Ignition fault finding

[DavidB]

Yes Peter, I mount them on the bulkhead alongside the coil. They are relatively discreet.
David.

[Dave Mann]

I've removed/bridged out countless Bosch distributor rotor arm resistors, they are a waste of time like child proof caps on bottles which children can open and us geriatrics can't open.

[John Mason]

I use Duncan's light bulb method always when setting the contact breaker gap (points).

John Mason

[Peter Naulls]

The distributor is a DK4A so no issues with hidden resistors, but yes, I have replaced the rotor arm anyway.

I have now replaced the condenser with a new, tested one. No difference. Also replaced the distributor base plate with one that was in use a couple of months ago. No difference.

Andrew, I disconnected the coil and put a voltmeter (rather than a bulb) across the low tension wires. As I crank the engine it flicks between 12v and 6v - not what I expected. Does this suggest there is some leakage across the points? I'm beginning to think there is a short in the loom somewhere given that all the other components have been replaced.

Peter.

[Duncan Grimmond]

Are your points opening evenly? I had a worn distributor which opened perfectly on no.1, not so well on 4, very poor on 3 and almost non existent on 2. It got worse as it warmed up.
Much fiddling, no improvement so into the scrap bin (to be cannibalised later no doubt) because I couldn't find the (obvious?) fault and I couldn't face yet another frustrating investigation. Replaced with a Bosch unit and never looked back.

[John Cornforth]

The distributor is a DK4A so no issues with hidden resistors, but yes, I have replaced the rotor arm anyway.

I have now replaced the condenser with a new, tested one. No difference. Also replaced the distributor base plate with one that was in use a couple of months ago. No difference.

Andrew, I disconnected the coil and put a voltmeter (rather than a bulb) across the low tension wires. As I crank the engine it flicks between 12v and 6v - not what I expected. Does this suggest there is some leakage across the points? I'm beginning to think there is a short in the loom somewhere given that all the other components have been replaced.

Peter.

Hi Peter

The DC voltage between the points side of the coil and ground (i.e. across the points) when the engine is turned slowly should be be either nominally 12 volts with the points open or under about 0.2 volts with the points closed.  The voltage from the switch side of the coil to ground will be 12 volts in either state, though it might be a fraction of a volt lower when the points are closed due to the current drain of a couple of amps causing some voltage drop in the ignition switch and wiring to the coil.  I recommend using the starting handle so you can stop at exactly the right point.

Bear in mind that at the instant the points open there is a very short lived 400 volt spike across them, which hopefully your DC voltmeter will ignore.

If you are reading 6 volts across the points when they are closed it suggests to me that either your points have high resistance and their faces need cleaning, or there is a high resistance in the circuit from the earthy side of the points back to the engine block.  In the latter case measuring the voltage from the earthy side of the points to the engine block can be instructive - I would expect less than 0.1 volts if all is well.  I have found when cleaning points that a diamond spatula is good - if you use fine emery it's best to clean things up with switch cleaner (not WD40) afterwards.  A strip of thin card soaked in switch cleaner and drawn between the points is useful.

Cheers,  John.

[Peter Naulls]

Hi Duncan, no, no such problem and the car was in regular use until the breakdown (though I have had this with a faulty replacement cam on another car so I know the symptoms).

John, I'm not sure I fully understand this and I'll need to have another look at my readings but it seems to me that the points aren't making a complete break in the circuit when they are open. It's bitterly cold today though so I'm having an evening indoors. Back to it tomorrow, hopefully with renewed enthusiasm!

Peter.

[Duncan Grimmond]

Good point about switch cleaner, emery is conductive and can cause shorts. Silicon carbide as per wet&dry in non conductive I believe but still needs a good clean afterwards. WD 40 becomes hygroscopic after a while, if you use it in ignition systems it must be as a temporary water dispersant to be cleaned off when you get home!

[daveg]

Andrew, I disconnected the coil and put a voltmeter (rather than a bulb) across the low tension wires. As I crank the engine it flicks between 12v and 6v - not what I expected. Does this suggest there is some leakage across the points? I'm beginning to think there is a short in the loom somewhere given that all the other components have been replaced.

Peter.

Peter,

You need to leave the coil in circuit or use a bulb when you take the measurement.  Your voltmeter will have a very high input resistance, typicaly 10M ohm if you are using a digital multi meter. If you are reading 6V with the points open it implies you have a leakage in the distributor equal to the input resistance of your voltmeter, which is probably negligible.

["Slack Alice" Simon]

Simplifying the system as much as possible: What happens if you connect the coil HT lead direct to a plug, one coil terminal direct to the battery, and one coil terminal on - off to earth using a piece of wire with a bare end?

If spark, then coil and plug ok.

Replace the on - off to earth with the contact breaker, operated by hand ... and so on.

Simon