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24v Coil trouble


In Second Gear
Hi all! New member here-
I am at my wits' end trying to sort this, not helped by the cold wet constant drizzle up here!!
My 24v L/W conked out, and following the manuals, it seemed that I was getting no High tension current, so, to cover ones butt, replaced coil, cap, points and condenser.
Guess what? Still no HT!!
I am getting current from either side of the brand new coil SW and CB connections, though.
Any suggestions?
Measure the voltage between the CB end & earth with the points open it should read 24v & with the points closed this should drop to 10v. That will show that the primary winding is ok.

With everything switched off, get a little jumper lead, clip one end to the carbon brush in the distr cap, the other end lay it so there is a few mm gap to an earth point. Don't hold it as the HT will leak through the thin insulation. Flick the points open & there should be a spark. So the secondary winding is ok.

The fault could be that there is a leakage between the HT cable & the earthed sheath, you need a megger to test this an ohmeter is NBG. Get a plug out of the engine connect to its ht lead & clip the outside of the plug to earth. When you turn over ther engine there should be a spark. So the leads are ok.

Just because there is a spark in the air, doesn't mean there will be a satisfactory spark under compression. I would ensure your plugs are new or very clean, not with a wire brush but a sand blaster. Also if the plugs are well used the mica insulation may be chipped/abraded & allow HT to earth out.
If it helps i have a complete set of coil filter box leads distributor and plugs on the for sale cant remember what i was asking but of the top of my head i will say £30 plus £10 post.
may be a quick fix plus give you a spare set
Gents, Thanks for the replies.
Clive - I will run through your checklist, what I have done so far is the plug & lead to block,( plugs are in vgc) plus a spare HT lead from coil HT to block.
Neither show sparks, and yet one set of leads is brand spanking, the other known ok.
The SW and CB show a light on a test screwdriver.
The ignition cut whilst in motion.

I'm no sparky, so how would I go about doing a megger test, and what is the kit likely to set me back, or is it possible to get the coil jig tested at an auto electricians?

Mike, interested in you offer for spares - how to proceed?

Thanking you all once again!

Sorry my firtst line was bollox it should have read: "Measure the voltage between the SW terminal of the coil & earth with the points open it should read 24v & with the points closed this should drop to 10v. That will show that the primary winding is ok."

A quick test would be to put your meter or test light on CB, with the points open it should read 24v & with the points closed 0v. If it reads 24v all the time the points are not closing or there is an break in the lead from coil to the distributor. If it reads 0v then the points are closed or one of the insulator washers in the points is incorrectly assembled or there is a short circuit in the centre lead to the earth screen of the coil to distributor lead.

If you have already replaced the coil, its probably unlikely that both are defective. Meggers a acouple of hundred pounds but you can pick them up in their various models of sophistication from £10-£30 at carboots. They are the only way to test condensers as well.

But don't get bogged down in meggers & taking the coil to be tested just see what happens with the simple tests above. Bear in mind when the current flows to the coil it is "charging" it up with magnetism. When you open the points it is the collapse of the magentic field that induces a magnetic field of 10s of kvolts in the secondary but also a few hundred volts in the primary - that is what causes the slight spark at the points as they open. Which is why you need to watch your fingers are not opening & closing the points when you are testing! Use something insulated.
Hi David its all on the for sale section of the forum
I think its under 24V spares

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