Thread: 3.5 EFI V8 - only 1 single spark

Originally Posted by Slunnie

Just hoping to tap into some better minds than my own...

I have a 3.5EFI V8 Disco1 parts car - it's just a parts car, I'm trying to get it started. Just going through the various checks. The fuel pump is dead, so not expecting an actual start yet.

I've tested for spark, and this is where it gets interesting and am keen to hear your thoughts.

No spark.
Tried a new spark plug, still no spark.
Earthed the plug directly to the battery with jumper lead - the very occasional spark at the very end of cranking.
Checked the distributor cap and rotor - both look fine (to me) with nothing obvious showing.
tested power to the coil+, there is 12v to the + side
Changed the coil for a different coil - the very occasional single spark at the very end of cranking - same as the with the previous coil.
connected spark plug via HT Lead directly to the coil (to isolate out the distributor), spark at the start of cranking and possibly at the end of cranking.
I tried intermittent earthing of the coils-, that did get some sparks at the plug (and also the earthing wire!)

My thoughts are that this has something to do with whatever acts as the points in the distributor..... does that sounds right or could it be something else? Are there parts that are known to fail that cause this same problem?

Thanks for any advice or guidance.

Not familiar with this particular car, but if it was a points dissy I would suggest the condensor, and/or the ballast resistor. So your thinking seems to me to be in the right area. Good luck and let us know.

if its the EFI with the electronics module on the side of the coil its the electronics module failed.

If its points in the dizzy, its the points failed, usually they fail by corrosion becoming partially conductive and preventing the snappy drop in voltage which makes the coil fire off correctly.

How good is the the battery? The starter could be taking all the power.

EFI Range Rovers EG 1991 have 2 wires to the coil, one directly from the ignition for starting (via the starter solonoid) and a white one which is a resistance wire to drop the volts to 7 for running. The usual cause of a spark only at the end of cranking is that the plain wire for 12 volts is broken. The usual cause is simply that the spade connector on the starter solenoid has fallen off due to heat making it lose its temper. So look to see that you have two wires to the positive side of the coil and that there is not a loose wire down near the starter.
Regards PhilipA

Originally Posted by Slunnie

Just hoping to tap into some better minds than my

I've tested for spark, and this is where it gets interesting and am keen to hear your thoughts.

No spark.
Tried a new spark plug, still no spark.
Earthed the plug directly to the battery with jumper lead - the very occasional spark at the very end of cranking.
Checked the distributor cap and rotor - both look fine (to
....... spark at the start of cranking and possibly at the end of cranking.........


Thanks for any advice or guidance.

Originally Posted by PhilipA

EFI Range Rovers EG 1991 have 2 wires to the coil, one directly from the ignition for starting (via the starter solonoid) and a white one which is a resistance wire to drop the volts to 7 for running. The usual cause of a spark only at the end of cranking is that the plain wire for 12 volts is broken. The usual cause is simply that the spade connector on the starter solenoid has fallen off due to heat making it lose its temper. So look to see that you have two wires to the positive side of the coil and that there is not a loose wire down near the starter.
Regards PhilipA

I second Philip's diagnosis and will offer a technical explanation, the HT coil works by the points switching the LT to create a commonly termed "collapsing field" in the HT windings. However this is actually a moving field, so at the start of cranking the field energises and creates a spark, remains stable during cranking, no EMF movement so no spark, and at the end of cranking de-energises or collapses and creates a spark.

Thank you very very much!!!

I'll check that out, thats a really good lead. The wiring diagram wasn't really helping me and the tests I had done weren't really showing anything much unusual apart from that described. I'm really pleased that it has led to that diagnosis, I'll head into the shed and sus it out.

The coil has 3 things plugged into the positive side. Power, condensor and a lead which I think goes to the dizzy. The neg side has 2 things plugged in, one off to the dizzy and one into a loom - not sure what that one does.

My optimism has returned!

The battery is a pretty good one, it has barely been used and it is goes onto the charger between tinkerings. The cranking is pretty good from it, hopefully based on that it has plenty of poke in it to run the ignition too.

Actually, the more I think about this, the more I think it is the "points gap" as you are creating the switching via the key at the start and end of cranking, (although the key does switch between the 12 volt and ballast wire), and when intermittently earthing the coils.

If the "gap" is ok, it could be the condenser/capacitor as it's job is to quench the arc between the points, so if it isn't doing that, then the points have an air gap but the arc is maintaining contact, so no switching. I expect solid state electronic ignition wouldn't need a condenser as no HT air gap involved.

You could try heating or cooling the electronic module to improve a bad solder connection as a test. Options are hot or cold (fridge) water, heat gun, put part in freezer then quickly install it.

I think so too. Good point about the condensor, I checked that was grounding properly too this afternoon, a problem fixed I think but no change. I'm hoping not to go deeper into the distributor than the rotor..... hoping, but I think it is on the cards!!! I'll get under it on the weekend to see whats happening with the wire to the starter also. Hopefully something simple to fix.