12V ignition on 24v 101 - please explain?

[Lotz-A-Landies]

I'm just trying to update my knowledge on the 101.

IIRC the radio and electronic warfare versions of the 101 still have a 12v ignition system.

Can someone explain how this is achieved?

Is it via a 24V to 12V ballast resister
a tap off only one battery (with a relay)
a DC to DC power supply
something else

[Mick_Marsh]

Well, It's not 2 or 3.

I always thought it was a 24v coil but there is a little box of electrical wizardry in series.
I'll have to look at the wiring diagram.

[Lotz-A-Landies]

I remember working on the fuel tank sender on the ex-RTC 101 at the Engineers Museum. The new sender was a standard 12V one so somewhere in the system there is a 24v -> 12V drop. But I am only assuming that it also included the ignition but may have just been the gauges.

[Mick_Marsh]

I remember working on the fuel tank sender on the ex-RTC 101 at the Engineers Museum. The new sender was a standard 12V one so somewhere in the system there is a 24v -> 12V drop. But I am only assuming that it also included the ignition but may have just been the gauges.

I've just bought a fuel thank sender. It has 24V on the box.
The sender is voltage independent anyway. It's the resistance that is important. It's got to match the resistance of the gauge. The 12v 101s have a voltage regulator on the instruments (10v I believe) but the 24v 101s do not.

[101RRS]

Yes the ignition on all 101s is 12v (well actually about 10v) - on the 24v volt versions like our GS models - next to the coil there is a box about 3" x 3" and inside is some hi resistance wiring that drops the voltage form 24v to 12v. The voltage needs to be read with the coil in the circuit - if you measure the voltage just across the resistance unit it does not read 12v but something higher but with the coil (not resistor type) in the circuit the voltage is about 10v. If you covert the ignition to 12v you need to use a resistor coil or a ballast resistor to get the correct voltage.

This is the box.

130123

The fuel pump is also 12v with a dropper also located on top of the tank (so RRC pumps can be used). I thought the fuel sender is 24v though that is why Series 3 instruments cannot be used.

Garry

[Lotz-A-Landies]

Good thing you can use a 12V fuel pump. I recently had to purchase a 24V round Facet Gold Flo and it cost over $400.

[Mick_Marsh]

The fuel pump is also 12v with a dropper also located on top of the tank (so RRC pumps can be used).
The "dropper" at the top of the tank is not actually a "dropper". It is a suppressor to supress the radio signal generated by the pump. Mine has been removed. The pump operates on 24V.
The Perentie has the same suppressor on the heater fan.

[101 Ron]

Without checking I thought the fuel pump was 12 volt and the voltage dropper was hidden in the space under the spare wheel.
My 101 has-been converted to 12 volt and the 24 volt stuff removed.
Most 24 volt suppressed British ignition systems run 12 volt coils though a voltage reducer, the reason is to reduce the capacitance effect of suspressed high tension leads which even with 24/12 volt and points produces a feeble yellow spark at the plugs.
If it was a full 24 volt system you would be lucky to get enough spark to run the engine properly.

[101RRS]

The pump operates on 24V.


I could have sworn that is not correct but the more I think about it I think you are correct. What I am getting confused with is that the one one pump fits both 24v and 12v vehicles as the one pump can work on both voltages. I discussed this with AJ a couple of years back as the one he sent to me at the time was a 12v unit and I questioned whether it was suitable.

In the parts handbook it lists different fuel gauge senders for 12v and 24v (different part numbers) however there is only one part number for the fuel pump for both vehicles - 589322* (the * indicates the part is not unique to the 101) and is for use on both 12v and 24v vehicles.

Garry

[101 Ron]

Supports more the theory a voltage dropper is under the spare wheel ?

[Mick_Marsh]

Craddock lists the coil as a 24V.
552765 - Ignition Coil 24 Volt. Military Vehicles (https://www.johncraddockltd.co.uk/other/101-forward-control/parts/electrical/distributor-and-ignition/552765-ignition-coil-24-volt-military-vehicles.html)

My parts book lists the 12V coil as p/n GCL111

It lists the 24V coil as 552765. It also lists the box Garry has shown as "Filter Unit", not voltage dropper. Think about it. The 24V 101s may have had radio gear. You didn't want noise generated from ignition coils and fuel pumps interfering with the radio signals.

[101 Ron]

Just checked the wiring diagram..........24 volt vehicles.
Both fuel pump and coil appear to be 24 volt with filter boxes on the supply.

[shippers]

101repair manual introduction is as follows
12v...alternator 16 acr 34a...6 volt coil...12v distributor.
24v...alternator 24v 90a fully suppressed with a shunt box if radio batteries fitted....10v coil with filter V.dropper.24v distributor with double contact breakers fully screened and engine speed limiter...fuel pump has V.dropper

[101 Ron]

Normally for a 24 volt system suppressed type , for it to work well the coil needs to be 12 volt or lower as it reduces the capacitance of the supressed High tension leads by half and therefore you get a spark at the plug that will do something.
Some one will have to find a 24 volt 101 which has not been played with, to get the true answer.