Understanding Lightweight FFR 24v ignition system

[fraser61]

I'm trying to convert my 1979 S3 Lwt FFR vehicle to pointless ignition by retrofitting a 24v magnetically triggered module, from Jolley Engineering, as I was under the wrong impression it would be a relatively easy task.. 

So I can understand what I'm up against I thought it prudent to seek any expert advice I may be able to garner from a site such as this one.  I've been told that the ignition system is actually stepped down to 12v from the "black box" mounted on the valve cover to the coil and onward to the distributor. 

Can anyone initially confirm this to be true and if so point me (no pun intended) to any documentation which may show and explain the workings of this system.  Thanks in advance

Fraser

[sirhc]

It should all be here:

 

[ruxy]

The black-box  (filter box)  contains a resistor that drops to 10 volts , so take care of coil selection after that. The rest of guts choke & capacitors are for the rf screening side.   It may be best to use a commercial  24 to 12 volt dropper.   The Jolley Eng.  24v magnetically triggered module  (QUOTE)  , is that 24 volt true & correct  ?      I don't know Jolley ,  Lumenition have the longest/best reputation in this field  IMHO  ,  the Lumenition Optronic is very good and well proven, their Magnetronic  - has the entire conversion contained within the dizzy to maintain Classic under-bonnet visual authenticity , this will be similar to your Jolley.

[fv1609]

On 8/2/2021 at 7:57 PM, fraser61 said:

I'm trying to convert my 1979 S3 Lwt FFR vehicle to pointless ignition by retrofitting a 24v magnetically triggered module, from Jolley Engineering, as I was under the wrong impression it would be a relatively easy task.. 

 

Fraser it is easy. I have fitted a couple of their 24v modules to Rolls Royce 'B' Series engines with no problems. These engines run with a 12v coil & ballast (dropper) resistors in a similar way to a Land Rover albeit at 10v.

A 24v module just needs a steady 24v supply to power it, but it doesn't care whether distributor voltage it is switching is 24v, 12v or 10v. It is performing a similar task to a relay that needs to be energised but can switch whatever voltage is required.

There is no point in trying to eliminate the ballast resistors by providing a separate regulator. Apart from the fact that 24v to 10v regulators are not a easy items to find, those that do try to do this miss the whole point of the design of the primary circuit.

A 10v coil has not been chosen because Lucas or Land Rover were incapable of designing a 24v coil. The purpose of having a coil in series with resistance is that it reduces the Time Constant of the circuit . The Time Constant in seconds = Inductance of primary winding in Henries divided by the Total Resistance (coil res + ballast res)

So the greater the resistance then the lower the Time Constant ie the time for the coil to build up & discharge its magnetism this contributes to a more abrupt change than would be achieved with a 24v coil.

The rapidity in change results in a higher HT output, which was considered helpful to offset the damping effect due to the capacitance of the screened HT cables.

I would try to avoid significant alteration of the original system as that loss of originality will have a serious effect on the value of your vehicle when at some stage you come to sell it. Generally people find 24v systems scary enough but a system that has been modified can be a turn off, which I can understand as I have seen some quite alarming changes done to 24v systems.

[fraser61]

Thanks Chaps for the insights, now I understand better what I'm dealing with the next challenge ideally is to integrate it with the existing hardware, which is where I'm struggling.   The screened cables and connections do not lend themselves to easy modification.

I'm a Brit living in the US and as such my truck is offroaded in the canyons and mountains of Nm, Arizona  and Colorado, up passes in excess of 12,000ft as I use it adventuring.  

I'm not a purist who seeks to keep the engine bay 100% authentic, but I am trying to retain my perception of the OFFROAD robustness of the screened system particularly, but is there any merit in that really? Has any one adapted the existing military ignition system to maintain the robustness or just reverted to the civvy 12v equivalent coil and distributor. 

Thoughts welcomed

Thanks

Fraser

 

[fv1609]

Fraser my first Lightweight was 24v that had been converted by a dealer to 12v. (I now regret not having had a proper 24v vehicle left intact but this was 45 years ago & there was no information available to the public, hence my articles)

Anyway it retained the screened ignition system albeit running from 12v. I used to do a fair bit of mildly adventurous green laning & never had any ignition problems. With the prospect of having to fork out for some new screened plugs, I replaced the whole ignition system with a standard 12v set up. Went green-laning & found that I kept conking out passing over deeper puddles. The alternator fan was spraying water over the HT leads, distributor & plugs.

So I bought some new screened plugs & re-installed the screened systems. Thereafter had no more ignition failures when green-laning.

[ruxy]

The  DZS4A   FFR dizzy is CAV  (the quality end of  "commercial" Lucas)   it is all centrifugal advance and a quality instrument.   25D4  and 45D4  Lucas 12 volt dizzy (according to time-line) & that includes the Ducelier 660 600 'sliding contact' and the  45D4  (41831A)  a special for the Defender  2.5 (2495cc) with 'sliding contact'   - these are all Micky Mouse in comparison.     One common problem I find with the 12 volt distributors is that people press down and bend the top-plate when finally securing the contact set,  you will not do that on a  DZS4A  - it is abt. three times the thickness.   Comparing dizzy in same condition - I can't say it performs better or worse ,  mpg - no difference.     I use a dummy double-cap with basic 12 volt wires bonded in , along with 12 volt engine spec plugs and a 240 volt power-strobe , then re-fit the proper FFR plugs/dizzy cap & screened wires after Ign. timing. You can do similar with a 12 volt strobe.

[fraser61]

So I'm checking the circuit prior to hooking up my 24v trigger module in the distributor as I had a short on my first try and if I put my multimeter on the screened output lead core from the filer box with the ignition on I should see around 10V, but I'm actually seeing around 24V.  Am I making a basic error or what is a typical failure mode for the filter box to cause this.

Thanks

[fraser61]

Sorry Clive I should have re read your guide before asking my previous question

"Note that this is not some sort of 10-volt transformer or converter, it is simply a resistor. When the contact
breaker closes a current flows through the ignition coil and the filter box resistor. The resistor is designed to
produce a voltage drop so that 10 volts will be supplied to the coil. This voltage must not be siphoned off in an
attempt to supply a radio or CD player. When the contact breaker opens there will be no voltage drop and
virtually the full 24 volts will be presented to any accessories that have been attached to the filter box output."