CDI spark unit question

Hi Guys,

I posted a long winded, confusing thread regarding the no spark problem on the front two cylinders of my '86 VF500 F and haven't gotten a response, so I figured I'd simplify my question.
Pulse generators check OK and when the primary leads to the coils are swapped I get spark on the front cylinders, but not on the back 2. All connections have been cleaned. So is it likely that the CDI (spark generator) could function OK for 2 cylinders and not for the other 2? Can you think of anything else that could cause these symptoms?
Thanks!

Jay

 

If you swap primary, you are refering to the high voltage pair right? To be simple, yes you can loose just one side of a cdi spark system, so if you swap the connections at the box and you get spark then its the box. Now think what killed it? Bad coils will still fire, but they take more power to do so. Your vom can not test for a coil and the cdi system looks like either a dead short or a open wire. So swap out another box and check spark color against a black background. It should be a deep blue spark with white on the ends, any yellow or red means bad plug or coil or leads.

Post what you have done and what the spark looks like.

 

The FSM identifies the primary side as the low voltage side that is fed by two spade lugs coming out the end of the coil. The FSM indicates that the resistance between these should be 2.75 ohms. These check OK on both my coils. According to the FSM and several you tube videos, the secondary resistance reading (taken from either of the spade lugs and a plug wire receptical) for a good coil should be around 14 kilo-ohms and mine both show as totally open, but give a hot blue spark. You are saying though, that a coil can't be meaningfully tested with a VOM, so does that mean that I should get some E Bay ones before I do anything else?
I don't see any way to swap the leads coming out of the CDI box itself since they go to a connector in the wiring harness with super small female leads that I can't hook jumper wires to. What I have swapped out is the low voltage connections to the spade lugs on the end of the top coil with those on the bottom coil. Doing that causes the front plugs to spark and the rears to do nothing.
Should I also buy a CDI box on E Bay or perhaps post here on the forum for the sale or rental of a known good CDI unit?
I keep worrying that since the bike appears to have been running on all cylinders when parked a couple of years ago that I'm missing some other issue. Any ideas about this?
Your help is greatly appreciated, there is no way I could get this bike running without help. Hopefully someday I'll be able to return the favor.
Thanks!
Jay

 

You can get a spark from a damaged high voltage side because of how transformers work. Your coils are nothing more than a saturated transformer. If you measure open on the high side then you are not getting continuity and so have little to no available current for which is needed to fire the plug under compression.

A vom can not measure a coil due to the fact it is basically a dead short. Technicians in a shop use a spark intensity meter which is a speacialized meter for measuring both voltage and current out of a coil. This is a rough test and only give an indication of the coil output not state.

I use a simple meter that injects a 1khz ac wave into the coil and reads the reactive capacitance (the reactance). Thus gives me a value that I can go to the stealership with and test a new coil against or from experience say its bad.

Ebay is nice but you might want dealer new if they have them. Since they have waranty, ebay parts usually dont as they came off someones bike. My question is why. Cdi modules can fail and can be taken out by a coil failure. Unless you can rig up a proper circuit to load that unit, your only available test is with an analogue meter (everything happens to fast for a digital)
In ac mode 20v setting, place your probes on one pair of contacts. Crank and watch the needle. It will bounce, note the bounce range and where on the range it is. Now do the other side. Same test, they should be less than 1% different. If you get nothing on one side then replace module.

 

Update: I was checking the coils secondary circuit incorrectly! The FSM has a picture showing the VOM leads touching one primary lead and one of the two secondary leads with the caption saying " the resistance of the secondary circuit should be 14.4 k ohms." Well the correct way to measure the secondary circuit is to check the resistance between the high output leads the plug wires are inserted into. When I did this, both coils checked fine, so I bought both CDI ignition modules on E Bay for $61.00, installed the A unit which was the one leading to the non-working coil and, viola, a nice hot spark to all 4 cylinders. Thanks to Zen Biker who in another post told me that a bad CDI box could cause just one coil to not fire. It is still a mystery to me as to why an ignition module would go bad on a bike that was running fine when stored but I'll not worry about it too much. Now, on to removing and cleaning the carbs.