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28V Screened Coil -- Am I missing something?

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I have a Lightweight 28v FFR  and have encountered an electrical problem when trying to replace the coil.

I've been using Clive's 'Bright Sparks for Land Rovers' fault finding charts but would benefit from somebody checking that I haven't missed or overlooked something.

Filter Box (Test 2)

With 'SW' lead disconnected from coil. Ignition 'ON', test points 1, to 6 all read 25 volts

Screened Coils

I purchased two replacement coils from the UK (one to use and one as a future spare) .  Both new, one still sealed in its vacuum packed plastic bag.

5C10 45120E and 5C10 45120 

Living in Sydney the advice of swapping out the coil for a known functional one isn't an option.

Testing the Ignition Circuit

  1. Connect 'SW' lead to coil. Connect HT lead to coil.  Disconnect 'CB' lead from coil.  Insert screwdriver into 'CB' hole in coil to short-circuit inner contact to base.  Reconnect CB lead, ignition points closed.  Voltage at capacitor/condenser terminal should be 10V but I'm getting 8.85v on one coil and 8.68v on the other coil. 
  2. Reconnected 'CB' lead to coil. Check ignition points gap set at 0.015  With ignition points open Voltage at capacitor/condenser terminal should be 24v but I'm getting 0.77v on one coil and 0-75 on the other.

Clive's charts indicate a fault in the coil but I'm thinking that the chances of there being a fault in both brand new coils is unlikely (isn't it?). 

I'm thinking my fault may lie in the Filter Box. Ballast Resistor and/or capacitors/condensers thoughts anybody........



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IIRC  - the coil is actually rated  28 volt

5C10 45120E

5C10  is a code often associated with Land Rover coils,  is there a NSN on the packaging.

45120  =  Lucas No.  could be prefix  LU/

Suffix E = possibly a slight change in spec. (manufactured over many years)

You may find a date code - search for tiny brandings - abt. 2mm high.

NOS - you would be unlucky if one deteriorated in storage.

The skin-wrapped one - at least you would see any oil loss.

The coils should be equally good for a few reference tests , then check against the original - it may still be OK.




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The data is all good for your Lightweight , you are testing & getting 25 volts on a pair in series.  The voltage would be better a bit higher , one battery could be poor condition pulling the other down.  When charging - you should be looking for near 29 volts.   I would get a discharge tester on both batteries (as singles) to see how how high they are on stated capacity , if sulphation has set in even on one battery - the old old problem (you need a good pair) , the REAL test is turning the engine with starting & firing up, you may be churning the engine & have nothing left to fire the plugs !

With a FFR - not so easy to fault find with screened wires / plugs.  You seem to have assumed you have a coil problem giving your high tension problem.

btw.  I use a DZ dizzy cap modified with bog-std.  12 volt wire set with rubber suppressor caps , then I can fit 12 volt spark plugs along with plug testers (Laser do a set of 4 qty. cheap) - then I can visually check for a healthy spark, speeds the process up.


Edited by ruxy
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Philip 28v is quite an easy mistake to make as specs for modern military vehicles refer to 28v systems rather 24v. I think a running & charging Rover 24v system should be running at 28.5v.

Once the ballast resistor has been drawing current for several minutes, the 10v to the coil drops 9v. It doesn't say that anywhere but I have measured it on a number of systems & that is what happens.

Have you measured the resistance of the coil primary? It should be 2.5 - 2.8 ohms

Have you measured the resistance of the filter box? It should be about 4.4 ohms when cold

It worries me that with the CB points open that you don't get 24v but instead you get 0.77 or so volts. I would check the continuity of the CB screened lead to coil.

If the continuity of the lead is ok, check that the points have been assembled correctly as there is some short circuiting in the distributor base plate assembly. It is unlikely that the condenser has failed & causing the fault, but if it has it would be getting very hot.


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Thank you everyone for your suggestions and wise council (which prevented me from reaching for the big hammer).

The problem has now been resolved.  I went back to the filter unit and:

  • Cleaned the brass spade fitting on the supply to the ignition switch, cleaned the contacts on the capacitors and connection on SW lead coil end.
  • Checked resistance or coil primary (Coil 1 = 2.5, Coil 2 =2.7).
  • Removed and Checked CB lead for continuity, cleaned contacts both ends.
  • Took out Distributor Base plate, thinking that the small internal lead from ignition Coil CB to Capacitor may have been shorting out against the 'U' shaped slot. Insulation was intact.
  • Turned over Distributor baseplate and the cause of my problem was staring me in the face - machine oil!  A very fine film of oil was shorting out the insulated fitting that connects the capacitor to the leaf spring.
  • Cleaned up baseplate, re-assembled and with contacts open I now have 24V at the capacitor!
  • Re-set points gap and the old girl fired up and burst into life, big smiles all round (and charging at 28.4 volts ).

For someone with limited 'spannering ability,' the lesson I forgot and had to re-learn is that in trying to trace faults -Pay attention to the little stuff that might not initially seem important. 

Thank you again.




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Well done Philip, you must feel well chuffed. Although it was painful at the time, you will have learnt a lot & have a glow of satisfaction every time you take her out for a drive.

It is good that you have doggedly slogged away searching for the fault. So often in these difficult times desperation leads to illogical ripping out of components in the hope that the fault will be cured. This results in a pile of components that may be perfectly good but can never be trusted or worse still more problems are introduced by including items that themselves may bring more problems. Not only is that tedious & illogical but it is expensive & nothing has been learnt.

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