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MG MGB Technical - Resistive Wire and Ignition Coil Input

I'm in the process of installing an Advance Auto Wire kit on my 1979 MGB. This is part of a complete overhaul and upgrade. It is a very nice system with fuses, relays, turn signal and hazard blinkers all on the same board inside the car.

This and the improved wiring size and quality should solve a lot of the old Lucas problems. Installation is a bit of work but should pay in the long run.

The kit comes with a wiring diagram that shows Ballast "if required" in the power supply to the ignition coil. When I disassembled the old wiring harness, I found a resistive wire about six feet long in series with the wire from the fuse panel (white) to the coil + side (wire W/LG). The wire was pink and white and had a resistance of about 1.8 ohms. The wire was probably 16-14 gauge.

I had my C.E.I electronic distributor refurbished as part of this overhaul. It has always been reliable with the electronic module located under the coil on the inner finder. The module has connections to the + and - side of the coil and then to the distributor itself.

Question - If I go with the 12 gauge wire supplied with the kit which goes from input to the fuse panel and then directly to the + side of the coil will this cause problems? If so what kind? Is there a specific voltage less than 12 volts required at the input to the coil and electronic module for the ignition?

Thanks in advance. I've also sent this question to the tech site for Advance Auto Wire.
Dick Field

Dick. Yes, the 75 onwards cars used a 6V coil and had a resistance wire in the wiring harness. If you do not use the resistance wire, you will have full time 12V input to your coil and distributor. Do not know what might happen with the dizzy, but the coil will tend to over heat and short out over some period of time. You need a resistor or resistance wire in the circuit if you are to use the factory coil.

As to "should solve a lot of the old Lucas problems", I have owned some of these cars when they were new. At that time, they were fully as reliable, electrically and any other way, as my Italian and US made automobiles. Lucas connectors are, today, the biggest problem and that mainly from people who do not understand that they have to be cleaned from time to time. Dielectric grease on the contact areas, after cleaning, will cut down on many of the problems commonly found.

Les
Les Bengtson

The original i.e. 6v coil would overheat with 12v connected to it all the time if you had fixed-dwell ignition like points, the earlier 45DE4 Opus ignition, or certain after-market ignition systems, but not with the 45DM4 CEI ignition. This is because the CEI has variable dwell, which means that at anything other than maximum revs the coil is only energised for a relatively short time compared to fixed dwell, around half the time at 2000rpm, and hence half the heating effect.

The switch in the CEI module will be handling double the current though, all the time instead of just during cranking, and with the later North American 32C5 coil having a primary resistance of 0.75 ohms, that is going to be more of a potential failure than the coil.

At the very least you should fit a 12v coil, but personally I'd keep the 6v system and if your new harness doesn't come with a ballast then fit an external ballast resistor of 1.5 to 1.8 ohms, which can make the difference between starting and not starting under adverse condition. Wire the resistor between the new ignition feed and the coil +ve, and provide a wire from your solenoid bypass terminal direct to the coil +ve, this would have been one of the white/light-green wires in the original harness.
PaulH Solihull

PS. The CEI module circuitry does contain a current limiting function, presumably to protect itself to some extent, but personally I'd prefer not to put it to the test!
PaulH Solihull

>Lucas connectors are, today, the biggest problem and that mainly from people who do not understand that they have to be cleaned from time to time. Dielectric grease on the contact areas, after cleaning, will cut down on many of the problems commonly found.

To follow up on this, the female sleeves that the bullets go into fail because the metal inside fatigues and cracks. Do not reuse your old ones when installing your new harness. Buying all new ones is cheap insurance.

On my B I changed them all (or at least all the ones I could find) and intend to do the same with the C when I reach that stage. The harness is fine so I don't see a need to replace it, but many of the old sleeves will crush under finger pressure. Better to junk them all....
Rob Edwards

Paul, Lee, and Rob;

Appreciate all the comments and advice. I'm replacing all the bullets on the components as I put them back in the car. Besides the new harness all connectors will be new. I have owned this particular car for 29 years. The wiring was getting brittle in some areas and I decided I wanted all the relays in a more protected area, more fuses, and upgraded wires. I agree with Lee on the use of dielectric grease. As I put on the new bullet connectors, I'm coating each one.

I'll look for a ballast resistor on Monday. Should be able to find one at the local NAPA. Worst case, I still have the resistor wire from the old harness.

Thanks,

Dick
Dick Field

The problem is that new bullet sockets I have purchased are not a patch on the originals, the biggest problem being that the 'pip' that is supposed to retain the head of the bullet is almost non-existent now, making it very easy to pull the bullets out when not intending to.

Secondly originals also had a raised section in the middle preventing you pushing one bullet in more than mid-way, so opposing bullets always sit equidistant in the connector. Again new don't have that, meaning that it is much easier to get one bullet pushed in too far, and the other not far enough, meaning that one end or the other can be closer to the end of the insulating sleeve and hence closer to shorting to earth.

Thirdly the insulating sleeve on originals is, if not actually rubber then rubber-like in that it grips the metal part and partly closes over the ends. New ones have a plastic sleeve that just slides on, and slides all too easily again meaning that one end can be exposed or nearly so increasing the risk of shorting to earth.

Not that I have studied them in any great detail ...
PaulH Solihull

So, old ones that have lost their tension and perished, or new ones that don't fit well. We're screwed! ;-)
Rob Edwards

Some items on this website may be of interest although I haven't any experience of their products.

https://www.holden.co.uk/displayproducts.asp?sg=1&pgCode=082&sgName=Electrical&pgName=Wiring&agCode=0474&agName=Bullet+Connectors+%26+Tools
Brian Shaw

The connector sleeves from British Wire are excellent. Nickle plated steel, so they do not crack, have pips, is a bitch to get the bullet ends in or out - if you get them in, they stay there. The sleeves are rubber but don't fit as well as the OE ones, so I use the old rubber if it is good. And ALWAYS use OxGard or similar.

There is no good reason to replace the original bullets, and you can barely duplicate the OE attachment. The only reason to replace them is that the wire is DOA.

FRM
FR Millmore

As far as getting the bullets fully seated goes, which is when the back of the bullet is flush with the entry of the metal sleeve, I use a pair of pliers with notches cut in the ends of the handles. Just wide enough to go over the standard gauge wire and bear on the back of the bullets, squeezing the handles, as long as they close to less than the length of the metal sleeve in the connector, always forces them home. Works just as well when inserting the 3rd and final bullet in a 4-way connector. http://www.mgb-stuff.org.uk/bullet.htm and scroll down a bit (I know you can buy a tool for the job, but where's the fun in that?). And I assemble mine with Vaseline which lubricates as well as protects from corrosion, a fraction of the price of so-called dielectric grease and a 250ml pot I've had for years will see me out ... but then I only use it on bullet connectors.
PaulH Solihull

I offer a suggestion, that if you are putting in new harnesses, or doing a re-wire, then ditch the bullet connectors and instead of bullets just solder the wires with a piece of heatshrink over the top.

After all, how many times will one be removing a harness? And the odd time that you do, then clip the joint and remake it. Quick, easy, cheaper, neater, effective and lasts for ever!
Hal Adams

Hal, the problem with that is somewhere down the line, a connection will need to be disconnected and more than one wire may need to be severed. RAY
rjm RAY

"...and a 250ml pot I've had for years will see me out ... but then I only use it on bullet connectors."

(lol)
R Taylor

Ray, yes, at some stage maybe - but its simple, cut the joint, trim the ends, remove the old heatshrink and do it all again! You will lose about 1mm of wire on each reconnection.
Hal Adams

The only bullets that I've ever seen give trouble are the ones for the front lights right behind the grille, all the others are relatively well protected, and those are fine if you assemble them as suggested.

That's an awful lot of connections so solder and heat-shrink otherwise, 140 bullets or so, and then there's the spades ...

A mad idea.
PaulH Solihull

Clean the original bullets with scotchbrite or similar and assemble with petroleum jelly as suggested by Paul H. The only time you "see" the build up of resistance on bullets is on the headlight circuit. As P H said it's the ones across the front that suffer most. AND you do "see" the resistance in all those bullets AND the lighting switch, when you put a relay on the lighting circuit. It's like a new dawn!!!
Allan Reeling

The best way I have found to clean the sleeve part of the bullet connectors is to use a gun cleaning brush. Makes sense since they called them bullet connectors, eh? I believe the correct size is a 0.17 caliber. You can chuck the brush up in a drill motor, and they will be quickly cleaned. To get the grease on the inside of these a Q-tip is handy. Photo below.

Charley


C R Huff

PaulH Solihull, why is this a mad idea? I am doing this past 20 years to all exposed joins in my Austin Cooper and MGB without trouble. In fact many of us here do this. We do not think it is mad.
H Chausson

I think eliminating bullet connectors by soldering and heatshrink is a bad idea, especially for those of us who like to do our own maintenance and diagnostics. It's always useful to be able to easily isolate a circuit or section of wiring when tracing an electrical fault.

Bullet connectors do occasionally suffer from poor contact, but I think a little maintenance as described in earlier posts is preferable to spending hours soldering and heatshrinking.
Brian Shaw

This thread was discussed between 07/12/2012 and 13/12/2012

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