• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

MGB MGB Ignition question

Share this page

EMGEBE

EMGEBE

Jedi Hopeful
Offline
I posted a while ago basically with a cold start problem,
Starting the B is sooo bloddy hard on cold mornings/nights.


My mechanic a while ago put in a resistor (think its called a ballast resistor) before my ignition coil because the last coil i had in my car lasted litterally a week.
Last night i couldnt get it started and the road side assistant guy bypassed the resistor effectively running 12 volts through my coil... It fired up straight away... not even a crank, just went BANG... START!

mgignition.jpg


My question is...
is there any harm in essentially setting up a switch system in my dash to bypass the resistor, for those cold mornings?

Just for example, start it up with 12 volts going through the coil, then switch the resistor back on when its all warm? dropping it back to 3 or 4 volts or whatever it is.
 
There are basically two types of ignition coils used. The first type uses a coil with about three ohms primary resistance & is fed directly from switched 12 volts.

The second type has a primary resistance of about 1.5 ohms plus a ballast resistor connected to switched 12 volts. This type uses a wire connected from the start terminal of the starter relay to bypass the ballast resistor during starting only. As soon as the starter is released, the ballast resistor is back in the circuit to drop the voltage to around eight volts.

It appears that you now have the ballasted type coil (1.5 ohms) instead of the original non ballasted coil (3 ohms). Just run a new wire from the starter terminal (small wire on the starter solenoid) to the connection between the ballast resistor & the coil. This will apply the full 12 volts to the coil during starting & drop the applied voltage to about eight volts when the starter is released.
D
 
Dave, there is a problem in doing what you describe. Ballast ignition systems usually have a "special" starter solenoid with an extra (fourth) terminal for the ignition. This terminal is only hot while you're running the starter. If you make the connection you're describing to a "regular" three terminal solenoid one of two things happens. 1) either the person connects the coil wire to the wire/terminal from the ignition switch, or 2) they connect it to the large lug going to the starter.

In the first scenario, this causes the starter solenoid to get a constant voltage feed even when you release the ignition key (engine running). This voltage may not be high enough to engage the solenoid/starter but it will heat the solenoid and draw more current through the ignition wiring. The second situation will do the same sort of thing but instead of energizing the solenoid, you'll be energizing the starter motor. There won't be enough current capacity to turn the starter over, but it's not good either.

The best solution is to buy a new/different starter solenoid that supports a "fourth", switched terminal. Use the fourth terminal to supply coil (+) directly on neg. ground cars. Look for later LBCs with ballast ignitions to determine a donor.

EDIT:
See the following links for Lucas solenoids. I'm sure you'll be able find them locally.
https://pegasusautoracing.com/ProductSelection.asp?Product=1160

https://www.rthursby.co.uk/acatalog/R_Thursby__starter_44.html
(you want the ones marked "cold start")

Generic solenoids in Oz at:
https://www.ashdown.com.au/ezyedit/fileLib/Catalogues/fuses3.pdf
 
dklason is correct. What he described is (make that was) the standard configuration on most american made automobiles prior to the advent of computer control systems that emerged around 1980. The starter solenoid had a second small post that was only active while the starter was engaged, and that post provided full 12v power to the ignition coil.

A slight modification to the suggestion that Dave Russell made can provide the same function as the more expensive (and time consuming) suggestion of changing the starter solenoid.

Start with Mr. Russell's suggestion and connect a lead to the starter terminal (to borrow Dave's nomencalature), but don't connect it directly to your ignition coil. Instead, connect the lead to the activation circuit of a horn relay. Use the horn relay to provide the full 12v connection to your coil.

Horn relays are available at most parts stores, and are generally very inexpensive. They typically have three connections:
- one goes to the power supply (positive battery terminal or equivalent);
- a second goes to the device being powered (horn typically, but in this case, the positive side of the coil);
- a third that is connected to a switched power source (the lead from your starter solenoid)

The relay functions the same way as your starter solenoid - power from the switched source is used to close a switch that connects the power source to the powered device. A soon as the switched power stops, the horn relay switch opens and the power connection is stopped.

NOTE: there are many types of relays similar to this, but basically two different configurations - one that is "switch open" like I've described, no power connection until the switched source is activated; and one that is "switch closed," where the power connection is disconnected when the switched source is activated. Either will work, but the type described is best suited to the application.

Hope this helps.


Mike
 
[ QUOTE ]

In the first scenario, this causes the starter solenoid to get a constant voltage feed even when you release the ignition key (engine running). This voltage may not be high enough to engage the solenoid/starter but it will heat the solenoid and draw more current through the ignition wiring. The second situation will do the same sort of thing but instead of energizing the solenoid, you'll be energizing the starter motor. There won't be enough current capacity to turn the starter over, but it's not good either.


[/ QUOTE ]
Hi Doug,
Good point. The feed back problem of coil voltage keeping the solenoid partially energized can be cured by inserting a diode in line with the previously added wire connected from the small solenoid terminal to the coil/ballast resistor connection.

This will allow current to flow from the starter switch contact to the coil but not from the ignition coil to the starter solenoid. I should have mentioned this earlier. Sorry.
D
 
Both the auxiliary relay and the power diode are excellent and economical solutions. Thanks for the ideas.
 
wouldn't it just be easier to drop the ballast resister and change the coil out for a non-ballasted type coil?
 
It would be easier except for the cost of a new coil. Also, the ballasted coil does have the advantage of better spark during starting. Some cars use it, some don't. Take your pick.
D
 
The cars should run fine with the stock set-up.
No sense in reinventing the wheel.
Make sure your choke is functioning correctly for easy start-up.
 
Back
Top