a CDI isn't too expensive then it's worth swapping in a new. At least you'll be narrowing the things that could be wrong!

Honda Rebel 250CC (1985)
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post #7 of 8 Old 03-29-2016, 02:46 PM
 
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Quote:
Originally Posted by alastair1985 View Post
The CDI is a piece of electronics that contains probably thousands of transistors on a chip.
According to:

How to Make a Capacitive Discharge Ignition (CDI) Circuit for Two-Wheelers - Homemade Circuit Projects

a CDI consists of a couple of diodes, a couple of resistors, a capacitor, and an SCR.

Quote:
Originally Posted by alastair1985 View Post
I'd guess that if a CDI isn't too expensive then it's worth swapping in a new.
I've been using a $7 CDI clone off amazon (see My schizophrenic 250) for about 6 months.

1987 Rebel 250. 60K miles. Ex-school bike.
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post #8 of 8 Old 03-29-2016, 04:36 PM
 
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Quote:
Originally Posted by dylan.hedden View Post
The instantaneous reading I would get would be in excess of the specifications for that connection and then would almost immediately go to and stay at "1."

Anyone know why I would get a reading for an instant and then go to "1?"
That is typical of the resistance you would see when your meter is charging a capacitor between the test connections. Capacitors look like DC short circuits instantaneously but as they charge the measured impedance goes extremely high.

Quote:
Originally Posted by dylan.hedden View Post
Safe to say the CDI is bad?
Hard to say, but CDI's contain heat sensitive components that can fail gradually resulting in intermittant operation. The only way to be sure of a CDI is to replace it with a known good one and see if your problem goes away. That's why you have to check everything else first.

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1996 Ducati 900SS CR (California Bike), 1973 Norton Interstate (in a box in the basement)
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