The MGA With An Attitude
Everybody wants MORE FUSES and RELAYS - ET-200

Can I talk you out of it?

I won't harp on anyone in particular here, but I have been getting lots of requests for information about installing additional fuses, and even more about installing relays. These are often mentioned in the same breath, usually refering to headlights and headlight upgrades. Occasionaly a relay is being suggested for the heater motor, or a fuse for the fuel pump. Oddly though, I almost never hear of any interest in fusing the parking light circuit, even though there are far more wires and devices there than within the headlight circuits. Well before you get too caried away with these ideas, maybe I can talk you out of it. This article will be about why people want to do such things, and why it might not be necessary, or may even be a bad idea (in some cases). I am about to play devil's advocate and take some practical pot shots at such ideas. Before you jump to some fast response early on, maybe read the whole thing first, as some of your questions or arguments may be addressed farther down in the memo.

The most common notion is that a couple of relays might improve the light output of the headlamps. Under the right circumstances this could be true, but adding relays also requires adding some wires, and more wiring means more wire terminals, and more wire connectors opens the door for more connector problems. And when you stop to think about it, most of the complaints about electrical problems in old British cars are not because of small wire sizes, but more often stem from bad wire connectors causing voltage drops or lost ground connections. Hmmmmm.

One headlight circuit diagram recently presented for my consideration shows four relays, one for headlamp high beam, one for headlamp low beam, one for a fog lamp, and one for a driving lamp. It also shows four fuses to go with, and a terminal block being used to split the new heavy gauge power supply wire into as many as 8 different circuits, two for each relay to supply the power contact and the actuator coil circuit in the relay (yes, it is rather strange). Each relay has 4 or 5 terminals, each wire has two ends that have to make an electrical connection somewhere, fuse holders and terminal blocks also have contact points. While attempting to count all of these new wiring connections on fingers and toes, I had to borrow three more people. About that time it dawned on me that this was about to nearly double the total number of electrical connections in the MGA, and I'm fairly well convinced that this is not such a good idea.

The general reason for wanting a relay here in the first place was to improve light output. The logic applied is that a new heavy power supply wire and a relay may increase the voltage at the lamp fixture by reducing voltage drops throughout the vehicle's electrical system. The first thing I would like to know is if you can either substiantiate or clarify or quantify the improvement in light output, which can only be a result of increased voltage at the lamp. I can certainly believe there could be some improvement, given the increased size of the new supply wires for the lights. I'm just wondering if all this new wiring can be justified on that point alone, as the rest of the effort might have very little benefit. You will get more of the flavor of this idea as I go.

Some years ago I replaced the original incandescent headlamps with H4 halogen lamps, which was quite obviously a big improvement in light output and visibility, well worth the $20 with no other modification required. I don't know for sure the output, but I seem to recall the original lamps were 45/35W, and the replacements might be 55/45W, or something to that effect. You might even use 60/55W headlamps, which I can certainly appreciate as being another step brighter, but so far having not much to do with relays and wiring additions. When you can have additional light output simply by installing brighter lamps, the relays are not particularly pertinent to that point. So here's a question. Would you get more light with 55W lamps and a relay, or with 60W lamps and no relay? And if those two cases might yield aproximately equal light output, then they would also be drawing aproximately equal current. Is it worth the added expense and effort to install the relay with the 60W lamp to increase the light output marginally more? Would it not be just as effective to install a 70W lamp rather than the relay (and maybe less expensive, considering you may be about to pop for the cost of the 60W lamp anyway).

Then again, at some point of increasing the power draw for more light, you suddenly realize that the original wiring simply won't carry the increased current load without overheating, in which case a relay circuit becomes a necessary upgrade to go with the uprated lamps. Or alternately you could tear the harness apart to install larger wires. But an obvious question here is, at what point do you concientiously make the decision to increase the lighting capacity to the point where it exceeds the load capacity of the original harness wiring? And Why? How much light is enough already? Or if more is always better, why not install 100W headlamps? My car seems to be quite happy with the $10 halogen headlamps and no additional wiring or relays. Is it worth going to all the trouble of adding wires and relays for the extra 5 watts per headlamp? Or is it even needed at that point?

The next question is about some 40 amp relays that were shown on the diagram (same circuit noted above). Keep in mind that the generator on the MGA is only good for 22 amps maximum, which is usually enough to supply the electrical needs of the entire vehicle (marginally). Most of the relays will carry 10 amps or less, even with upgraded headlamp bulbs. The one exception on this particular schematic is the High Beam Relay which is shown feeding power into a Driving Light Relay, and would therefore be carrying the power for at least three lamps instead of two. But that can be fixed. You could run power (via the new heavy gauge harness) directly to the Driving Light Relay, and send a signal from the High Beam Relay output to trigger the Driving Light Relay. Then rather than using the Driving light switch to send signal power to the relay, use that switch to control the ground circuit for the relay. No additional wires are required in the original harness. Just rewire the Driving Light Switch so the original wire going forward to the lamps now goes to the relay ground terminal, and the original power input terminal on the switch gets connected to ground instead of power. Then no relay will ever carry more than 10 amps, and the fuses and supply wires (and relays) may be sized accordingly.

Another petty point on the relays. For servicing convenience it may be best to use four identical 5-termianl relays, to avoid stocking extra spare parts for the 4-terminal relays. Or if you don't mind connnecting multiple wires to a single terminal, they could all be 4-termainal relays. This condition is a non-problem with screw terminal devices, and generally not a concern when using a relay socket. In the case of a relay socket, 4 terminals could also be more reliable than 5.

But these are still added relays and added wiring connections. The relays may also have push-on spade connectors, which are not the most reliable thing to have in a power carrying circuit. One of the high reliability points of original wiring in the MGA (chuckle) is that many of the wire terminals are screw posts, which are far more reliable than push-on connectors. This includes all of the dash switches (including the original ignition switch), the original control box, 1500 type turn signal relay, 1500 generator, ignition coil, distributor side terminal, original type distributor cap high tension wire connectors. wiper motor, brake lamp switch, dipper switch, flasher unit, and all fuse block connections. In all of my experience with MGAs I have never see a connection problem with a screw post terminal. Gotta love it.

In general, the Lucar connectors are a perpetual problem waiting to happen, and they tend to inconvenience you at inopportune moments (like there might be any other kind). Also a small voltage drop at any less than perfect push-on connector could negate any voltage advantage you were trying to achieve through the use of a relay. For new relays, you might give some consideration to using screw terminal relays, which can be had in the form of a generic starter or ignition relay (not the larger starter solenoid switch). When you do this, connecting multiple wires to a single termianl is no probem, so 4-terminal relays would serve quite well (even 3-terminal relays in some cases). Or you might consider using a high reliability relay socket and soldering the wires to the socket terminals. Here too multiple wires on a single terminal are workable, and 4 contacts between the relay and the socket will be more reliable than 5.

There are also new fuses in these new circuits. If the diagram represents any kind of spacial relationship, I presume the new fuses are mounted on the same panel with the relays (not a bad idea), somewhere near the bonnet latch (not such a good idea). A new unfused 10 ga wire (that's what it said) running from the starter switch forward the new relay panel represents a new possibility of something shorting out with a high current to cause a burn hazzard. If you are going to the effort of adding new fuses for separate circuits, it would be better to put them as near to the power source as possible to minimize the length of unfused wire, especially the large high current wire. So you might consider locating the new fuse panel near the starter switch, and running the multiple fused wires forward from there.

This diagram also doesn't note the existence or location of bullet connectors in the original (or new) wiring harness. There are bullet connectors located within about 18 inches of the headlamps and parking lights, and probably the same for a driving lamp or fog lamp with pigtail wires. I suppose you would not be replacing the wires on the lamp itself, so these connections should be shown in the wiring diagram.

Are you falling asleep yet?

Also consider that the primary reason for physically locating the relay panel near the front of the car might be because that's where you see the existing bullet connectors near the lamps. Or maybe this circuit designer was thinking to run only one unfused power wire forward rather than 4 fused wires? There is a small section of the front harness running between the left and right side lamps, connected with the same bullet connectors, and incorporating 3-way connectors near the right side of the car where the harness approaches from the inner fender. Rather than placing the relay panel near the center and running separate wires both left and right, you could put the relay closer to the right side and run a single wire to the 3-way connector. If you are concerned about voltage drop in the original wire carrying only 5 amps to the one lamp on the left side, you could, without too much trouble, replace the short section of cross harness with one using larger gauge wires.

Then consider the distance from the central location of the relay panel to the bullet connectors on the right front corner. With about the same length of wire runs you can physically locate the relay panel on the inner fender near the starter switch. There you can once again incorporate the relays and fuses into a common panel mounting. Now you can have the new fuse and relay panel physically located near the power source with no penalty in wire length. In case this hasn't dawned on you yet, this is exactly why the later model MGB cars had the fuse panel located on the inner fender. Don't you suppose the wiring would look a bit cleaner in that configuration?

About wire size, .... 12 ga wire to power a pair of headlamps is definitely overkill. In house wiring a 12 ga wire is used for a 20 amp circuit with up to 100 feet of wire running in each direction (200 feet total) with negligable voltage drop and no detectible heating of the wire. 14 ga wire services a 15 amp circuit with the same conditions. Automotive wiring always has shorter wire runs, and 16 ga wire can comfortably carry 10 amps with negligable voltage drop. Three numbers difference in wire gauge equates to doubling (or halving) the current capacity. For reference, most of the original wiring in the MGA is 18 ga. Kicking that up to 16 ga would be a big improvement, nearly a 60% increase in current carrying capacity, or nearly a 40% reduction of voltage drop (and also makes for larger wire bundles). Problems which people commonly perceive to be caused by voltage drop in the wiring are much more often caused by corrosion of the end connections. If you think your headlights are unnecessarily dim, or your heater motor is running slower than it should, check and clean all of the wire connections before you decide that you need a relay.

Now a few observations about fuses and burned wires from experience. I drove a few different MGA when in college in the late 60's, total about 20,000 miles. My current MGA had 150,000 miles on it (with original wiring harness) when I bought it, and never had the wiring burned. I restored it, including installation of a new wiring harness, and have since put another 225,000 miles on it. Through all this, I don't recall ever blowing a fuse, let alone burning any of the wiring. But it has had plenty of lighting problems due to bad bullet connectors.

For other MGA I have known which have had wiring problems (other than end connections), the problems are invariably the result of neglect or faulty installation of the harness, or some egregious and unforgivable bodge by the owner (or mechanic). Examples: Cigar lighter mounted in dash and connected to the ignition switch for power. Bypass jumper wire for parking lights running from light switch in dash to bullet connector at right front corner of car via shortest route across top of engine, because the DPO wouldn't fix the corroded bullet connector in the harness near the starter switch. Parking light wired in with bare twisted wire connections and no rubber boot or grommet on the back of the lamp fixture. Wiring harness wrapped around frame (with sharp corners) for support rather than being secured with proper p-clips or wrapping bands. Intermediate battery connection cable lying on top of rotating propshaft. Any number of single wires passing through the sheet metal firewall with no grommets. Many instances of radio connection wires running under carpet in areas subject to foot traffic. Any kind of add-on accessory in or behind the dash with new stray wires draped over metal cables and brackets or hanging without physical support. Any wire attached by twisting the ends or twisting one wire around another, with or without finish insulation. Any wire with a bare end stuffed loosely under a Lucar connector.

The list could go on ad infinitum, but you get the idea. Wiring problems, including voltage loss problems, are most often the result of some neglect or abuse of the wiring system. A well maintained MG will usually have very few problems with the wiring. Those most common problems are usually with bullet connectors and grounding points, which have little to do with relays, and may not even be cured by a relay. Bottom line here is that the time you might spend engineering and installing such relays and additional wiring might serve you better if you spent the same amount of time inspecting the original wiring and fixing any deficiencies you might find, like loose or corroded connectors or mising grommets.

When you do find it necessary or desirable to add any wiring to the MG, do try to do the best job possible to make it look nice as well as being reliable. For many people who know even a little bit about MGs, when they see any new wiring that looks like it doesn't belong there the first impulse is to rip it out of the car immediately and ask questions later. This is not necessarily because it doesn't serve a noble function, but more a reaction to long standing experiences with DPOs and DPMs and even DCOs who often make a total bodge of such projects. It will help your personal case immensly if the end result at least has the appearance of a clean and careful installation. To that end, you would want to cover bare wire terminals and wrap up new harness sections to look similar to the original wiring harness. Bright colorful wires in plain site will not help your cause when the car originally had most of the wires wrapped in a black harness cover.

In the event that you do add some accessory to the MG that might consume more than 10 amps of current, like some high power driving lights or a stereo radio greater than 120 watts, then you may not only be considering relays, but also the need to replace the generator with an alternator for more charging capacity. Keep in mind that the original generator is good for about 22 amps peak output, and just switching on some uprated headlights and a driving light or two may exceed the capacity of the generator. How long can you run at night on battery power if your car is consuming 30 amps when the generator is only putting out 20 amps? Just a thought. The corrolary to that logic is that if your total power consumption is low enough to allow your car to survive on the original 22 amp generator, then there is probably no single circuit in the car drawing enough current to require a relay.



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