CDI unit

[John]

Here's one I don't understand on my 78E (Organ Donor). I had gone through the carbs, and reinstalled them. Did the 'Earball' sync of the carbs, and it idled and ran great. Real quick acceleration! Better than my 80SG. I let it warm up at idle, and after a few hard twists on the throttle, it died. I checked power at the coils, had it there, so I pulled a spark plug, reconnected the plug wire, and checked for spark. No spark. Checked the wires to the pick up coils first. BTDT. No broken wires. Made sure the connector was making good contact with the CDI module. Started probing the connectors at the CDI module, using a volt meter, and at the connector for the pick up coils (PUC) I found that when I had the probe touching the wire (white w/ green tracer) leading to the PUC's, and cranked it, it would run, and when I removed the probe, it died. I had .5 volts leading to the PUC's. I was very lightly touching the wires, no pressure involved. Did this several times, same result. Removed the cover to CDI and started probing the solder connections for the terminals for the PUC connector. Same result. Would run when the probe was touching the solder joint, and die when probe was removed. Then to my surprize, one time I did that, and when the probe was removed, it kept running. I shut it down, and tried to restart, it lit right off. Tried it several times and each time it worked properly. Is there any possibility that the meter was supplying the ground for the TCI? Why would it take a constant ground on one of the wires for the PUC's make it run, when that ciucuit gets interrupted by the PUC's?
Anybody ever run into this problem before? What was your finding?

[TopCatGr58]

Hey there John,

I'm no electrical engineer, nor have I seen the innards of the TCI, but from the Schematic diagram and my feeble knowledge reading these things, it's a closed loop to the PUC's and back to the igniter unit, doesn't go to ground with the PUC's ! So...I think you probably still have a corroded or broken solder joint and you somehow put just enough pressure against it to keep it in contact even when you removed the probe. I would recommend resoldering that contact as many folks have done which has fixed their TCI unit !! The actual ground is in the "OTHER" plug going to the TCI...the black wire...according to the schematic !

Just glad it will run....resoldering should fix it!?
T.C.

[John]

TCI

The PUC's are nothing more than a Hall effect switch, and as the rotor passes each PUC it makes interupts the current causing the ign coils to discharge, just like a set of breaker points. Remember them? If you grounded a set of points, the engine would die, as the coils would not build up the secondary voltage. I know this all done within the TCI, but I cannot understand why when I had the meter probe touching the terminal end at the TCI it would run. After it began working properly, I wiggled the wires, and the connector, and had no change.

[TheDjost]

A hall effect sensor, like those that we have in the crank position sensor that we call the pick-up coil, can only supply a very small amount of signal to the TCI unit. Now if there is a bad connection somewhere in the unit, maybe the extra reactance induced by the probe (whether be it capacitive or inductive) might help the transistor trigger. I would just redo all the solder joints one the whole board.

[nashville_bill]

If it happens again, which it probably will, try the meter probe without the meter to see what happens. That way you will know if it is the pressure on the connection or the circuit through the meter.