INTERMITTENT SPARK

[ramon alban]

Hello Tetly,

I;m not sure if you have a Misfire, (which could be a fueling problem), or an intermittent spark.

If the latter, you might try to adapt this cute little process to your problem

Engine stutters/stops then starts and runs on for long or short periods before repeating same.

Rig up two 4/5 watt 12 volt bulbs on test leads into the cabin and connect one between coil +ve and earth and the other to coil -ve and earth. Mark the bulbs POS and NEG. Beware cables dont foul hot/moving parts.

Start the engine and observe:

POS sensing the ignition input voltage is permanantly on,

AND

NEG sensing the pulsed voltage across the Ignition Amplifier flickers or is dimly lit.

NOTE - ChrisJC adds, that an LED + resistor shows flicker better. A 2V 15ma LED needs a 680 ohm resistor for 12 volts, all from Maplins or Ebay.

Drive and have a helper note what happens when engine stalls.

If POS and NEG go out together there is an open circuit interruption of the 12 v feed such as a faulty wire/ignition switch,

OR

A short on the same circuit due to burned insulation.

If NEG comes full on the Ignition Amplifier (due to excess internal heat) or its associated wiring/earth has failed in open circuit mode,

OR

The pulsed input (ie Hall Effect) to the amplifier from the dizzy has failed or the associated wiring is faulty/shorting to earth,

OR

The Coil has failed in a short circuit mode (unlikely cos it would surely kill the amplifier methinks?)

If NEG goes out the Ignition Amplifier has temporarily failed in short circuit mode due to excess internal heat,

OR

the local wiring is shorting to earth,

OR

The coil has failed in open circuit mode due to insulation breakdown or internal heat.

Please feel free, anybody, to point out Errors and/or Omissions of the above (work in progress) process!

Supplimentary:

Early Ign Amps have a finite life of approx 60-80000 miles and must always be refitted with thermo conductive paste.

Condensation inside dizzy plays havoc with electrics, Spray with WD40 and wipe dry, Likewise all external ignition components.

Ramon
http://www.vintagemodelairplane.com

[ChrisJC]

As an alternative to bulbs, get your local electronics bloke to set up some LEDs & resistors. A bulb has a bit of a lag as the filament heats up / cools down, whereas an LED is instantaneous.

Chris.

[ramon alban]

As an alternative to bulbs, set up some LEDs & resistors. A bulb has a bit of a lag whereas an LED is instantaneous.

Good idea Chris!

I've added it to the introduction.

You've been around ignition systems a while, what do you think about the logic/process suggested?

As you'll know, it's not original, I just distilled it from stuff seen elsewhere to suit the system on late 1982 -86 SD1's.

Ramon

[Eliot]

You missed out the prayer to the prince of darkness.

[Dangerdoc]

Hi Tetly,

As previously stated, Ramon is a wizard! And not a sign of anyone called "Harry" in view.

The LED idea is pure genius. Was thinking that thre are loads of pre-rigged 12VDC LED's avail from the dreaded flea bay and maybe use two different coloured ones.

Suggest Red for Pos and any other colour that takes your fancy for the Neg.

That's my splutter.

Doc

[ChrisJC]

You've been around ignition systems a while, what do you think about the logic/process suggested?

Excellent - I think you've got it well covered. The LED trick is exactly what I would do if I had any kind of intermittent fault.

The only thing I'm not sure about is when you suggest that a short circuit coil would blow the amplifier. I would hope that the amplifier has a a protection circuit built in for exactly that eventuality, Not that I've ever tried it.

Chris.

[tetly]

thanks for all your help after a lot of reading & phone calls i was convinced it was not the ECU so went and brought new distributor /cap/leads/plugs connected all up turned key and it fired up little tweak with timing now runing fine again thanks for your help

[ramon alban]

went and brought new distributor /cap/leads/plugs connected all up turned key and it fired up little tweak with timing now runing fine

Hello Tetters,

Well I'll go to the foot of our stairs! In your first post you told us

Have tried new cap rotor arm 2 different distributors with 2 sorts of ignition module STILL got intermittent spark -------- anybody got any ideas

So after two whole pages of diagnostic ideas and creativity in which nobody suggested to buy a new dizzy, you cunningly bypassed the ideas and bought a new dizzy. Now who's the clever one?

Yet I wonder if both sets of original parts being swapped were BOTH carrying the same fault, or if the original fault is still lurking around in a dark corner waiting for a cold wet night on the M4?

Lets hope its fixed, otherwise it'll be back to the flickering LED's methinks.

Well done anyway. Everybody can go back to sleep again - "Foooor nooowww!" (cr, Sparky and his Magic Piano)

Ramon