MGB & GT Forum

Good Afternoon,

I am in the process of upgrading the ignition system on my 74 MGB (dist. cap, rotor, wires, coil, plugs) . The PO had already installed a Pertronix Ignitor with a standard Lucas Coil (which they previously crushed a bit via the bracket).

I checked the original coil and it measured 3 ohms with 12v to the positive post with the key in the on position. Ordered the 3 ohm Flamethrower per recommendations.

Discovered today (attached to the positive side of the coil and out of sight) the twin white/light green wires that would seem to indicate a ballasted ignition system.


PLEASE NOTE: when i purchased the vehicle the tachometer was wired but not operational. Might be related.

From previous posts / instructions , it seems the following wiring is correct:

From Distributor:

Pertronix Black wire to Neg Coil Post (it is currently connected to a white/ black wire from the loom, i would guess this is then connects indirectly to Neg Coil post)
Pertronix Red wire to Pos Coil Post

From Wiring Loom:

White/ Black to Neg Coil Post (two wires connected together at spade terminal)


Questions:

1. Am i correct in assuming that the white/light green wires indicate a ballasted system?

2. Why would the a ballasted system use a 3 ohm coil? Any advantages or disadvantages?

3. Assuming it is a ballasted system, how best to wire the power to the coil?


I have reviewed previous posts but really would like some "up to date" opinions.

Thank you very much.

Which '74 do you have? There were three different styles of them. Mine has been called 1974 1/4. No ballast wire in the car. If you have the original Pertronics (Pertronics 1) a 3 ohm coil would be correct.

I looked at your profile and you have a late 1974 and I think it would have a ballast wire in the harness. The ballast wire should not be used with the Pertronics 1 and 3ohm coil. Red lead to + side of coil and switched 12 volts to the + side of coil.

Yes, it is a late 74.

What issues would the 3 ohm coil have when used with a ballasted system?

Could this be the issue with the Tach as well?

Thank you!

You CANNOT put the red wire to a ballasted coil. It won't have enough voltage. You cannot run a 3 Ohm coil with a ballast wire - that'll be 4.5+ Ohms and it'll run terrible.

Since you have a 3 Ohm coil, wire it per the non-ballasted cars. Tape off the white/green wires and run a new white wire from the fuse panel to the coil. Red wire to the coil +.
Done.

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jeff@advanceddistributors.com

Thank ya'll,

I will be installing the coil per non-ballasted instructions. Will delete the ballasted wire to the positive coil terminal and will wire in a "switched power" wire from the fuse box (post 5).


Just wondering, is there any benefit to running the ballasted system?


Alleigh


UPDATE: Thanks for all the great advice. Started first turn of key!



Edited 1 time(s). Last edit at 2024-04-13 06:21 PM by Safety Fast Girl.

Thjank you so much Jeff