Intermittent Spark

[Range Rover 2 Door]

Greetings one and all.

1986 Range Rover 3.5
Twin Solex 175CD carburettor.
9.35:1 high compression.
Lucas 2CE ignition module (amp under the coil).
Lucas 35DM8 electronic distributor.
69,000 miles (genuine, one owner, great car).

Getting an intermittent spark on all cylinders. Underpowered, doesn't drive very well. Comes to a halt and dies.

Tried replacement/new parts:

Brand new battery (big and strong).
Three coils, one a brand new Lucas DCB 198.
Two ignition modules/amps, one a brand new Britpart module from LRdirect.com (Land Rover no longer supply them).
Two distributors.
Three distributor caps, one a brand new Lucas part.
Three rotor arms, one a brand new Lucas part.
Three baseplate pickups, one brand new (all air gaps good).
Two sets of plugs, the original Champion N9YC plugs and brand new Champion RN9YC plugs.
Three sets of plug leads. one set brand new.
Four coil leads, two brand new.
Additional engine earth strap.

Changed these parts one at a time to see if they made any difference. In all cases they made no difference.

Just got the car and she was driving unevenly, underpowered, sounding like one or more cylinders was down. She died on me several times. She usually starts easily and will tick over reasonably smoothly (even on seven or less cylinders). Increase the revs and she goes flat/uneven/hollow, sometimes "catching" and sounding quite good. First noticed the intermittent spark when I was checking the timing. The timing light went out. Thought it was strange, hadn't had a problem with the light before, suspected the light might be goosed. Disconnected no1 plug lead and put a spare plug in it and earthed it on the rocker cover. That's when I noticed the intermittent spark. Literally, sometimes it sparks, sometimes it doesn't. I should say, when it sparks it's a bloody good spark, hit me few times! If I lift the plug from the rocker cover when it's sparking, I can get a spark of half an inch or more. Strong and blue. Then, as if by magic, it disappears.

The behaviour appears to be random. I have checked all cylinders and the same thing happens on them all. When I start the engine, the plug under inspection might be choosing to spark, might not. She can sit ticking over with the test plug sparking or completely dead. When testing (and I've done a lot of it) they never all fail at the same time, the engine (unloaded) continues to run. There is only one thing I have observed that seems to make any difference, engine revs. If the spark on the test cylinder has failed, I can get it back by revving up the engine. She revs up unevenly and eventually "catches", sounding quite healthy at times, and the plug will spark. Decrease the revs and it appears to be pot luck as to whether the plug will continue to spark. Sometimes it does, sometimes it doesn't.

Replaced all the parts above (one at a time). Made no difference. Back to basics. Took the left rocker cover off to see the valves on no1 so I could check the timing marks and distributor alignment. Removed the spark plugs to ease turning the engine and allowed me to see no1 piston coming up to TDC. (Tip: It was a swine trying to get a socket into the deeply dished crankshaft pulley to turn the engine. Put her in fourth gear and jacked up the front nearside wheel. Was able to turn the engine easily by turning the road wheel forwards) TDC marks are good and the rotor arm points to the no1 segment in the distributor cap when No1 is at TDC on the compression stroke, both valves closed. All leads are going to the correct cylinders and observing the clockwise rotation of the distributor. Set the timing to 6 degrees BTDC at 650 RPM, vacuum advance disconnected.

Thouroughly cleaned all the engine/chassis earths, starter to chassis, block to NS wing (also doubles as the earth for the ignition amp), battery to OS wing and chassis. Fitted a shiny new earth strap between the engine (engine lifting eye bolt) direct to the battery negative terminal.

Checked the voltage at the coil. At all times the positive terminal is getting a tad under battery voltage, approx 13+ volts (when running). Slow tickover, high revs, always the same battery voltage at the coil positive. Tried a power lead direct from the battery positive to the coil positive, still getting the intermittent spark.

I appear to be getting strange readings on the negative side of the coil. According to the official Land Rover workshop manual, with the ignition off, the voltage between the battery positive and the coil negative should be zero. In all cases, with my various replacement parts, I am getting battery voltage here. If I swich the ignition on, I get about 0.1 volts. I don't know what I get when I am cranking because I am working on my own and can't be in two places at one time!

Getting further strange readings on the negative side of the coil. When the engine is running, I am getting 12 volts minimum from the negative coil terminal to earth. I believe the voltage should only be a pulse of 0.1V or so. Further strangeness. When I increase the engine revs, the voltage goes up as high as 19V here. Interestingly, when I tried the original amp again. I still got all the results above but when revving the engine, the voltage stayed around 12V.

The only wires on the coil are from the ignition amp harness. Positive ignition feed is going to the positive terminal. The two amp connections are going to the correct terminals (they really only fit one way). Out of sheer desperation and not giving a monkey's for sizzling anything, I swopped the two amp wires. She flatly refused to start then. Put them back to where they were, and she started first time. Still getting the same problems.

Something else I noticed. When rigging up alead to test for a spark, say the coil lead to the rocker cover, I noticed that when I switch the ignition on, I get one spark. This is BEFORE cranking. Simply switching the ignition on, gives me a spark. Tried switching it off and on, every time I get one spark. Is this normal?

I had a read through this forum, very interesting. Noticed a lot of talk about dodgy rotor arms earthing through the distributor, and solutions involving insulating the underside of the arm or raising the arm slightly. Tried raising it (couple of different rotor arms), made no difference. Put a thin slice of rubber inside the arm, made no difference. I have to say, I have got my doubts about these rotor arms theories, although at one point I figured it was the weak link.

I am getting a very strong spark from the coil lead to earth. I can't tell how consistent it is, it goes too quickly. Same too with a timing light, I connected one to the coil lead and ran the engine, but it flashes so quickly it's difficult to tell if it's missing there.

Checked the lead from the distributor baseplate pickup to the amp (two wires, bullet connectors to the pickup, earthed sleeve). Removed it and cleaned it. Very good physical condition. Good continuity on the wires and none shorting to the earthed sleeve. Earthed sleeve is also earthed on the NS inner wing earth. There are no HT leads near this lead. The connection to the amp is clean and sound and can only go on one way (large and small spade terminals).

All HT leads are kept apart. Flash shields in both distributors are intact and good. Reluctor rings are physically good. Both shafts have no lateral play, one has a touch of endfloat. Problem is exactly the same regardless of which distributor is fitted. Just a point, the spare distributor was from Ebay and was described as "working order", for what it's worth. I can't believe I got a complete distributor assembly at random with exactly the same fault as my original, so I am mainly ruling this out. On my original distributor I thouroughly cleaned it and lubricated it. Mechanical advance weights move as they should and the springs are good. All three baseplates I have tried, move freely and there is no fouling of the pickup leads, inside or outside the distributor.

Naturally I haven't considered any fuel issues as these won't cause an intermittent spark on a dry plug outside the engine, but as a matter of routine I have cleaned out the fuel tank, filter and carb float bowls (one of the bottom nuts on the carbs is a total swine. Had to chop the end off a spanner and use a ring from another spanner to get it off! Next time get a curved spanner!).

I would love to try a points distributor on it, to rule out the electronic wonder-gizzmos but that could be a pain for me, getting the correct oil pump drive (needs the wobbly female type (don't we all!)), ballast resistor and coil issues too potentially. My most likely next course of action is to try an A+R amp.

As you might imagine, this is driving me nuts, appreciate your thoughts.

[Ian Anderson]

How much play is there in the rotor arm when fitted in the ormal position.

It may be dizzy drive is not good and allowing play / rptation of engine without same movement in rotor
spark would then not be going to correct pin on the distributor / missing intermittently the chosen lead.

I have heard that the timing gear and chain still function with only stubs on the sprockets but would give wierd timing

Ian

[CastleMGBV8]

This I.m afraid and you kick yourself for all the hassle youv'e gone through may be a very simple problem. it's certainly worth a try anyway.

Apparently some of the replcement rotor arms sit a little low on the shaft and therefore can have a bad connection with the connector in the cap which would explain why there seems to be a consistent spark from the coil lead.

Try putting a little packing inside the rotor arm to raise it slightly on the shaft so it makes a better connection with the cap, only take 5 mins and you never know it might do the trick.

Kevin.

[ramon alban]

Getting an intermittent spark on all cylinders. Underpowered, doesn't drive very well. Comes to a halt and dies. Tried replacement/new parts:

Brand new battery (big and strong).
Three coils, one a brand new Lucas DCB 198.
Two ignition modules/amps, one a brand new Britpart module from LRdirect.com (Land Rover no longer supply them).
Two distributors.
Three distributor caps, one a brand new Lucas part.
Three rotor arms, one a brand new Lucas part.
Three baseplate pickups, one brand new (all air gaps good).
Two sets of plugs, the original Champion N9YC plugs and brand new Champion RN9YC plugs.
Three sets of plug leads. one set brand new.
Four coil leads, two brand new.
Additional engine earth strap.

Changed these parts one at a time to see if they made any difference. In all cases they made no difference.

Hello Chris, (?) big assumption, sorry about that, but then, so is your intermittant spark diagnosis.

When the engine dies, thats not intermittent, that's full on "dead". Ignition would then display nil sparks so it would then be a cinch to diagnose by substitution - methinks.

BUT, lets say it is ignition, and as Mr Holmes said to Mr Watson, when you have eliminated all the possibilities, then what ever remains, however improbable, is the culprit.

In your case have a look at this circuit:



Yes, its for Efi (mostly the W/B outputs) but the rest of it applies to your setup too, and note that the only thing you have not mentioned is the Suppression Condenser.

This little jigger can break down when pooped, due to the "back emf" from the coil current collapsing, finding an alternate partial path to earth and inhibiting the generation of reliable sparks by the amplifier/coil combo.

But then again if this whole deal is not an ignition issue, and the observation of missing strobe flashes is just a trick of the light, or the efficiency of the strobe pickup it leaves the possibility of it being a fuel issue.

ie, " Underpowered, doesn't drive very well. Comes to a halt and dies."

I know nothing about carbs, but those symptoms could well apply to fuel flow, crud in the tank, fuel lines, filter blockage, mixture set-up rich or weak, carb behaviour, manifold air leaks, temperature sensitive components, blah, blah, blah.

Yes, you may have eliminated some of that already, its just a list of things to look at - as you said ----

"Naturally I haven't considered any fuel issues as these won't cause an intermittent spark on a dry plug outside the engine, but as a matter of routine I have cleaned out the fuel tank, filter and carb float bowls"

---- but I would keep going on the fuel issues for a while yet, so you'll need a carb person to go a bit further perhaps?

Just my three pennyworth, but good luck anyway.

[Range Rover 2 Door]

How much play is there in the rotor arm when fitted in the ormal position.

It may be dizzy drive is not good and allowing play / rptation of engine without same movement in rotor
spark would then not be going to correct pin on the distributor / missing intermittently the chosen lead.

I have heard that the timing gear and chain still function with only stubs on the sprockets but would give wierd timing

Ian

Hi Ian.

Thanks for your reply.

No obvious play. Even if there was, when setting the timing dynamically, wouldn't the slack be taken up? This theory would also mean that both distributors have the same wear on the drive gear. Not impossible but I'm disinclined to believe that the replacement distributor I got at random has exactly the same fault as my original.

[Range Rover 2 Door]

This I.m afraid and you kick yourself for all the hassle youv'e gone through may be a very simple problem. it's certainly worth a try anyway.

Apparently some of the replcement rotor arms sit a little low on the shaft and therefore can have a bad connection with the connector in the cap which would explain why there seems to be a consistent spark from the coil lead.

Try putting a little packing inside the rotor arm to raise it slightly on the shaft so it makes a better connection with the cap, only take 5 mins and you never know it might do the trick.

Kevin.

Hi Kevin, thanks for the reply. I'm praying for a "kick myself" moment!

You can kick yourself now, I stated in my original post that I raised the rotor arm! Two theories here, one to get nearer the carbon brush, two to get an air gap or room for insulation material to prevent the dodgy rotor arm earthing through the distributor shaft.

[Range Rover 2 Door]

Hello Chris, (?) big assumption, sorry about that, but then, so is your intermittant spark diagnosis.

Hi Ramon. It's Charles, actually!

Nothing could be clearer. This is an intermittent spark. I can see it with my own eyes. I can see it on a test plug and I can see it on my timing light (an old fashioned one that connects in series with the plug lead). I can see it in the wet plugs that haven't been firing and I can see it in the raised oil level, reeking of petrol, from the unburnt fuel going to the sump due to the lack of sparks.

When the engine dies, thats not intermittent, that's full on "dead". Ignition would then display nil sparks so it would then be a cinch to diagnose by substitution - methinks.

Sorry Ramon, that's semantics. Despite writing a meaty first post, I wasn't clear enough. She only "dies" when driving. As stated in my post, under test conditions in the garage, she never dies, she never fails on all cyinders at the same time, she always runs albeit with sparks coming and going lottery style.

When I say she "dies" on the road, I mean she is so underpowered that she cannot drive and I come to a halt. Sometimes the engine stalls, sometimes it doesn't. It always starts again though. Classic lack of power symptons, in this case caused by an intermittent spark on all cylinders. That I know. Nothing could be clearer. It's the cause I'm after!

Indeed, it's easy to fix something that doesn't work, it's a bugger to fix something that works sometimes!

BUT, lets say it is ignition

As before, no question of it (unless I have a very serious problem with my eyes!).

In your case have a look at this circuit:

Yes, its for Efi (mostly the W/B outputs) but the rest of it applies to your setup too, and note that the only thing you have not mentioned is the Suppression Condenser.

This little jigger can break down when pooped, due to the "back emf" from the coil current collapsing, finding an alternate partial path to earth and inhibiting the generation of reliable sparks by the amplifier/coil combo.

That's interesting Ramon, thanks for the diagram. I don't have a Suppression Condensor, should I have one?? I stated in my first post, I only have two conections on the coil positive, one from the ignition switch (white) and one going to the amp (white/black). I only have one connection on the coil negative (white/black), it goes to the amp.

But then again if this whole deal is not an ignition issue, and the observation of missing strobe flashes is just a trick of the light, or the efficiency of the strobe pickup it leaves the possibility of it being a fuel issue.

No question, it's an ignition issue. That said, my strobe light could have packed in at the same time all my test plugs packed in, just at the time I got this car.

Just my three pennyworth, but good luck anyway.

Greatly appreciated. Wasn't it your good self that wrote the excellent article about using test bulbs/LEDs connected to the coil to highlight amp failures? I was kind of hoping to get a reaction to my coil voltages. Any thoughts?

Cheers.

[ramon alban]

Great, Now Charles, lets go over some of that stuff. Observations in RED.

Nothing could be clearer. This is an intermittent spark. I can see it with my own eyes. I can see it on a test plug and I can see it on my timing light (an old fashioned one that connects in series with the plug lead). I can see it in the wet plugs that haven't been firing and I can see it in the raised oil level, reeking of petrol, from the unburnt fuel going to the sump due to the lack of sparks.

Wet plugs and contaminated oil might also mean an over rich mixture, so bad, that it starts ok when cold, then dies as it warms up. Wet plugs will also quench individual sparks such that the spark detecting lamp may not register it if the high voltage goes strait to earth via a contaminated plug. Just keep an open mind, Eh?

Sorry Ramon, that's semantics. Despite writing a meaty first post, I wasn't clear enough. She only "dies" when driving. As stated in my post, under test conditions in the garage, she never dies, she never fails on all cyinders at the same time, she always runs albeit with sparks coming and going lottery style.

Still with an open mind, Chas? An engine off load in the garage may seem to run so-so ok and only under load gives up the ghost because the mixture is so bad the lousy combustion yields no usable torque.

When I say she "dies" on the road, I mean she is so underpowered that she cannot drive and I come to a halt. Sometimes the engine stalls, sometimes it doesn't. It always starts again though. Classic lack of power symptons, in this case caused by an intermittent spark on all cylinders. That I know. Nothing could be clearer. It's the cause I'm after!

The cause? Hmnnn? I'm really sorry but its beginning to look less like sparks all the time. You have already replaced everything virtually triple times over so unlike Playtex bras, conincidence can only stretch so far. What would you do if you replaced everything a fourth time and you still saw these missing sparks. Come back for a fifth replacement? Please look beyond the sparks for just a while.

But staying with your convictions a little longer, you could set some in-car ignition monitor as you know with lamps/leds etc and see what if anything is actually dying.

Indeed, it's easy to fix something that doesn't work, it's a bugger to fix something that works sometimes!

As before, no question of it (unless I have a very serious problem with my eyes!).

The eyes may indeed tell of missing sparks, thats true, but what if the cause was quenching as mentioned, thats not an eye problem, its predjudice.

That's interesting Ramon, thanks for the diagram. I don't have a Suppression Condensor, should I have one?? I stated in my first post, I only have two conections on the coil positive, one from the ignition switch (white) and one going to the amp (white/black). I only have one connection on the coil negative (white/black), it goes to the amp.

Well, theres a thing? The trubble with new information it often invalidates all that goes before. So no suppression condenser? What follows is my interpretation of its function from my, as yet, unpublished essay on ignition systems.

A Condenser (or Capacitor) connected into the low voltage circuit across the coil/amplifier combo to suppress Radio Frequency signals otherwise destined upset in-car entertainment systems and offend un-neighbourly soap addicts watching their TV’s. It is also perceived to assist in rapidly collapsing the coil primary voltage for a sharper high voltage discharge.

Note. that because it has this perceived second function of aiding spark generation, please fit a condenser and report back.

I'm the interested party here because that second function is not defined anywhere that I can discover so the perception is mine, thus far, and I'm very interested in a real live case study to show up its validity or otherwise.

No question, it's an ignition issue. That said, my strobe light could have packed in at the same time all my test plugs packed in, just at the time I got this car.

Nah! I doubt that coincidence and you are very likely correct, after all, you have the benefit of being the observer, whilst we forum infesters only see alternatives to other peoples' cast iron certainties - dont you know?

Unfortunately closed minds tend to cut off lifelines. Anyway keep plugging away.

I was kind of hoping to get a reaction to my coil voltages. Any thoughts?

Now, untill a few weeks ago I knew nothing of ignition systems apart from the sparky bit had to be properly timed, so I never worked with the voltages before nor now, but lets look at your results from the earlier post from the simple process of expectation.

Checked the voltage at the coil. At all times the positive terminal is getting a tad under battery voltage, approx 13+ volts (when running). Slow tickover, high revs, always the same battery voltage at the coil positive. Tried a power lead direct from the battery positive to the coil positive, still getting the intermittent spark.

As one would expect

I appear to be getting strange readings on the negative side of the coil. According to the official Land Rover workshop manual, with the ignition off, the voltage between the battery positive and the coil negative should be zero. In all cases, with my various replacement parts, I am getting battery voltage here.

So with ignition off you get 12/13 volts at coil neg, therefor something is feeding that to coil positive methinks, suggest you try to find out from where. because the basic circuit says only the white ignition wire feeds the coil. Check your workshop manual circuits for variances, I dont think there should be any.

On the other hand perhaps you have a different electronic ignition amp that has a permanent feed and that voltage is being seen at coil neg via the amplifier innards. Easy to check by taking wires off one at a time.

Alternatively take out system fuses one by one to see if the voltage mentioned will go away.

Then again does the car have an alarm system that is permanently on when everything else is off and it does its job by communicating with the ignition system somehow?

So many possibilities so little knowledge!

If I swich the ignition on, I get about 0.1 volts. I don't know what I get when I am cranking because I am working on my own and can't be in two places at one time!

Thats prolly ok as the amp is switched on and the coil is passing current. Also explains why at switch-on you see a single spark.

Getting further strange readings on the negative side of the coil. When the engine is running, I am getting 12 volts minimum from the negative coil terminal to earth. I believe the voltage should only be a pulse of 0.1V or so. Further strangeness. When I increase the engine revs, the voltage goes up as high as 19V here. Interestingly, when I tried the original amp again. I still got all the results above but when revving the engine, the voltage stayed around 12V.

You might want to verify these because it will depend upon the ability of the meter to average what it sees and digital voltmeters are really bad at this because they only sample their readings.

An anlogue meter (needle) will show a better average and it will read something between 0 and 12/13 volts changing with engine speed.
Frankly IMHO its not important as long as it is neither nil nor battery volts.

Reason I'm not too sure about that last bit, I have not done the research yet, just top of the head stuff.

So there you go, just following clues I'm afraid.

[Range Rover 2 Door]

Now, untill a few weeks ago I knew nothing of ignition systems apart from the sparky bit had to be properly timed

Hi Ramon.

Thank you very much for taking the time to reply. Very good of you.

I first started playing around with sparky bits as a lad, 30 years ago on lawnmower engines. Take my word for it, it's an intermittent spark. It's the solid-state electronics I know little about. I usually find it easier just to replace them.

I will get this car running, you can be sure of that, and I will report back. I was hoping to get some definitive information re my coil/amp voltages. You have given me some pointers.

Cheers.

[ramon alban]

I first started playing around with sparky bits as a lad, 30 years ago on lawnmower engines. Take my word for it, it's an intermittent spark. It's the solid-state electronics I know little about. I usually find it easier just to replace them.

OK - I give in and promise not to stray off topic any more. No more fuel nonsense - right and I wont ever mention quenched sparks at the plugs ever again, nor the likelyhood that an engine with poor mixture can run just fine off load but lack any torque when loaded?

In the meantime check this SD1 efi ign module image aganst your module behind the coil, for any component similarity - you may be surprised.



And also check your actual circuit against the diagram on my earlier post.

I will get this car running, you can be sure of that, and I will report back. I was hoping to get some definitive information re my coil/amp voltages. You have given me some pointers.

I guessl 31 years in electronics with TI tends to help with sorting thro' the odd voltage question.

Did you know we used to make the power transistors for those early modules and the also the mid '80's ECU's.

[Range Rover 2 Door]

Thank you very much, Ramon! I'll check it out.

Cheers.

[ChrisJC]

OK, some random thoughts (some might highlight that I haven't read your original missive thoroughly enough though!)

- You need more instrumentation. I would fit two LEDs, one to coil +ve, one to coil -ve (with appropriate series resistors). I would then expect to be able to tell if the low tension circuits were in order (or not) as one LED should be permenantly on, the other should flicker regularly.
- Measuring coil -ve with a voltmeter (which it sounds like you are doing) is pretty meaningless on a running engine. The voltage will move between 0V (when the amp is charging the coil), and about 400V at the instant that the amp turns off. The only meaningful measurement is that with the ignition on, the coil -ve should be at the same potential as the coil +ve (i.e. +12v). And of course with the ignition off, it should all be dead.
- To examine what coil -ve is doing, you need a storage oscilloscope. That way you can log the voltages, and make sure that there are no missing pulses etc.
- The LR workshop manual makes sense, it is basically saying the same as I have above regarding the only sensible measurement with a voltmeter.

My suggestion is the 2 LED trick, to eliminate or otherwise the low tension circuits. You might need a big zener diode potentially on the coil -ve resistor to prevent the 400V spike over-currenting the LED.

Chris.

[Ian Anderson]

Start it at night
Let it run and see if you have arcing anywhere around the distrbutor cap and leads

Could be something that simple and just need to move wires apart or away from chassis / rockers

Ian

[Range Rover 2 Door]

Hi Chris.

Thanks for that.

- You need more instrumentation. I would fit two LEDs, one to coil +ve, one to coil -ve (with appropriate series resistors). I would then expect to be able to tell if the low tension circuits were in order (or not) as one LED should be permenantly on, the other should flicker regularly.
- Measuring coil -ve with a voltmeter (which it sounds like you are doing) is pretty meaningless on a running engine.

What's the difference between using an LED on the coil negative and a voltmeter? How come one is recommended and the other meaningless? I don't have any LEDs to hand and figured it was worth seeing what was going on there. My readings were all 12V or more. Wouldn't that make an LED light full on, indicating a problem?

What about the reading from battery positive to coil negative (ignition off). According to the Land Rover manual, there should be zero voltage. I get battery voltage.

Thanks again.

[Range Rover 2 Door]

Start it at night
Let it run and see if you have arcing anywhere around the distrbutor cap and leads

Could be something that simple and just need to move wires apart or away from chassis / rockers

Ian

Hi Ian.

Did that before. In a pitch-black lockup at night. Not a glimmer.

I have decided to try a points distributor in it. I will report back.

Cheers.