|The MGA With An Attitude
LUCAR CONNECTORS, and why I hate them - ET-103A
On September 23, 1004, during a discussion about using a relay, I wrote:
>".... Push on wire terminals (Lucar connectors) could be troublesome in time. Screw posts with bare wire ends, or binding screws with ring lugs are almost infinitely reliable by comparison. ...."
At 11:06 AM 9/23/04 -0700, Mark L. Lambert wrote:
>"Not sure why the spade connectors would be unreliable. .... If you have the right crimp tools, the spades grip the wiring just fine."
If properly installed, a crimp connection can make a gas tight joint, equal to a weld for reliability, as long as it doesn't get physically pulled loose. That's not the problem.
>"I suppose the friction fit is suspect, but there isn't any load on the connectors, so the only problems I can think of are vibration and corrosion."
Yup. Now you got it. Lucar connectors carrying a high current will eventually have some problems. The concept of "high current" is relative. While five amps at 12 volts is nothing at all to a starter motor, and seems only normal to a headlight, it could be sudden death to a micro circuit, or it can melt wiring or start a fire if shorted to ground in a certain fashion. The long-term success of an electrical connector depends on the ability to maintain a very low contact resistance indefinitely (as in forever), but Lucar connectors are not very good at that.
The concept is like a red-hot wire in a toaster. When you pass an electrical current through a resistor it drops the voltage and dissipates energy in the form of heat. Doing that with a long wire, the wire may get warm. When the resistor is physically very small, all of the heat gets dumped in a very small area, and the temperature rises dramatically. A faulty electrical connector with high resistance and a small contact point is like that. It will generate heat in a small spot and can get quite hot with only a moderate amount of energy. For energy reference, you may already know that you can burn you fingers by touching a four watt incandescent night light bulb, even though the bulb is relatively large. Concentrating that much energy in the small contact area of a Lucar connector can make it much hotter. With a five amp current it only requires a 1 volt drop across a corroded connector to make five watts of heat. That would happen if there was five ohms of resistance in a corroded or loose contact. A solid connection is near zero ohms, while an open circuit has nearly infinite resistance. A corroded or loose connector could have resistance anywhere in between (yes even five ohms).
It starts with minor corrosion. When the electrical resistance starts to rise at the point of contact it generates heat, which accelerates corrosion. Moisture can accelerate corrosion. Vibration can also work to loosen a connector and increase resistance. When resistance is high enough it can generate a lot of heat. Then the female Lucar connector looses it's spring temper and relaxes its grip on the spade, and eventually you loose the connection. Vibration certainly doesn't help. Sometimes the connectors just fall off the spades. This may take anywhere from a few years to a decade or two, but it is pretty much inevitable that it will eventually happen with Lucar connectors.
The problem stems from the fact that contact point between the Lucar connector parts on the flat side is large with low pressure, does not make a gas tight joint, and is therefore subject to immediate corrosion. On the curled side the contact is small and has high pressure, and may even be a gas tight joint to begin with. But the contact point is very narrow, so a little corrosion sneaking in from the sides doesn't have far to go to undermine the connection, which is the part that commonly takes a few years.
When you have only a few of these connectors, like on the back of the generator for instance, the periodic attention to fix them may be only a minor irritation. But when the car is loaded with dozens of this type of connector it gets pretty obvious after a while. About 10 years on you end up with connector problems one after the other, and sometimes more than one at a time, until it drives you batty. Then one day you may decide to spend a whole day just cleaning and/or replacing connectors to reduce the problem to a manageable level for a few more years.
The MGA is (was) originally equipped with screw post connectors for bare wire ends on the voltage regulator and dash switches. The early generator and ignition coil used screw post and hex nut connectors, as do the dash instruments. In other places the MGA uses ring lugs on the wires secured with binding head screws, as on the flasher unit, wiper motor, and dipper switch. These connections can be reliable for many decades without a failure. I have never had a single problem with these original screw type connectors in my MGA. The crimp connections between the wire end terminals and the wire should not be a problem, as long as they are properly installed with gas tight crimp joints.
There are of course other places where the MGA uses press together connectors. For example, the in-line bullet connectors in the wiring harness, and bullet wire end connections to corner lamps. These are often problematic for the same reason of corrosion and relaxed grip. Over time I have replaced many of the troublesome in-line bullet connectors with crimp on butt connectors, which generally hold up very well. Occasionally I may solder a wire directly to a contact in a lamp fixture to eliminate a plug in bullet contact. So my MGA electrical system is getting more reliable with time.
But occasionally those "insidious" Lucar push on connectors like to sneak into the system. The first to call were on a replacement ignition switch, which was not available with original screw posts when I needed it. Shortly thereafter a couple more arrived with a replacement generator, where replacement units are commonly all Lucar terminal type to reduce vendor inventory requirements. Some more tried to get in with a new 40KV Lucas Sport Coil, but I quickly slammed the door on that proposition by removing the spade adapters and attaching the original screw eye terminals directly to the screw posts on the coil. A bunch of them tried to jump in all at once with a replacement voltage regulator, but I had enough patience to persist and insist and ultimately buy the proper original type regulator with screw post terminals. Again the same with a replacement flasher unit, keeping patience to buy the correct part with screw terminals. Three of the little beasties did sneak in with the Mallory Dual Points distributor, one on the outside and two inside.
With periodic maintenance of the Mallory points I can check and adjust those connectors as needed, so they have been behaving (under the constant supervision). The connectors on the back of the generator are in plain view, so I get to poke those nearly as often as I check the oil, and attend to adjustment as necessary to avoid problems.
Over the past 15 years I have had to change the Lucar terminals for connection to the ignition switch a few times, which is a bit of a pain when working high up behind the dash. I gave up shopping for an original type ignition switch with screw terminals and finally installed a barrier strip, to take all of the lighting circuit load off of the ignition switch terminals, as well as some live accessory circuits.
Considering the amount of niggly little problems I have with the small handful of Lucar connectors on my MGA, I shudder to think what it may be like having to live with the Lucar problems of a slightly newer model, like a mid production MGB for instance, where almost everything is plug-on, and there are many more wire ends for more devices. Through two decades and 200,000 miles of driving my MGA, it has never had a single problem with a screw terminal wire connector, and I like it that way. Sometimes you can sit back with a grin and say, "Man they don't make 'em like they used to".