Coil experts

FMAN · Post by **FMAN** » Fri Jun 22, 2007 10:07 pm

Hi,

Sorry for the more complex questions. When

In simple tests

- Look at all the wires going to the CDI box, coil and distributor- disconnect them and see if they look clean and the wire is attached firmly to the connector.
- Does the CDI whistle?
- Flick the points with the ignition on as Tim describes, do you get a fat spark?
- Crank the engine do you still get a fat spark?

I have a more unlikely theory below but try the above test and let us know how you get on.

As Tim said the CDI is not as susceptible to low battery voltage as the normal type points system but has its own peculiarities. Perhaps Tim can confirm as I haven’t had to strip my CDI apart yet, but I assume the points input goes to the base of a NPN transistor with a pull resistor and some protection circuit. What this means is there is a threshold voltage level below which the CDI box thinks the points are closed. If there is a large enough voltage difference between the points earth and CDI earth when cranking it will not see the points close hence no spark. This is why there is a small earth wire between the coil earth and the CDI box. If the main earth strap between the gearbox and the body has some resistance there will be a volt drop over it, the small earth wire from the coil will try to take the current so you might prove this theory if that wire has signs of getting hot. When the engine gets easier to crank the volt drop will reduce and hence it will then spark.

Why did you change the starter motor, was it cranking slowly or just not working? Did this problem start before or after the starter was changed? Does the car charge alright?

Good Luck

Neil

bahama68 · Post by **bahama68** » Sat Jun 23, 2007 12:23 am

I changed the starter because I had trouble starting the car on a few occasions when the engine was hot. It happened a couple of times when I pulled into petrol stations, filled up and then nothing - the car was completely dead. If I rocked the car back and forth in second then tried it again it would start so I diagnosed it as a lazy starter and replaced it. Since then I've had no problem with the starter, but this was about twelve months ago. Then about three months ago this problem started. It seems to have started off as taking a long time to fire up to not firing up at all.

John.

impmad2000 · Post by **impmad2000** » Sat Jun 23, 2007 8:14 am

FMAN wrote:Perhaps Tim can confirm as I haven’t had to strip my CDI apart yet, but I assume the points input goes to the base of a NPN transistor with a pull resistor and some protection circuit. What this means is there is a threshold voltage level below which the CDI box thinks the points are closed. If there is a large enough voltage difference between the points earth and CDI earth when cranking it will not see the points close hence no spark. This is why there is a small earth wire between the coil earth and the CDI box.

Neil,
This is a most useful thread on Pelican for CDI issues.
http://forums.pelicanparts.com/showthre ... ost3333823
Cheers
Tim

FMAN · Post by **FMAN** » Sat Jun 23, 2007 10:02 am

Very interesting.

Thanks

Neil

bahama68 · Post by **bahama68** » Sat Jun 23, 2007 4:27 pm

Firstly, thanks for everyone's help so far on this.

My new coil turned up this morning so I fitted it and not a lot happened. My fuel light was on so I went to the petrol station, got £5 quids worth of petrol, put it in my car and tried again. After about thirty seconds, Bob's your uncle, the engine splutters to life

It would be the understatement of the year to say I'm very happy. I took it out for a bit of a blast around, to warm the engine up, and put a bit of life back into the battery, which probably wasn't a good idea as it reminded me why I really shouldn't sell it. Anyway, I've ordered a new battery, which should turn up mid week then I'll leave it for a week and see if the problem is solved or whether it was just a bit of luck. The CDI box is definitely whirring so that seems to be working. I'll let everyone know how it goes.

John.

charlieboy · Post by **charlieboy** » Wed Sep 05, 2007 1:25 pm

Hi impmad2000 your input as always has been very useful, I am iterested in your test rig, any chance of some details, trig pulse set up etc.

impmad2000 · Post by **impmad2000** » Wed Sep 05, 2007 1:32 pm

Not a simple test rig !! I'll post a picture ! A picture speaks a thousand words !
But it is what it is 'cos its stuff I have lying around.
I've used a PIC micro controller to generate the trig pulse, fed via a FET to trigger a maplin (Velleman) ignition module, and then to trigger the CDI. I was fooling around with the idea of putting some ign mapping in the PIC, hence a motor, wheel and hall effect trigger.
I'll post a picture tonight.
Cheers
Tim

impmad2000 · Post by **impmad2000** » Wed Sep 05, 2007 8:02 pm

This is what I use

Heath Robinson would be one way to describe it. but it does fire the CDI at multiple frequencies though. I too use a real coil and spark plug.
I see you've been on the pelican thread too. The guys there have a more conventional test set up.
IMHO, the only real way to test them is on a real car. That is the only way to be sure that they are not missing. By listening to the sound of the spark, you do start to hear what sound like problems. I have a unit at the minute that will start fine, but once warmed up, the sound changes becomes progressively worse, and then starts to drop to about half frequency.
Good luck

Tim

charlieboy · Post by **charlieboy** » Wed Sep 05, 2007 11:08 pm

Thanks for all the help Tim, gulf blue rsr is my son with the car, I am the so called electronics man.

Yes pellican is very useful.

Have more or less sussed the rig, we have a fully Plasma LCD DVD capable repair workshop so I have all the kit available.

Bit green on these boxes my knowledge is TV and domestic electronics, but am getting there, your input will get me up and running.

The first CDI I looked at has what appears to be the usual 31/1 burnup with the zenner diode short and some other bits further in suspect, it was run with the coil fitted back to front, (a&b reversed), ran the car for 12-15 miles before failing to start after a stop.

Will let you know how I get on.

impmad2000 · Post by **impmad2000** » Thu Sep 06, 2007 8:39 am

charlieboy wrote:The first CDI I looked at has what appears to be the usual 31/1 burnup with the zenner diode short and some other bits further in suspect, it was run with the coil fitted back to front, (a&b reversed), ran the car for 12-15 miles before failing to start after a stop.

On some 3 pin CDI's this is a single Zener, later changed to two in series.
I now also use two in series as I've had the single ones burn out and short again after a short period of time.

Cheers
Tim

charlieboy · Post by **charlieboy** » Sun Sep 09, 2007 1:12 pm

Hi impmad2000,

Firstly thanks for all the help you have given to my father, from a mix of your help and pelican he was able to repair/prove ok, both the cdi boxes, and I know he was very appreciative of the help!

The next step then proved that it is the coil sender in the dizzy that is giving me the intermittent problems, this we have now managed to prove without doubt as when we took it to pieces we could get the coil to test ok and then go open circuit (at least that's what I think he said!! I'm not good with electrics!)

What I need now is the new coil for in the dizzy, I have posted wanted add's for a dizzy on here and phoned all over the place, davidppp says he has one we just need to check part numbers, the problem I have is time, I'm hoping to get the car to silverstone next w/e and so I'm trying to cover all angles of sorting this problem! early s man on pelican has posted before that you can get the coils in the US from http://www.napaonline.com/masterpages/N ... ck-Up+Assy my question is do you know if this is available from some where in the UK? I will be trying bosch tomorrow but as the clock is ticking all help is very much appreciated!!

Thanks STEVE