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Discussion Starter #102
There should be a test procedure in service manual?
CDI is Capacitor Discharge Ignition.
Capacitance testers are uncommon but are available. The diode tests for 'black box' were always relevant when I was working at dealers. The only real difficulty, your not using the exact same meter Honda used in 1978 so expect the numbers to be within 20% of factory spec. I would test generator low speed coil as it was a known failure point, bike would start and run but when turned off would not re-start as low speed charging coil had broken down. As it's a simple continuity test it won't take long (remove left side cover and test at generator plug) It may be worthwhile removing sprocket cover just to make sure wiring didn't get trapped or cut through by drive chain?
There is a test procedure in the FSM for the CDI's using one of 2 analog VOM's, different spec's depending on which meter you use. From the FSM
UPPER ROW: MEASURING RANGE
(SANWA TESTER) x 1k ohm
LOWER ROW: MEASURING RANGE
(KOWA TESTER) x100 ohm
And here's the chart for the later "C" unit.
308592

I have bought both testers, analog VOM's, that need complete overhauls to become functional.
I don't have nor been able to find a new CDI to take readings from with a digital VOM.
That's why I say we don't have valid tests for the CDI.
Now if someone has a new CDI they'd like to send me for testing I'll spend the money to get the meters repaired and reliable so I can verify the test procedure and then run the same tests using multiple digital VOM's.
 

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Discussion Starter #103
I posted the C chart since it's the most current version you see in the CDI's but the earlier A and B version have different specs on some circuits.
 

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That's the chart I was thinking of, first time I've seen it in probably 35 years
You need an old style (analog) meter with a needle. The sample rate of a digital meter can't give correct readings (eg.'needle swings')
Even some more modern bikes need an old style analog meter, it isn't possible to test some fuel pumps or fuel pump relays with a digital (you get no reading or open circuit reading with a digital)
Not sure if OP knows how to use chart?
It's showing the reading when you connect various colour wires, (Brown to light blue, etc) There are diodes inside black box which is why the actual polarity is given (+/-) Majority of meters will have 1.5~3v when measuring resistance but some may have 9v from the internal battery. It's quite rare but a very small percentage of meters can have a 12v internal battery
Real old SUN workshop mains voltage (automotive shop, etc) will probably give correct results if you can find a40~ 50 yr old one in a junkyard or closed down shop (but a $10.00 analog will do just as well. )
One thing I always found somewhat interesting is the reverse EMF going back to the box.
I've measured over 320v on a 12v system (and had a 'knowledgeable' instructor BSing about it when listening in to a 'discussion' I was having)
It's actually simple to understand if you know how an ignition coil works. When secondary coil magnetic field collapses to give spark at plug, it re-induces into primary winding's. If you check coil on oscilloscope. the trace looks like the heartbeat rather than just up and down
You need a peak voltage adapter for meter though (simple enough to make one, there are a couple of diagrams online)
I don't think I would attempt to buy a Snap-On (Bluepoint) adapter though, they were very unreliable. I guess tha'ts what happens when you dictate price to manufacturer? (and why Lucas has such a bad reputation)
 

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Well I m lost confused and fed up... 🥺
New coil reads 5 ohms and old reads 0.8 ohms. Now my cdi may be dead.
great!
Electrical stuff can be tricky. What id do is follow Jim’s detailed instructions and go through it slowly. He made a thread that says what to test and what the spec should be. I think I might make a you tube video about this topic. Worth a shot I’m always one for trying new things. Ps it took me a few times of going bsck and trying to test again before I understood what I was doing and testing
 

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So I have been told by Hendler (the new coil 540712H):
“Hi Ed,
Looking at our suppliers data. Yes, this is the correct coil for your bike. The primary resistance should be around 5 ohms.
Many Thanks,
Hendler”
But then Marcus from Rexs says
“Hello
5 Ohm coils are not correct. We only have one for the Suzuki GT500 that is suitable electrically. The mounting is not right though.
CDI unit is £120, the coil is £34.50.
Many thanks
Marcus Rex”
Where do I go from here?! does anyone want a project?!
 

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These tests are used to check the CDI system. There are no valid tests for the CDI unit so this is an elimination process. If all these tests are good then the CDI is at fault. Before replacing a CDI all these components have to test good.
Use a multi-meter and set it to measure electrical resistance in Ohms.

Coil Test:Primary Side.Measure the resistance between the Yellow and Green wires.
The Yellow and Green wires being measured will be coming out of the spark plug coil.
The resistance should be less than 1 Ohm. Spec is .35 to .55 ohms
Secondary Side: Measure the resistance of the spark plug wires with the plug end caps removed, they unscrew. Insert a probe into each wire. Spec is 7.2K ohms to 8.8K ohms
Plug End Caps: Measure the resistance of the plug end caps. Spec is 5K ohms. Replacement part number is NGK XD05F

Stator Tests:
The connectors used are found under the left side cover
Measure the resistance of the Pink and Green wires.
The Pink and Green wires to measure will be going into the bundle that goes along the frame and eventually to the stator. This is the advancer pickup sensor
The resistance should be about 135 Ohms. New Spec 129 Ohms

Measure the resistance between the White and Blue wires.
These wires to measure will be going along the frame and eventually to the stator also. This is the CDI power source
The resistance should be about 85 Ohms. New Spec 84.2 Ohms

Measure the resistance between the Brown and Light blue wires.
These wires to measure will be going along the frame and eventually to the stator also. This is the primary pickup coil, the aluminum piece outside the rotor
The resistance should be about 207 ohms. New Spec 203 Ohms

The stator ohm readings listed can exceed the old spec by no more than 1 ohm, more than that will have a negative effect on the timing advance.
The New Spec was obtained by measuring 2 NOS stators, fresh unused old stock early model with the bullet Blue and White wire connectors.

Run/Kill and Ignition switch Tests:
Connect one probe to the Black w/White tracer pigtail wire coming out of the main wiring harness that connects to the CDI unit. Connect the other probe to the negative battery terminal. The results should be:
Ignition switch on, Run/Kill switch in Run position: Infinite Ohms reading meaning an open circuit
Ignition switch on, Run/Kill switch in Off position: Zero ohms or close to that showing the circuit is closed/completed to ground
Ignition switch OFF, Run/Kill in any position: Zero ohms or close to that showing the circuit is closed/completed to ground

Alternator Tests: Select/connect to one Yellow wire with a probe. Connect the other probe to one of the remaining 2 Yellow wires, resistance should be below 1 ohm. Repeat twice using a different wire each time. If the readings are good then test each of the Yellow wires with one probe connected to ground. There should be an Infinite ohms reading meaning none of the charging coils is shorted to ground.

NOTE: Some VOM's are auto-ranging meaning they switch ohm scales as needed and some are range selectible. If you have to select the range be sure to do that matching the expected ohm range.


I think I m gonna go back to surfing... 🥺
 

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Discussion Starter #109
The coil you have is definitely incorrect regardless of the seller saying it's correct.
Honda spec's say the same thing as I and Rex's are saying.
 

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Discussion Starter #110
Note: the one offered by David Silver has been reported as having too high of resistance also.
 

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'High' primary resistance is for points ignition, the Suzuki GT500 doesn't use CDI but TPI.
 

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Discussion Starter #113
Electrically that coil is good, you'll have to get creative on the mounting and don't use the ground wire eyelet. It should achieve ground thru the CDI. Don't cut it off until the bike is running though.
 

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Discussion Starter #115
Of course, that's what the forum is all about. Helping people get the vintage Honda twins on the road again.
 
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