Ballast resistor removal

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Philip Tibbitts
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Ballast resistor removal

Post by Philip Tibbitts » Tue Jun 09, 2020 9:58 am

The starting problems on my 1976 MGB Roadster seem to be down to a faulty Resitive Wire (Balast Resistor).

The usual suspects can't supply one, I either have to buy a replacement loom(!) or a TR6 ceramic resistor.

I think I'd like to replace the 6v coil with a 12v set up to avoid this non- starting situation again.

Is there a recommended wiring scheme to carry out this operation or would a 12v supply from the ignition relay to the proposed replacement 12v coil direct, suffice?

Has anybody done this before?

Dave Wheatley
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Location: SW Scotland

Re: Ballast resistor removal

Post by Dave Wheatley » Tue Jun 09, 2020 10:51 am

Deleted - twaddle.

Ignition relay seems to appear in 1978 according to Bentley wiring diagrams.
Last edited by Dave Wheatley on Tue Jun 09, 2020 11:21 am, edited 1 time in total.
1978 MGB GT
1931 Austin 7 RM
1955 Standard 10
1972 Ford Cortina
No modern!

Philip Tibbitts
Posts: 17
Joined: Mon Nov 28, 2016 12:18 am
Forename: Philip
Surname: Tibbitts
Location: Telford, Shropshire

Re: Ballast resistor removal

Post by Philip Tibbitts » Tue Jun 09, 2020 11:12 am

Dave, thanks for the speedy reply.
Although my car is 1976 registered it seems to have the later wiring loom connections. As the white ignition supply goes directly (unfused) to power the ignition relay, which feeds the fuel pump, wipers, heater blower and the ballast resistor wire - but not directly to the fuse box.
Once the ignition relay coil is energised by the white wire, a brown wire supplies 12v power the white/ brown trace wire to the fuses.

Normally methinks, the ignition circuits are unfused, so I think a direct supply from the live side of the white/ brown, either from the live side of the fuse box or directly, to the new replacement 12v coil?

RSVP if you have further thoughts Please?

Dave Wheatley
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Re: Ballast resistor removal

Post by Dave Wheatley » Tue Jun 09, 2020 11:35 am

Hi Philip, as you were posting, I was busy deleting my post, after I realised that you did have the ignition relay.

Which wiring diagram are you using?

I think you could run a wire fro No5 white/brown live side of the fuse box to the coil+. There is still the old 6 volt resistance wire to think about as it will be getting 12v between the fuse box and the coil. From the wiring diagram, if this has to be cut, it should be cut at the coil end because it appears to feed other stuff immediately before the resistance bit of wire.
1978 MGB GT
1931 Austin 7 RM
1955 Standard 10
1972 Ford Cortina
No modern!

Philip Tibbitts
Posts: 17
Joined: Mon Nov 28, 2016 12:18 am
Forename: Philip
Surname: Tibbitts
Location: Telford, Shropshire

Re: Ballast resistor removal

Post by Philip Tibbitts » Tue Jun 09, 2020 11:14 pm

Well I've sorted the starting problems!

I found the resistive wire sticking out of the front part of the loom, adjacent the ignition relay, it's a pink fabric covered single core wire similar to the type of wire found wound on to an electric fire element, it extends from the positive+ on the coil thru' the loom and connects to the 12v power white wire, on the ignition relay.

I unwrapped the wiring loom and traced the resistive wire under the front slam panel to the nearside horn position. Then back again (presumably the correct 1.5m length to create the 6v resistance required)

It then connects to a white/ light green trace wire which re-connects back to the coil.

With it all reconnected properly ( Hands up - I must of inadvertantly disconnected it when replacing the ignition coil!!)

The motor started and continued to run when the ignition key was released after turning the key thru' the cranking cycle.

All better now!

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Paul Scott
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Re: Ballast resistor removal

Post by Paul Scott » Wed Jun 10, 2020 5:13 am

Well done Philip.

That will fix your issue for sure.

Regards
Paul
1975 MGB Roadster
Webguru for MGB Register

Dave Wheatley
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Forename: Dave
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Location: SW Scotland

Re: Ballast resistor removal

Post by Dave Wheatley » Wed Jun 10, 2020 8:47 am

Good news Philip.

Very interesting, the route of the resistance wire!
1978 MGB GT
1931 Austin 7 RM
1955 Standard 10
1972 Ford Cortina
No modern!

Vic Butler
Posts: 514
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Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: Ballast resistor removal

Post by Vic Butler » Wed Jun 10, 2020 10:22 am

Well done Philip.
I wondered where the resistive wire was.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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