Engine only starts with direct 12v to coil

[db7gtgrigio]

Hi all

S2 FHC UK spec - will only start from cold with a wire connected direct from battery to coil. In this condition it starts easily. With no direct connection to coil it just cranks without any sign of firing

Once warm it will start without the battery connected to coil, but only when you release the ignition key. This suggested to me that there is no spark at coil when starter motor cranking. You can feel the engine catch just as you release pressure on the key.

Following advice from this forum I replaced the ignition switch, as it was suggested that there could be a faulty connection isolating the 12v to coil when cranking

Unfortunately this has not fixed the problem.

Really at wits end on this when, can anyone suggest what else it could be?

Many thanks in advance

[abowie]

Chris have you tried shorting the wires at the back of the ignition switch ("hotwiring") to see if it will start with the switch out of the circuit?

[mgcjag]

If it starts as you release the key then it sounds like the switch wireing is wrong...I think from your previous post that you fitted a non standard/aftermarket switch...with the key in the position to operate the solonoid and turn the starter it should also be providing 12v to the coil..Steve

[db7gtgrigio]

Thanks for the suggestion Andrew - no I have not and I can try that at the weekend however the original ignition switch has never been touched previously in the 15 years since I have owned the car, and both it and the OEM Lucas replacement tested identically with my multi meter on the bench, so am struggling a bit to see how that could cause the issue

[db7gtgrigio]

If it starts as you release the key then it sounds like the switch wiring is wrong...I think from your previous post that you fitted a non standard/aftermarket switch...with the key in the position to operate the solonoid and turn the starter it should also be providing 12v to the coil. Steve

That is exactly what I thought but the switch wiring has never been touched in my 15 years of ownership - I checked with multi meter the functions of the original switch and the OEM Lucas replacement (not a non standard or aftermarket switch - very hard to find!!) and they both performed as expected and as the wiring diagram indicates.

Its a real mystery!

[abowie]

Is there a relay in your circuit, as in this diagram? If so there's a few other places to look.

[mgcjag]

It shouldn't be difficult...your car starts with a direct 12v to the coil......so when cranking why is there no 12v at the coil...what ignition switch replacement have you fitted..in a previous post you mentioned a 39415 Lucas.....disconnect the lead at the solonoid...then you can play with your low tension wireing and ignition switch without the starter spinning... Steve

[db7gtgrigio]

Is there a relay in your circuit, as in this diagram? If so there's a few other places to look.

Yes there is a relay located on splash panel near battery but its an earlier car with no ballast resistor.

[Series1 Stu]

Hi,

Could it be as simple as the starter motor drawing all the power at cold cranking and causing a large voltage drop at the coil? The starter would draw less power when the engine is warm so allows a higher voltage to the coil. The direct feed from the battery will equalise the voltages at the coil and the battery.

It's a simple enough job to test the voltage across coil feed and ground while cranking the engine (disconnect the lead from the coil first).

If this is the case then either the battery or the starter motor or, (unlikely) both, is failing.

Regards

[MarekH]

...If this is the case then either the battery or the starter motor or, (unlikely) both, is failing.

If the battery has enough energy to crank the engine, then it is hardly failing. The coil is unballasted during cranking, so ought to have plenty of time and voltage to recharge at 120rpm. Clearly that is not so, so from the circuit diagram posted, I would clean the c1 contact inside the relay and check the integrity of the white/blue path all the way from the relay contact to the coil positive.

To test, disconnect the white/red at c4 (so the starter does not operate), turn the key to the start position and either:-
1/ with the battery disconnected, measure the resistance between the disconnected battery positive lead and the coil positive, or;
2/ measure the voltage drop between the battery positive and the coil positive with the key in the start position.

In both cases, the answer wants to be near zero. Any significant resistance on that path inhibits coil charging.

kind regards
Marek

[db7gtgrigio]

It shouldn't be difficult...your car starts with a direct 12v to the coil......so when cranking why is there no 12v at the coil...what ignition switch replacement have you fitted..in a previous post you mentioned a 39415 Lucas..... Steve

Yes, I fitted a 39415 switch which is effectively the same as the original 39679A (I bench tested both with multimeter and they gave exactly the same conditions at each key setting)

[db7gtgrigio]

...If this is the case then either the battery or the starter motor or, (unlikely) both, is failing.

If the battery has enough energy to crank the engine, then it is hardly failing. The coil is unballasted during cranking, so ought to have plenty of time and voltage to recharge at 120rpm. Clearly that is not so, so from the circuit diagram posted, I would clean the c1 contact inside the relay and check the integrity of the white/blue path all the way from the relay contact to the coil positive.

To test, disconnect the white/red at c4 (so the starter does not operate), turn the key to the start position and either:-
1/ with the battery disconnected, measure the resistance between the disconnected battery positive lead and the coil positive, or;
2/ measure the voltage drop between the battery positive and the coil positive with the key in the start position.

In both cases, the answer wants to be near zero. Any significant resistance on that path inhibits coil charging.

kind regards
Marek

Thank you for this suggestion - I will try that at the weekend - my car is pre ballast resistor however I assume that wont make any difference to the tests you suggest

[MarekH]

Correct, but it does give you an idea of the correct working parameters. A ballasted coil is designed to normally run from a voltage lower than 12v and it is "overdriven" during charging, so when the battery voltage depletes, the coil still happily charges. During cranking, an unballasted coil wants to charge up to the highest amount it can, since it doesn't benefit from the "bypass boost" voltage a ballasted one does. This means it is doubly important for it to have no resistance in its 12v charge path as that acts to cap its charging voltage.

kind regards
Marek

[mgcjag]

Hi...so you do have a non original switch fitted...I understand that you have tested it on the bench but it does have different connections than an original switch... are you sure you have the correct wires on the correct terminals.....also note that its not impossible that these "new" manufactured switches can be faulty straight out of the box....as mentioned in several posts above it should not be difficult to trace your 12v feed right to the coil when your switch is in the cranking position...Steve

[mgcjag]

Extraxt from Chris Vine Blog
Ignition Switch
As I was sorting out electrical bits and pieces, attention turned to the Lucas ignition switch, which is marked 157SA 39415A. I’m not sure if this has been replaced at some stage but the terminals bore no correlation to the wiring diagrams and had intermittent connections when tested with a multi-meter.

The terminal connections on the wiring diagrams indicate:

1 – Brown : supply from Battery, under permanent current
2 – White : under tension only after ignition switch is on
3 – White/Yellow : via starter solenoid relay, delivers power to the starter motor
Therefore, when the key is in position II, terminals 1 and 2 should be connected and when the key is in position III, terminals 1, 2 and 3 should all be connected.

To achieve this with my 157SA switch, the White/Yellow wire can only be connected to terminal 1. The other wires using terminals 2 and 3, in any order.


There’s no ability to change the terminal connectivity so I’ll just have to adjust the terminal wireing accordingly

[db7gtgrigio]

Last night I decided to re-test the ignition switch that was in my car when I bought it 15 years ago. Lucas 39679A. I noticed it has what looks like a manufacturing date of 44th week 1970, which given my car's build date of 5/6/69 suggests it is not the original one.

I tested continuity across the numbered terminals as follows (with Position I being key in but not turned):

Position II - continuity across terminals 2 and 4 - I assume this is to provide power to accessories ie radio
Position III - continuity across terminals 2, 3 and 4 - I assume this is the run position which provides power to the coil
Position IV - continuity across terminals 1, 2 and 3 - I assume this is to provide 12v to the starter relay

This indicates that the brown 12v wires should be on terminal 2, because that terminal is common to all positions.
Because terminal 3 is connected in the ignition on and start position that must be the white wire to coil
Because 1 is only connected in Position IV (start) that must be the white/yellow to starter relay
Because terminal 4 is isolated in Position IV (start) then that must be for accessories.

This is different to the wiring positions in the diagram earlier in this post, which has
1-Brown
2 - White
3 - White yellow
But this maybe because it is for a ballast resistor car which mine is not.

[mgcjag]

This is a diagram for a S2 shows standard switch and connections for the Britex lock which i think your car has...Steve

[db7gtgrigio]

This is a diagram for a S2 shows standard switch and connections for the Britex lock which i think your car has...Steve

Thank you for that, its really strange because if I wired my switch up as the diagram shows there is no way the car would start because terminal 1 on my switch is only connected to 12v when key is turned to start but the diagram shows terminal 1 taking the direct feed from the battery. I know from the lucas number in my S2 parts book that my 1970 manufactured switch is precisely the correct switch for my car so this is really weird.

I have again retested my switch, this time using a 12v light bulb in circuit, and can 100% confirm that the only time positions 1,2 and3 are all connected and 4 is isolated is at start, which means 4 must be accessory and 1 must be starter relay

[mgcjag]

I'm starting to loose track on this...you mention "your switch" but you have 2..the original and a replacement...the way forward...identify what your wires are......test a switch that gives the correct internal connections....fit wires to it accordingly...Steve

[MarekH]

Continuity and resistance are not the same thing. A meter on the continuity setting will happily tell you everything is fine if the resistance is down around or below ~1ohm, but even a resistance that high is like putting a massive block in the way of the coil because it is like putting a ballast in the ignition path at startup and never charging up the coil.

Take your switches and make a connection table for each of them. Resistance between those terminals needs to be negligible between brown and white/yellow when your key is in the start position, but most meters cannot accurately measure below 1 ohm, even though the scale may display 0.1ohm increments. Continuity means NOTHING when you are dealing with such tiny resistances because it doesn't tell you how well you are connected. The meter test leads may be up to 0.5ohms. Whilst the meter cannot accurately measure resistances this low, it can measure voltage very accurately, so this is what you have to use to the bottom of the problem.

Since you have success with a jump lead to the coil, make up a spare cable run of your own and swap it in in places and retest. If you get a better result, e.g. by hotwiring across just the ignition switch with your jumper lead, then you'll know where some of the fault lies and how to achieve a better result, rather than just from battery to coil.

To be certain, you then need to be systematic and ascertain that there is no voltage drop all of the way along the paths to the components and back to the battery negative. Do that by disconnecting the white/red (so you don't run the starter needlessly) and measure the voltage along each section of every run with the key in each position. Measure your battery voltage between both terminal posts and then add up all of the subvoltages along the path. For the first line, a perfect car will see no voltage between battery positive and distribution point on frame; for the second line, no voltage between distribution point and brown wire at back of ignition switch; for the third line, no voltage across ignition switch between brown and white and whit/yellow spades on back of ignition switch; etc etc until you get to the coil, where you should see full voltage just like between the battery posts, and finally no voltages between battery negative along earth path to the coil.

It's called a circuit for a reason, because it has to trace all of the way from battery positive to battery negative. If you are losing voltages somewhere on this path, your coil doesn't get to charge up as much as it should.

You've had partial success with your direct coil connection and now you need to drill down and find out more details as to where that is having an effect, e.g. is it just the ignition switch or are connectors dirty or is it brittle wiring AND something else all added up.

kind regards
Marek