Car wont start.

fregster · Post by **fregster** » Tue Oct 09, 2007 1:56 pm

I have installed MegaJolt in my MGB. The system works well but I can not start the car on the MJ.

After my inital investigation as to why it would not work, cranking the car showed a week spark, but as soon as the car started it gave a nice strong spark.

If I start the car on the dizzy then 'carefuly' move over to the MJ the car runs well.

A voltmeter shows the V drop to the coilpack to around 10.5V when cranking then 13.5 - 13.9 V when running.

Now incresing the cranking voltage is going to be difficult, I have got a new starter motor from the MG Owners Club, which is said to use half the current draw of the OE device, which should help.

Is there any thing else I can do, I have a new battery (The old one was still in good nick but I thought I would try, and a new high out put alternator to make sure the battery is kept topped up), I was thinking about a MSD Blaster coil, to see if the out put voltage is high enough when cranking to still produce a spark.

Many thanks for any help and sugestions.

Paul

mwman · Post by **mwman** » Tue Oct 09, 2007 2:31 pm

I do not know much about MGB's, I am a Triumph man however, on reading your post I would suspect you have a ballast resistor feed too your ignition coil. This may explain the loss of volts!!

Simon

fregster · Post by **fregster** » Tue Oct 09, 2007 2:35 pm

Thank you for your reply, the input voltage is taken direct from 12V permant line the voltage drop is (I think) caused by the current draw on the starter motor.

Does any one know what's the minimum spec voltage for the coil pack?

Paul

mwman · Post by **mwman** » Tue Oct 09, 2007 2:40 pm

The Bosch coil spec sheet showed 10 to 16Volts from memory.
I am supprised you get that much drop, is the earth OK on the battery, it must turn over very slowly!

fregster · Post by **fregster** » Tue Oct 09, 2007 2:44 pm

No, it cranks like it should, start fine on the dizzy.

If I connect just 1 spark plug to the dizzy, it starts and then run's on the MJ, I can put all 4 on the MJ once its started.
I have been through checking the earth's and all 'seem' fine, I have tried running +ve direct from the battery and an extra earth strap from the engine block to the battery -ve with no difference.

P

mwman · Post by **mwman** » Tue Oct 09, 2007 2:47 pm

Have you got the 20uF cap on the coil DC feed?

fregster · Post by **fregster** » Tue Oct 09, 2007 2:55 pm

There is a cap on it but it might only be 3uF off hand though, the MJ software does not lock up when I crank it and works fine too I should have added.

mwman · Post by **mwman** » Tue Oct 09, 2007 2:59 pm

I kept the Ford capacitor, luckily!
Well unless your EDIS 4 is duff or the coil, I have no other suggestions!
Sorry.

fregster · Post by **fregster** » Tue Oct 09, 2007 3:08 pm

I have tried 3 coils all give the same problem.

I am convinced that the input voltage is not high enough during cranking, thanks for your help though.

Anyone else got any sugestions to check, is it worth swaping the cap out for a 20uF? There does not seem to be much noise on the power line.

MartinM · Post by **MartinM** » Tue Oct 09, 2007 4:35 pm

The capacitor is a red herring - 3uF vs 20uF is absolutely negligible in terms of supplying extra current for sparks when cranking...and the perceived wisdom is that it is there more for RF suppression than anything else anyway. And a huge number of installations don't have it at all

But I don't know the answer to the problem...

...but like you, I would have thought that 10.5v was sufficient...

fregster · Post by **fregster** » Tue Oct 09, 2007 5:10 pm

I did not think about the cap as to help the voltage but only as to reduce the RF, I would of thought that the charging time and the discharging time would not be enough to make much of a difference to the juice received by the coil pack though.

I will have to wait and see if the current draw on the new starter is low enough not for the voltage to drop so much.

I do not want to give up on this as I am fed up of setting my points and having cold damp starting problems!

P

KLAS · Post by **KLAS** » Tue Oct 09, 2007 6:18 pm

did you try to supply the coil with a seperate batterie? if you have one in spare it would be worth a try

mwman · Post by **mwman** » Tue Oct 09, 2007 9:11 pm

The capacitor forms a series tuned circuit with the coil, so as well as reducing RFI emissions it ensures the AC signal (from the switching semiconductor) is fully applied across the primary winding of the ignition coil.
These same rules apply to RF amplifier stages.

Simon

fregster · Post by **fregster** » Wed Oct 10, 2007 8:20 am

I knew about RF circuits (Tuned arials etc), just never translated the theroy (Silly me). Any way I do have a spare battery now, as I replaced the one in the car and fully charged the old one (You never know with an old car when the battery will go flat!).

I am going to swap the start out tonight, but I will also have a go with the second battery as once again the points have gone out of spec. I hate points now.

MartinM · Post by **MartinM** » Wed Oct 10, 2007 8:24 am

mwman wrote:The capacitor forms a series tuned circuit with the coil, so as well as reducing RFI emissions it ensures the AC signal (from the switching semiconductor) is fully applied across the primary winding of the ignition coil.

Unfortunately not.

This is how a standard Kettering inductive-discharge points-based ignition system works, where the capacitor is in parallel with the points ie connected between the coil negative and earth. When the points open, the capacitor predominantly stops the arcing across the points and there may be some elements of series tuned circuit trickery in there but my electronics knowledge is a bit too far in the past to work through the equations

In the EDIS setup, the capacitor is across the coil i.e. from coil +ve (ie battery +ve) to ground and is purely there for RF suppression (and it makes, if anything, a parallel tuned circuit, but that's just a coincidence of how they are connected rather than a design aim).

There are no AC signals from the switching semiconductor in the EDIS module - they work as a simple switch - turned on to build up a current in the coil primary and turned off to stop the current flowing, which causes a collapsing magnetic field in the coil that acts on the secondary winding to produce the high voltage that causes the spark in the plug.

Regards
Martin