The V8 Owners Forum -

I have a interesting problem which I have been pestering the Cobra Club with.
The 4.6 is now high compression at 10.3:1.

I am struggling to start it, the spark is very weak when cranking. I have a plug light that flashes as the plug sparks. It is weak at cranking, but very strong flash once started.
I am using a standard starter, which was serviced and checked before I put it on the engin.
Battery is new, and of the start/stop variety, 100AMP and 950 amp capacity. When I measure voltage to coil it shows 12.7V, but cranking drops it to 9.8V, was lower on the old battery.
I am using Lucas DM Dizzy and 12V coil.

I also have a Hi-Torque starter in a box never used, I was wondering if that may help by not drawing as much current?

Everyone tells me that a 12V electronic ignition system doesnt need a 9V coil and ballst resistor, so I have stuck with the 12V coil.

Timing is 12BTDC

Confused.com.

Mark

Sounds like a classic case of a 12 volt coil instead of a lower voltage coil & ballast resistor - have you got a suitable coil & ballast to try?
FWIW I've got Lumenition ignition on my 3.5 and when I fitted a geared starter I had to retain the coil & ballast because it wouldn't start with a 12 volt coil.

No I have not got a new 9V coil or resistor. The old one I have from the 3.5 may be faulty??? But it does show as 1.2Ohm across the terminals, so should be ok??

I know I have been told by many people that a 9V coil isnt necessary, because I am using a 12V electronic ign system, but I simply cannot get my head around why its at 9.8V on cranking.

I have ordered a 9V coil and resistor, in a vane hope that this will work. Added to that the new starter, and I might get somewhere.

I am at a bit of a loss.

Mark

Did the old battery have a short life? I ask because I once had a starter with shorted turns (or whatever) that did still work - but drew a lot more current than it should have. Which would also cause the volts to drop. And knackered batteries. However, the ignition should still work ok above 9v - to allow for starting with a less than fresh battery on a cold day.

Where are you measuring the low voltage when cranking?, is it directly across the battery terminals?

If not, you might have a poor connection in the earth somewhere.

Chris.

The starter draws a lot of power, enough that the voltage available for anything else drops.
If you have a 9 volt coil, when the engine is running the 12 volts available are reduced to 9 volts by the ballast resistor, when cranking the starter the ballast resistor is switched out of the coil feed so it still sees around 9 volts and still gives a good spark.

May not be relevant but I had low voltage and it turned out to be because I used stainless steel bolts for all my loom / chassis earths and some other connections. SS acts as a resistor (and I melted some contacts because of this). Zinc plated bolts solved the problem.

Engine started ok this evening. This is what I had to do.

I have a 12v Lucas coil DLB 198, and is fed by a 12.7V supply from switched ignition. But as previously said this drops to 9.2V with old battery.

I have just spent £100 on a new battery, great big mother. Now volts stay at 9.8 V during cranking. But here is the twist, I also took a reading of the main battery feed into the engine bay, this read 10.8V during cranking? So I supplemented the 12v ign supply with this supply via a relay switched by the white/yellow wire off the starter. When engine fires, and I switch the starter off, additional feed is removed. Guess what, the engine started easily.

I have no idea why the ignition feed drops to 9.8v? Perhaps it is because the feed to the fuse box etc comes off the same feed that connects to the starter from the battery and a great big thick red wire.

So don't really understand why this has happened, but giving the coil another better additional supply seems to have worked.

Mark

I am putting a 52mm2 earth wire direct to the starter tonight and then to the chassis of the car. Hopefully that will rule out any earth issues.

Mark

Checked my SD1 EFI this morning - it has a accurate voltmeter wired across the actual battery. Read 10.6v when cranking - battery is a Halfords one and many years old. Wouldn't be surprised if it died this winter.

But to read the actual battery voltage, it does have to be wired across the battery itself, when drawing a lot of current.

The factory wiring on my SD1 takes all the feeds from the battery terminal - rather a rat's nest. The only thing that shares the starter cable is the alternator.

Thanks for doing that Dave. So it looks like my set up isn't far away from yours, and my 10.8 V ok?

While I was under the car this evening putting on the earth lead, I made the call to swap out the starter motor for a Hi-Torque item I had in a box.

Will see what its like later in week.

Mark.

If you have the constant energy ignition (i.e. amplifier on the side of the distributor), there is no need for a ballast resistor. It automatically compensates for battery voltage by adjusting the width of the pulse used to charge the coil.

You can see it on an oscilloscope quite easily.

Chris.