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MG MGB Technical - MegaJolt Electronic Ignition

I fitted my MegaJolt from Matt Kimmins (Quality made piece of kit).

But the cranking voltage to the coil pack drops to 10V, which is not strong enough to produce a spark.

If I start the car on the dizzy then move "carefully" over to the MJ it runs really well as the V returns to aprox 12.5V once the engine is running.

I have tried a new battery, running power direct from the battery but the voltage drop is to much.



I had though about a new hi-torque starter motor as I understand these require about 1/2 the ampage, which might prevent the V drop so much.

I had also thought about some form of in line step up power transformer but I can't seem to find an appropriate system (re: parts).

Is there anyway I could take power off the starter solenoid during starting to boost the starting voltage?

Anyone got any ideas?
Paul Fryer

You should check all your electrical connections as well as the alternator output. When running above idle your voltage should be between 13.5 to 14.5.

The power for the car is already taken at the starter, it is the group of brown leads.

An actual transformer will not operate on DC power. You would need some type of solid state voltage booster.
John H

I had not tested the voltage output of the alternator but will do (Do not think this will help as once it is running it is fine just starting) or resistance from the block to the battery to check for bad earthing but I will do, anyone know what sort of impedance the chaise normally gives or could anyone quickly test there's.

I did suspect a poor earth before as my fuel gauge only reports 3/4 full at full tank and 1/4 full at empty but suspected that this would be a local earthing or sender problem.

Obviously a transformer is 2 circuits which would never work here, but you knew what I meant. Sorry my bad.

I have been looking at voltage boosters but this would only be a sticking patch for possibly a bigger problem.

Paul
Paul Fryer

Paul,

there have been some B's with ballasted wires to the coil. May be your car is fitted with this feature.
I would run a wire from the starter solenoid via a ralay to the coil pack and switch the relay with the starter switch (white wire), just for a test.
There also might be a drop in voltage due to the interface for the tach, but i am not sure upon this and would have to read about it again.

Did you contact Mat allready and asked him for his hints to your problem?

Ralph
Ralph

I asked Matt, but I have had no reply. I think he might be a little busy at the minute.

There is not an external resistor on the coil but it does have 6V across it.

Paul
Paul Fryer

If this is a late model car the resistance is built into the wire harness in the form of a high resistance wire. You can cut the harness back and replace it or tap into the white wire for the fuel pump at the junction point of the rear harness just behind the coil. You will need to change the coil also.
John H

The coil works fine, I think were going a little off trac here, or I am not understanding you.

Its the voltage to my new coil pack which currently its power is taken from (I think) the brown wires, permanently live, but there is a large voltage drop during cranking.

The voltage drop for the coil is countered by switching from a 6V to a 12V input (10V in my case) during cranking is it not?

I need to be able to provide 12V to the new coil pack during cranking.
Paul Fryer

The Ignition on the car should work with the battery voltage down to about 9 Volts.

This is why there are circuits such as a ballast resistor are used providing only 6-9V during running, and when cranking the ballast resistor is bypasses and providing full batter voltage (what there is of it)

More likely your problem is not the voltage, but how the coil or Megajolt is wired. The suggestions above address that.
werner haussmann

12.5V running is way low.

System V running should be around 14 - 14.5. If not, then there is a fault in the charging system or the battery is bad or flattish.

Batt V fully charged and not running should be around 13.25- 13.5.

After you get those fixed, you will be starting with 13.5 at the battery, instead of whatever you have now - sounds like it's well under 12.5 if you only have 12.5 running.

Your main problem is probably bad connections throughout the car, but especially in the N & earth circuits.

Might look up the recent threads "VDO gauge problem" and "VDO gauge problem II".

You will always have a V drop when cranking, but I cannot believe that the system will not work with "normal" voltage drop on cranking.

FRM
FR Millmore

The old battery provided around 12.5 directly across its terminals which is why I changed it.

The new battery I have not tried now it has been in the car it had 12.5ish when I put it in from new.

I need to check the V directly across the alternator to test that, but I suspect bad earthing.

Paul
Paul Fryer

Hello Paul,

I have a Matt Kimmins Megajolt installed on my Moss Supercharged MGB engined MG Special car. If the initial MAP timing window (load bin zero) is set to ten degrees advance with 500 RPM set, this will provide a very low torque starting to the engine and it will both turn over, fire easily and provide a rock steady idle - been there done that!!!

I beleive that the later Megajolt firmware load does incorporate a selectable option within the computer software for "cranking timing" or is labeled something similar.
John
J Harle

I have to sort out some wiring problems first and then I will try setting the initial RPM load bin.

Thanks for the advice.

On a side note I am thinking about supercharging, would you mind if I emailed you about your set up?

Paul
Paul Fryer

I have a copy of the map installed on my work laptop,

The 500 load bin is 23 BTDC and then drops to 19 at 800 & 1200 at 0% throttle

The 500 load bin is 13 BTDC, 15 at 800 & 16 at 1200 at 100% throttle

Would you recommend dropping the whole advance at 500 to 10 BTDC?

I did try the whole map at 5 BTDC and 10 BTDC before I found it was not sparking at crank.
Paul Fryer

This thread was discussed between 17/09/2007 and 20/09/2007

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