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JAGUAR STARTER, 12 / 24 V, One for Clive?


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Dear All,

 

The so called Jaguar J60 engine fitted to the Conqueror is really a 4.2 litre car engine with automatic gear-box.  It has J60 ignition and inlet manifolds.  I did not think it would be easy to fit the FV 24 volt starter (it is not) so I left it on 12 Volt starting and used the 12 Volt system for the main engine EFI system.  I found the usual dual voltage problems!  Therefore the tank will now be all 24 volt.

I would like to keep the 12Volt starter, use 24 Volts but keep the current down by means of a very heavy duty resistor.  I could either use some proper heavy gauge resistance wire or use steel by cutting out a 'tortuous path' in a sheet of 1.5  mm steel. Alternatively I could try just using another starter motor field wing in circuit.

Probably one for Clive to advise on!

Does anyone have a figure for the current going through a starter on an Jag engine?  If so I could calculate the required length of 1.5 x 10 mm steel strip (equivalent of) to give the desired resistance.

 

John

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John I don't know what the current demand of a 12v Jaguar starter would be, other than it would be in the order of twice the current demanded by the equivalent 24v starter. For example a 24v  Starter No.3 draws in the order of 100-120A.

But bear in mind before the heavy current is drawn you have to apply the appropriate voltage to the starter to operate the solenoid that then initiates the starter to draw its heavy current. The solenoid may not take kindly to what may be nearly double its operating voltage at the start of the process given that any dropper resistor to satisfy the starter would be fractions of an ohm.

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Looking at the spec for an XJ6 4.2 starter motor, a hot start may take as little as 100A but a cold winter start can draw 500A. so it's not easy to specify a series resistor to drop form 24v to 12v. The resistor would have to dissipate roughly the same power as the starter was taking - so maybe 5kW. Mild steel would double its resistance for a temperature rise of 200°C (someone check my maths?) , another complication (unless it starts very quickly). My conclusion, it's not simple. Sorry. Clive?

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3 hours ago, andym said:

Presumably your 24V comes from two 12V batteries in series, so why not just run the starter off one battery?

Andy

Doesn't work Andy, the battery that has been used for 12v will not recover if connected to another fully charged battery in series.  My local DAF dealer did a similar job on a large fleet many years ago and it cost him dearly in replacement batteries.

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Dear All,

 

Thank you for your input it is much appreciated.

The problem with the J60 24 volt starter is that it is an ordinary 4.2 litre XK engine and not a J60.  I don't think that it is straight forward to fit the J60 starter.

"Clive" is Field Marshall Clive Elliot our Chief Electrical Engineer.

I am not too worried about the starter throw in solenoid as it will only be overcurrent for a moment until the starter starts turning the engine.  In other words, it will not have time to overheat. The external starter sol / relay is already 24 Volts.

The positive temp co-efficient of a steel resistor could be beneficial as if the resistance was selected by experimentation the current through the starter would be reduced if the steel resistor got hot.

However, what I have done is to order (quite cheaply) 2 metres of 2 mm Manganin resistor wire.  I need a resistance of  between 0.12 and 0.04 ohms.  By chance the resistance of 2 mm, 12 AWG Manganin wire is 0.04 ohms per foot.  With a bit of experimentation I should be able stop 24 volts from sending the starter into orbit.

I will let you know how I get on with it.

John

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14 hours ago, radiomike7 said:

Doesn't work Andy, the battery that has been used for 12v will not recover if connected to another fully charged battery in series.  My local DAF dealer did a similar job on a large fleet many years ago and it cost him dearly in replacement batteries.

If you purely rely on charging the batteries from the vehicle, yes.  But if like me and, I assume, many others, you keep them topped up from a separate charger it shouldn't be an issue.

Andy

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1 hour ago, andym said:

If you purely rely on charging the batteries from the vehicle, yes.  But if like me and, I assume, many others, you keep them topped up from a separate charger it shouldn't be an issue.

Andy

Would another way is to use the battery balancing box from a Cent?

That provides a 12v feed for the wireless system

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If I recall correctly (and a quick google says yes) Ford Escort rally cars often used a 12v starter with 24v to spin the engine really fast. This probably shortened the life of the starter a bit but, hopefully, it's only getting intermittent use and should get plenty of time to cool down.

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