Electrical question...
Discussion
Can anyone think of anything other than the starter which takes a lot of load (current) from the electrical circuit on a 7?
I'm currently in the process of hard wiring the starter to the battery, so it's permanently live (it's switched by the action of the solenoid being actuated), and have wired a 30A relay direct from the battery to the solenoid using the low current +12v feed from the starter button as the low tension switch on the relay. What amperage cable would be the consensus recommendation from the battery to the starter? Anyone know what the Magnetti Marelli type (solenoid on top) draws?
Therefore does the FIA switch *need* to handle anything with a high current if the starter motor circuit is 'cut' out of the circuit?
Has anyone had any problems with either the ballast resistor or the FIA master switch itself?
I'm currently in the process of hard wiring the starter to the battery, so it's permanently live (it's switched by the action of the solenoid being actuated), and have wired a 30A relay direct from the battery to the solenoid using the low current +12v feed from the starter button as the low tension switch on the relay. What amperage cable would be the consensus recommendation from the battery to the starter? Anyone know what the Magnetti Marelli type (solenoid on top) draws?
Therefore does the FIA switch *need* to handle anything with a high current if the starter motor circuit is 'cut' out of the circuit?
Has anyone had any problems with either the ballast resistor or the FIA master switch itself?
I would go for a minimum of a 20 mm2 good quality copper cable for the starter. Peak cranking current can be very high (hundreds of amps) so it is important to have a good low resistance path so that all your battery power is turning the engine rather than heating up the cable!
I doubt there would be any more than 20 amps for other items such as headlights although there may be some high peak currents drawn by fuel pump and cooling fan.
But my concern would be that the starter cable is the most likely to be damaged in an accident and, as is it a heavy un-fused cable, it would cause a fire. The FIA switch reduces the risk if you've got one.
I’m not 100% sure what the ballast resistor does other than safeguard the alternator and electrics in the event of it being turn off with the engine running. What was your problem?
I doubt there would be any more than 20 amps for other items such as headlights although there may be some high peak currents drawn by fuel pump and cooling fan.
But my concern would be that the starter cable is the most likely to be damaged in an accident and, as is it a heavy un-fused cable, it would cause a fire. The FIA switch reduces the risk if you've got one.
I’m not 100% sure what the ballast resistor does other than safeguard the alternator and electrics in the event of it being turn off with the engine running. What was your problem?
David Long said:
I’m not 100% sure what the ballast resistor does other than safeguard the alternator and electrics in the event of it being turn off with the engine running. What was your problem?
David, cheers. The ballast resistor limits the current in the circuit when there is no heavy load to effectively drop the resisdual current down to lower levels. If there is no ballast resistor, the cables tend to get very hot and eventually fail!I'm trying to get rid of a load of weak points in the system, and also provide the most direct, robust feed to the bl00dy starter circuit - I generally have a very slowly turning over engine rather than the click when things get hot, which implies more current is needed to the solenoid to help kick it harder to overcome the interal resistance generated through heat (even though I've got a heat shield fitted!).....
I may wire in a 100A relay for the feed to the starter, where the +12v low tension driver for the realy is on the switched side of the master switch
Fergus, I am confused by your problem (as I probably was before).
Is this a correct summary...?
"When things get hot, the starter cranks slowly"
If so, the solenoid wiring is ok as it either makes the circuit or not. It doesn't affect how ,uch current goes to the starter.
So if the problem is the wiring to the starter (as opposed to the solenoid), that means a high resistance somewhere when it is hot.
That is an unusual problem. I think the slow cranking problem is more likely to be an electrical or mechanical problem with the starter or its installation. It might be better to measure the cranking current hot and cold which will help pin-point the problem.
As a thought, I am not convinced that 100a is going to be enough for cranking.
Bert
Is this a correct summary...?
"When things get hot, the starter cranks slowly"
If so, the solenoid wiring is ok as it either makes the circuit or not. It doesn't affect how ,uch current goes to the starter.
So if the problem is the wiring to the starter (as opposed to the solenoid), that means a high resistance somewhere when it is hot.
That is an unusual problem. I think the slow cranking problem is more likely to be an electrical or mechanical problem with the starter or its installation. It might be better to measure the cranking current hot and cold which will help pin-point the problem.
As a thought, I am not convinced that 100a is going to be enough for cranking.
Bert
furgus said:
The ballast resistor limits the current in the circuit when there is no heavy load to effectively drop the resisdual current down to lower levels
No. The ballast resistor is there to prevent damage to the alternator rectifier and voltage regulator. If the master switch is turned off whilst the engine is turning (and more to the point, the alternator), if all load electrical load is removed (ie, ignition, fuel pump, ECU, cooling fan, indicator lamps, etc) Then the alternator is still trying to produce power, but no current flows, then it can increases the voltage to the point that the solid state components can fail. The ballast resistor provides a load to ensure that the output of the alternator is never open circuit.furgus said:
If there is no ballast resistor, the cables tend to get very hot and eventually fail!
There is no logical reason why the cables would get hot if there is no ballast resistor.If you fit a 100A fuse the the thick cable to the starter motor, there is a good chance it will blow!
Additionally, any further connections that you introduce into the circuit all have a resistance and an associated volt drop across that connection.
Your problem sounds more like a poor connection somewhere(eitherthe lead from the battery to the stater motor, or an earth from the motor back to the battery), or a knackered starter motor.
Trying to put a "cleaner" load onto it sounds good in theory but if your increasing the load onto a motor that is failing due to whatever reason, heat, resistance change etc would just increase the chances of blowing the motor as well as decrease the life of the unit.
Perhaps shielding the motor more or getting some cooling to it might be a more simple option to try before looking at the wiring. There isnt a lot of air flow through the bonnet louveres/ engine bay. The flow comes through the nose cone and the whole area becomes a big air brake of high pressure. A duct from the nose cone or wherever you fancy going to the starter and then an exit through the tunnel somewhere might provide the air flow needed to keep the temperature down.
Perhaps shielding the motor more or getting some cooling to it might be a more simple option to try before looking at the wiring. There isnt a lot of air flow through the bonnet louveres/ engine bay. The flow comes through the nose cone and the whole area becomes a big air brake of high pressure. A duct from the nose cone or wherever you fancy going to the starter and then an exit through the tunnel somewhere might provide the air flow needed to keep the temperature down.
Finchy172 said:
Just get a Brise starter motor and some silver composite heatproof matting to shield it from the engine heat and you wont have any problems.
There is a specific heat shield already designed!
I've already got a pre moulded alloy plate which is bolted to one of the starter motor mounting bolts, which crucially allows air to flow around both sides (unlike the blanket approach) whilst shielding the unit from direct heat from the #4 primary.There is a specific heat shield already designed!
What's the best price people can suggest for a brise unit? I've heard that htey are not as reliable as they're perceived to be though?
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