cdi

[Gary Brahan 1]

Hello Brian

Thank you for the reply.The cdi unit makes a sound like a camera flash charging when you turn it on then doesent do anything else until you switch the system off then it sparks once.This engine was running fine one or two flips of the prop and it was running Now if you take out the plug and rotate the prop past the sensor nothing happens if you did this before with the plug out it used to spark everytime the magnet passed the sensor but it does nothing at all just sparks once when you switch the power off do you think a replacement cdi like the rxcel or what do you suggest

Once again Thanks for the help

 

Regards Gary

[Brian Parker]

Gary,
The noise from the unit shows that the unit is charging itself.

If you have access to a multi-meter you can test the sensor. With power on, the sensor centre wire(usually red) should show the line voltage (usually battery volts or perhaps slightly lower). Avoid shorting out the leads. Use a pin or similar down the sensor plug case to enable access to the centre cable with the (red) multi-meter lead, multi-meter negative lead to battery negative.

If voltage is present, then transfer the pin/meter lead to the sensor signal wire (usually white or orange) and pass the magnet across the sensor. The meter volts will show a voltage rise and fall as the magnet passes if the sensor is OK.

 If line voltage is absent then the cable or the unit is faulty.

 If line voltage is present then the fault is with the sensor.

(I take it you have tested the magnet?).

'Just Engines' supply replacement sensors for about £4.

 I also see from their Web site that they  stock a ‘CDI’ tester for £7.99
Hope this helps, good luck.

You seem to have spread your postings across two threads, perhaps a moderator will consolidate them into one thread.

[Peter Beeney]

Brian,
    In Gary's other thread, in your post you make the little statement - ‘To have two new CDI units fail is probably quite rare.’
    In the past, and under normal circumstances I would entirely agree with you, but having seen some recent events regarding these type of items I’m beginning to have some slight doubts.
    A colleague’s new petrol engine ran perfectly for a while and then suddenly started to seriously misbehave. It would start, run for a few seconds and then stop. Switching the battery supply off and back on would reset it. To start with he was not entirely convinced so he changed the battery, switch, spark plug etc. but all to no avail. He returned the faulty unit for a new one and happiness was restored. But not for long, now the new one is going exactly the same way. For the moment he’s put the model to one side.
    Of course, this may be a one off, but it might be interesting to see what the failure rate is with these Chinese CDI units. At least, I think they are Chinese! Certainly I think there are also other items, too, that changing faulty ones for new does not necessarily mean that you will get a trouble free result.

    Bring back the Briggs and Stratton contact-less magneto, I say.       PB

[Brian Parker]

Peter,
 As with many imported Chinese items these CDI units probably share a generic PCB across the various ‘brands’. So if you are correct then faulty units could crop up anywhere.

 

However in my experience invariably when two units fail one after the other the true fault remains undetected.

 

Although come the Chinese industrial revolution who quality checks the quality checks?

[Steve Hargreaves - Moderator]

Not well versed in model petrol CDI units but I do believe that turning the engine over & causing the CDI to "fire" without the plug earthed in some way can kill them very quickly as the voltage intended to cause the spark effectively has no-where to go & damages various components in the output stage of the CDI.

 

I understand that best practice is to always earth the CDI to the engine with a wire lead & never turn the engine over with the ignition switched on unless the output (plug lead) is grounded in some way such as through the plug.....

 

As I say I'm no expert on petrol motors but have read a fair bit on them as a prelude to getting involved.....

[Martin Harris - Moderator]

The ground return on a lot of these units is via the plug cap and the shielding on the plug lead.  You need to ensure that the plug is fully inserted (some feel in when they aren't) and that the body of the plug is in contact with the metal part of the plug cap.

[GONZO]

You may find the following test proceedures. for RcExel units, useful. I found them on the RCU forum. One of the experienced contributers comment on testing without the plug connected was that if that caused it to fail then the CDI was no good anyway.

''Take an old expendable servo extension and cut off the male plug end, separate the wires, strip a bit of insulation from each of the 3 leads and plug the other end into the sensor lead on the ignition module. Put a good CM-6 plug in the cap, doesn't need to be all the way seated but far enough for the hex of the plug to make contact with the metal shell of the plug cap, plug a fully charged 4 cell battery directly into the ignition. Test for battery voltage at the red and black wires of the test extension you made and plugged into the sensor lead of the module, if there is no voltage replace the module, if there is battery voltage......Short the white and black wires together, every time you break this connection there should be a spark, no or intermittent spark=bad module, has good spark......Remove the test lead and plug the sensor on the engine into the module and turn the engine over, no spark, replace the sensor, good spark.... Remove the spark plug, look down into the plug cap and turn the engine over, if you see spark arcing through the silicone boot to the metal shell, replace the spark plug cap, no visible spark but you hear a snap.....Put a small screw driver into the bottom of the cap, turn the engine over and you should observe spark jumping from the screw driver to the plug cap shell outside of the silicone boot, an arc of about 1/4"-3/8" if it does this there is nothing wrong with the ignition, if you hear an arc but it's not in the cap with the screw driver it could be a problem with the resistor in the cap or the high tension lead where it goes into the cap, replace the plug cap. ''

Hope this may be of some use to anyone.

Regards.

[Steve Hargreaves - Moderator]

Also probably worth pointing out that these CDI units produce some very high voltages..up to arounf 20-30KV I believe so do be careful not to touch anything when testing the output....that sort of voltage will straighten your hair & stop your pacemaker!!!!