Subject: [HUKRC] CDI unit repair
Date: Tue, 20 Jun 2000 13:26:46 -0000
From: [removed]
Reply-To: hukrc@egroups.com
To: hukrc@egroups.com

The following describes how it is possible to repair a CDI unit on a Honda.
I hope it may be useful to some of you.

Repair of CDI unit
This is about a CDI unit which some of you may have in your bikes and how to possibly repair it.
The usual Capacitative Discharge Ignition unit is a sealed box of electronics which lies, electrically between the generator and the ignition coils. They seldom go wrong but when they do fail the only remedy is to buy a secondhand unit or a new one.
I was quoted sixty UK pounds to eighty pounds from breakers and one hundred and eighty pounds Sterling for a new unit for a Honda CB400T. This was frustrating since I could see by the discolouration on the surface of the unit that one of the components of the circuit board had failed.

To cut a long story short this was the component on the end of the circuit which does all the work and transfers the voltage pulse to the coils and is the component most likely to fail first. It is called a thyristor. This is a cascade device and in my case was 10 mm square and about 5 mm thick with three legs for electrical contact. The device passes current across two of the legs until the current drops then switches off and does not pass any more current unless a triggering impulse is received by the remaining leg. This triggering impulse comes from the electromagnetic pick up mounted on the stator part of the generator in the Honda. The voltage ultimately passed to the coils is supplied separately to the CDI unit from the generator.

Now, CDI units are usually sealed in rubber or resin. I was fortunate in that mine was sealed in rubber which could be carefully cut away to reveal the board and components. However, don't despair if yours is in resin because the thyristor may well be mounted at the side or corner of the board (in the corner in my case) since it is at the end of the circuit electrically. If it is, then it should be possible to chip away the resin to get at the three contacts for the legs. A small wood chisel should do the job (use safety specs because resin chips in all directions). Avoid heating the new thyristor when you solder it in.

Many manufacturers have these circuits assembled from unmarked (cheap ?) components. In the case of the Honda I had to refer to the workshop manual to estimate the specification required for the thyristor replacement. You need to find out the maximum voltage supplied from the generator for feeding to the coils - I think it was 400 V for the Honda. Then you need to find out the triggering voltage from the pickup - with the Honda it was 1 or 2 V at a few milliamps. Of course, after replacing the component I had to build a new mounting box for the freely flapping unit since the original case had to be hacksawed off !

I used the following component : TIC126M (a P - gate thyristor from RS supplies in the U.K.) spec: 600 V at max. 7.5 A, conduction angle 180 degrees, case temperature 70 degrees C, triggering or gate 2.5 V at 20 mA (milliamps). The burnt-out original thyristor had no code markings and I am sure had a much closer spec. to the operating conditions.

Cost ? - one UK pound and the bike is still doing fine !

Peter Lamont, Isle of Luing, by Oban, Scotland