Cdi Ignition And Distributor Help Please!!

gulf blue rsr · 08-29-2007, 02:18 PM

After much searching and reading I'm now going to have to ask for help please.........

The car is stripped out and lightened rsr look 1982 3ltr sc

The symptoms are these.....

It all started with the car out on a run in France, we came across a Lamborghini and decided to give chase, car was quite hot and nearing the red line in 5th gear, engine started popping and back fireing so we eased off and tried to let it cool down a bit, it kept coughing and spluttering , generally running rough with lots of back fires and eventually cut. I got out did the usual checking of leads and relays etc found no faults so tried the starter and it instantly ran like a watch

5 miles of gentle driving and same again but this time refused to start!!! no spark so towed to the garage, change cdi box and coil started and ran fine, 2 hrs later same symptoms!!! but this time kept restarting and off again probably 10 times or so then died no more go!! no more spark!

Tow truck home back to the UK

Tow truck company in France managed to get the battery completely flat, recharged it had the cdi tested and coil tested both using info of this forum both check out ok, replaced them back on the car and it ran fine???

Spent weeks with it sat then trying it it goes when it wants and stops when it wants but with no pattern, sometimes when its hot sometimes when its cold, sometimes after starting 5 times in a row it will decide that's it no more! the car is unusable because I never know when or where it will stop!!!

It is now refusing to give spark at all so I think at least if I find something I can fix it, I've changed the cdi box for a mate's known good one, no difference same with the coil, I've replaced the green wire from the distributor, checked all the wiring at the front in the fuse box still no joy!

2 things I'm unsure on and hopefully some body can confirm for me are....

1. The voltage on the + side of the coil... I seem to have 6.1 volts? is this correct? I would have thought 12 volts but then what does the cdi do to this voltage with just the key in the on position engine not turning over? Help!

2. What should I be getting from the end of the green wire? I want to test the distributor but can't seem to find anything on here that tells me should I be getting/testing for Volts...dc/ac or pulses or what? I can get the thing tested with an oscilloscope if that is what I need but what reading should I expect to see and in what form please??

Sorry for the long post but this way you get as much detail as possible to work with and hopefully I can get some useful help!!

Thanks STEVE

Jim Williams · 08-29-2007, 05:26 PM

The output from the CDI box to the coil is a pulse of about 400 volts from the discharge of an internal capacitor. I don't know what steady state voltage that would appear here without the distributor sending its pulsed output to the CDI which only happens, of course, when it is triggered by the distributor.

The reluctor in the distributor sends a positive/negative pulse to the CDI. I don't remember whether is is the positive or the negative part of the pulse that does the triggering. The pulses are about 10 volts in amplitude and can only be viewed with a 'scope, and only with the distributor turning.

gulf blue rsr · 08-29-2007, 11:55 PM

Thanks for that Jim,
I will pass this onto my father (he's the electrical man for me) and hopefully that will make sense and be of use to him!

Thanks again

STEVE

gulf blue rsr · 09-01-2007, 01:57 AM

Just a quick bump,

Planning to work on the car tommorow, my fathers coming over with the 'scope and I was just wondering if the symptoms sound like they could be the dizzy? I could do with some idea's where to look next if the dizzy checks out ok.

any help appreciated

Thanks STEVE

Jim Williams · 09-01-2007, 05:08 AM

Electric-wise, the circuit for the CDI is relatively simple. Look on our host's site at

http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_82SC_Part1-2.jpg.

There you will find the portion of an '82 wiring digram that covers the CDI. On pin 15 comes the voltage to supply the CDI box. "15" is from the ignition switch, anytime the ignition is in the "Run" position, battery voltage should appear here. The black violet lead is to the tachometer.

Carefully check the continuity for all the leads to and from the distributor, CDI box and coil. It sounds as though there is an intermittent connection somewhere within the wiring connecting these units. Wiggle the wiring around while making the voltage and resistance checks.

Hope this helps.

An ohm check of the reluctor in the distributor should show under 1 Kohm or less, at a guess. This is simply many turns of wire with a magnetic field being passed by when the rotor is turning.

Early_S_Man · 09-02-2007, 01:23 PM

The following thread has extensive discussion of the green coax signal cable from the distributor, and references for the CDI unit, including schematics and replacement parts for the CDI unit:

http://66.236.61.177/showthread.php?t=358877

The pickup coil inside the distributor should be 600 Ohms +/- 10% at room temperature.

http://66.236.61.177/showthread.php?t=333640&highlight=distributor+lube

http://66.236.61.177/showthread.php?t=203100&highlight=documentation

gulf blue rsr · 09-02-2007, 03:02 PM

Big thanks to both Jim and Warren,

Today we worked on the car again, last time we tried we put back on the original suspect coil (which we now know is definitely dead!) we plugged the 'scope in and ran tests on the dizzy and cdi box and everything seemed fine up to the coil - still no spark, then we changed back to the blue coil (the one I had to buy when I was in France) and bingo it works again!

So now it's either fixed it's self again or did we fix it??? only time will tell!!!! The only thing that we did different last time we worked on the car was change the ht lead from the coil to the dizzy, we just can't remember if this was before or after or at the same time that we swapped back to the old coil!!

Just as a recap in all we have changed - cdi box, coil and green wire, we have checked and cleaned all fuses and wiring from the front of the car, this whole time the cdi box has always made the whistling so that tells us that it has always had power, the 'scope has told us the the dizzy is fine and using the diagrams and other info from searching this forum the cdi box circuits are known to be good.

Our next plan if the fault is still here (which only time will tell!) is to make up some new wires between the cdi and the coil, cdi and earth and any where else we see fit hopefully that will sort it!

Do you think that there is anyway that this problem could be a fault with the rev counter at all? I only ask as this is the only other thing connected to the cdi box?

Thanks once again for all the help, with it I know we can get to the bottom of this problem until then we shall have to keep trying!! But when it is sorted I shall be glad to let you know, as the problem with a lot of these things is that once people have sorted the problem they rarely post back to the thread to tell of the cure!

STEVE

gulf blue rsr · 09-03-2007, 04:43 PM

Well time has gone quick..........

Got the car out of the garage, went to the petrol station, filled up all fine so far, stopped and started 3 or 4 times, 5 miles up the road park up for half an hour......... you guessed it game over back to no go!!!!!

Cut the wire to the rev counter, this is getting desperate now clutching at straws, no difference, left the car there and got a lift home, I'll go back to it tomorrow and see if it has fixed it's self, if not we shall take the scope and test some more.

Any ideas any one??

Thanks
STEVE