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MG MGA - Headlamp circuit relays

Thought I had better start a new thread as the subject has changed from the original "Extra fuses re-think and re-work". The headlamp relay circuit is up and running. I fitted it on Friday afternoon and evening and it would have been a quick job had it not been for the difficult access to the headlamp "pigtails" with the radiator grille in situ. I would have been better off removing the grille, but I just couldn't face it. I also had to remove the heater trunking to feed the headlamp power lead along the existing loom. Anyway, it is a fairly simple task, the two relays, one for dip and one for main beam, are mounted on the right hand side (from the driver's seat) of the bonnet latch bar and are operated by the existing wiring from the dipswitch. The power for the headlights is fed by a single 17.5A wire via a 25A fuse from the A1 terminal to one side of the contacts of both relays and the headlight wires are connected to the other side contacts of their respective relays. The result? the headlights are noticeably brighter, as borne ot by the following figures. With the engine running at about 1500 RPM and the votages stable and the headlamp dip beams on, without relays I got 12.6 volts on the A1 terminal and 11.55 volts at the lefthand headlamp, and with the relays I got 12.5 volts at the A1 terminal and 12.2 volts at the lefthand headlamp. That is a 1.05 volt increase at the headlamp, well worth the effort I would say!
Lindsay Sampford

Your maths skills seem to be different to mine I have to say, do I need to do my degree again Lindsay? :-)
Robert (Bob) Midget Turbo

Sorry, that should be 0.65 volt increase at the headlamp!
Lindsay Sampford

So that is about a 5% increase in voltage, fraid I am not an expert in luminaires so does that translate into a direct 5% more light or is it not a direct relationship?
Robert (Bob) Midget Turbo

Hi Bob, in my haste I was looking at the wrong numbers before! I just did the same test again (it's easy to patch the relays out and make the wiring as standard) without the engine running and without the numbers dancing about on my volt meter and this is what I get. Un-modded; 11.00V at the headlamp and 12.04V at the A1 terminal and with the relays 11.66V at the headlamp and 12.03V at the A1 terminal. So the voltage increase at the headlamp is pretty much the same as with the engine running. I did the measurements quickly and turned the headlights off between measurements to minimize the effect of the battery "going down". According to FRM's figures relating voltage to lumens it should equate to a 16.5% increase in the said number of lumens produced by the lamp. It definitely looks brighter with the relays, I'm sure I'm not fooling myself!
Lindsay Sampford

Lindsay

This is all very complicated for my brain! Unlike Bob I do not even have a degree to fall back on. I think I will stick to day time driving!! Quite possible now as I become a gentleman of leisure from 1 Oct - no choice. Still I should now be able to afford that elusive supercharger!

Steve
Steve Gyles

I did a similar relay installation. I think the lights are slightly brighter, but the real reason I did it was to reduce the load on the headlight switch. The original switch showed evidence of having been quite hot at times.


Ed Bell

Lindsay-
I think you meed to do some more work! Or maybe check your meter.

Your readings at A1 are way low, and indicate a basic charging problem - generator, wiring, or control box.
One thing to look for is high internal resistance in the (connections of) control box shunts, which are riveted through the back side of the box. The control box settings require that the generator not even cut in until 12.7-13.3V, since that's what a good 12V battery should give. That's measured between D & E at the control box, but should be very close to the reading from D terminal at gen to case of generator. And A & A1 to Earth should be very close to this also. If not, then there are bad connections or contacts to or in the box. All these should be no more than 0.2V max different, my opinion.

I can't find a wiring diagram for MGA just now, but if it is wired as Magnette, the power goes from starter switch/batt cable to ammeter to fuse block on its way to control box A; these could be significant drop point(s).

I rather expect that you will find that all these grub screw connections are blackened with oxides due to heat. If there is sufficient cable, cut off the ends a bit; otherwise twist the bare ends around a bit with OxGard and reconnect. OxGard is a special electrical grease designed to dissolve oxides; something similar ought to be available there - usual use is in alum/copper connections in mains service connections, so look at a home electrical place. Use it on all electrical connections, especially the grub screw ones. Use it on thee screw threads, Do not overtighten the screws, do retighten a couple of times after some use - they do loosen as the wires settle in, and that is a major cause of trouble with these.

As stated in the other thread, I have not had opportunity to measure a generator car like I have later ones, but open circuit test voltages are set at 15.4-16.6V, temperature and specific control box related. See manual for procedures.
In any event V at D & A/A1 should be 14-14.5 at least. Generator at 1500 or 3000 rpm depending on control box - the difference is minor.
As Barney and the shop manual state, a moving coil (analogue) voltmeter should be used for these setups.

The battery voltage fully charged should be above 12.75V, and this should show up at A & A1, so you are getting a significant drop somewhere between there and the batt.

Check all grounds first, as detailed in Electrobabble. Final check should be gen case to batt ground post, no more than .25V, everything ON.

Then, all power, final gen D to Batt hot post, again not more than 0.25V.

Final charge system check, gen D to case and Batt + to - should be absolute max 0.5V different - I normally get this to 0.2.

I normally expect to see about a 1V increase solely by fitting the relays, IF everything else is "good". That is no magic of relays per se, but is the result of bypassing a lot of wire and decent connections; if bad connections/switches are also bypassed you will see the up to 3V I have posted. However, the voltage increase caused by the relays will generally increase the drop across bad connections still in circuit. so you may not see the expected improvement until other areas are attended to. In this case, that would be the power feed to A1, and the pigtails/plugs at the lamp.

I measure the headlamp voltage directly across the lamp connector blades. Differences between the hot headlamp blade and A1, or the earth headlamp blade and engine earth will tell you where drops are happening.

FRM
FR Millmore

FRM, I don't think I had enough revs on when I did the initial tests. I just tested the voltages on the A1 terminal, and before starting I had 12.6v, with the engine running at about 1500-2000 I had over 14v and with the headlights on it was about 13.5v. I have never had a flat battery and the lights do brighten when the generator cuts in so I think the low voltages on the test were down to the operator! Without the engine running I am getting a 0.66v improvement in headlamp voltage with the relays. My wiring harness is new and the connections were clean and tight and the earths good before I started so I'm not going to see the improvement that you would get if starting with a poor system. When I get time, I will run the tests again with the engine running faster. I am very glad I did the modification, even a modest improvement is an improvement and it greatly reduces the current through the light switch and the dipper which has got to be a good thing.
Lindsay Sampford

This thread was discussed between 07/09/2009 and 09/09/2009

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