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MG MGB Technical - Why are stop lights fused?

I am planning to wire some relay switches into the head lights on my 69 roadster. No lights seem to be fused except the stop lights by means of an inline fuse mounted to the inner fender well. Does any one know of a good or bad reason why this was done?
Dana Wilson

I think you will find that the only bulbs that are NOT fused are the headlights.
David "unless Spridgets were more technologically advanced than Bs..." Lieb
David Lieb

Go to http://www.advanceautowire.com/ and click on "STOCK SCHEMATICS" for excellent diagrams of how the wiring is supposed to look on your car.
Johan Garmer

Johan,
Thanks for the link. I have a copy of Bentley's Complete Official MGB 1962-1974 that I have been studying. My curiosity is not about how my wiring should look as much as why this inline fuse is there. If the fuse box had room for it would they have simply run this circuit through there? I am looking at a photo of the engine compartment of a 1969 (same year as mine) on page 63 of Clausager's book and this fuse is not present. I wonder if it is necessary.
Dana Wilson

" I wonder if it is necessary."
May be they had been installed as a safety caution by PO, preventing the main fuses to blow out, but depending on the good fuse calibration. Might be interesting in case of rear impact and acting as a more selective protection?
Guy RENOU

The inline fuse is shown on the schematics in the Bentley manual so I don't believe it is the work of a PO.
Dana Wilson

None of diagrams 4, 5, 8 or 9 (the ones covering 1969) of the stock schematics on the page, taken from the Bentley manual, has an extra fuse for the stop lights after the fuse box. I'd go with Guy's guess.
Johan Garmer

Joahn,
You are correct, the inline fuse in diagram #4 on page 316 (item #67) is for the parking lights. I should have titled this thread "why are the parking lights fused"
Dana Wilson

This was a transitional period, from no fuses on light circuits to some, ie side/tail/marker lights. Since it corresponds to the introduction of side markers in the US market, I suspect it may have been a requirement under US safety regs. Brake lights and TS are fused through the already marginal W/G fuse. Nobody cared if your headlamps died and you couldn't see; they only cared that other people could see you, so headlamps got no fuses.

FRM
Fletcher R Millmore

You've got to love those transitional periods.
Dana Wilson

Fletcher,
I think it was more of a matter of not wanting your headlights to go out because of a lousy fuse, leaving you in a precarious condition (BTDT). Minimizing the potential points of failure and all that.
David "Joe Lucas never really trusted fuses or relays AFAICS" Lieb
David Lieb

David,

In my humble opinion, you are right, this extra protection reduces the potential to loose head lights in case of a rear electrical issue. I go with you for this.
Guy RENOU

Depends on the year. Originally brake lights, indicators/turn signals and when provided reversing lights were fused, but not parking or headlights except headlamp flasher from 1968 (US) or 70 (UK). In 1968 all cars got fused parking lights, originally one in-line for the fronts and one for the backs (my Leyland schematics show this as two years prior to side-markers for the US in 1970). In 1970 this changed to one fuse for each side in the (now 4-fuse) main fusebox. Headlights were never fused.

I had to smile at the question, mostly people ask why so *few* circuits were fused! Shorts at the fuel pump and overdrive are not that uncommon, and can total the harnesses if not the car. If adding relays to headlights the improvement in current availability can cause a fire much more quickly than via the original 'weak links' of dip-switch and main lighting switch, which were bad enough. So fuses really do need to accompany relays. One per filament if you don't want to be plunged into darkness from a problem at one corner. Whether you then use a master fuse or fusible link from the source (usually the solenoid) to the relays is up to you, but will usually only be protecting a couple of feet of wire, and only then if the fuse is right at the solenoid which isn't the most convenient position.
Paul (too many Pauls own MGBs) Hunt

Paul,
I would add that a fuse per filament is not a bad idea even without relays. Quality of replacement dip switches, at least in the USA, is not up to that of the original and it hurts to lose an original to a short.
David "my MGB had the classic melted dip switch" Lieb
David Lieb

David,

You go to Advanced Autowire web site and you scroll in the main page. A toggle button on the left page side is titled stock schematics. You choose your car year drawing and the modification is straightforward. IMHO,and unless you use relay, fusing each filament is not that relevant: they melt or open but don't short. Fusing relay is setup to protect relay or solénoïd which are not as Over Drive or fuel pump as mentionned by Paul.
http://www.advanceautowire.com/
Guy RENOU

Do you mean filaments can melt or open but not short? Quite right, usually, but fuses are there to protect against shorts from *anywhere*, including wiring damage, which is why they should be as close to the supply as possible.
Paul Hunt 2010

As I Told it Paul, "fusing each filament". But I do completely agree with you about fuses aim, of course.
Guy RENOU

Hi
I don't know if it's the same over there as it is here but a lot of the later cars here (not MG's)are fitted with bimetal strip circuit breakers. They are a lot better setup for headlights--- If you happen to get a short in your headlights they flash on and off instead of the dark thing happening Cheers Willy

Paul- an interesting little problem I had once.
A guy came in with his car and said that if he had his parklights on and his foot on the brake he couldn't turn his engine off.- He could turn the key off and pull it out and the engine would continue to run until he either turned the lights off or took his foot off the brake.
Turned out that there was a short in one of the taillight globes and if the lights were on and the brake on as well there was a power feed from the lights,back through the brakelight switch, powering the ignition.
WilliamRevit

Later American spec MGBs had thermal circuit breakers instead of fuses for some circuits, but whether they opened and shut automatically or needed to be reset I don't know.

I can quite see that happening, William, although not as disturbing as the starter continuing to crank when you have released the key when one of the isolation diodes has shorted.
Paul Hunt 2010

Oops-- Sorry Paul my mistake, The engine running car wasn't an MG it was a Ford thing which has the brakelight switch powered through the ignition. Cheers Willy
22'C here today-sun shining,very light sea breeze.
WilliamRevit

"it was a Ford thing which has the brakelight switch powered through the ignition"

So do most MGBs which is why I could see it happening to one of those (except very late UK models).
Paul Hunt 2010

Paul,

On my 77 USA spec, I had the problem of it not shutting off. I had to switch the wire on one of the under hood (bonnet) relays, per the instructions on your site or from an archived post of yours (thanks).

Does making that change do anything to prevent the starter run on that you are talking about here?

Thanks,
Charley
C R Huff

US cars with the ignition relay had a wiring error that meant the car wouldn't shut off if there was anything wrong in the emissions plumbing or anti-runon system. Oddly enough, and quite coincidentally, doing the mod that was made to UK cars i.e. moving the coil feed from the output of the relay to the ignition switch and moving the ignition warning light from the ignition switch to the relay, *will* prevent this problem of continuing to run if both the running and brake lights are on and there is a bridge between them, as it isolates the coil from everything else so you can't get voltage feeding back to it.
Paul Hunt 2010

Paul,

It sounds like you are saying that two wiring changes are required:

"i.e. moving the coil feed from the output of the relay to the ignition switch AND moving the ignition warning light from the ignition switch to the relay..."

I can only recall making the first of these two changes. Maybe I did both and just don't remember. I guess I'd better have another look.

Thanks,
Charley
C R Huff

In theory only one change should be needed, and indeed when the UK initially got the ignition relay in 77 only the ignition warning light wire moved. But by 1980 the coil wire had moved as well.

The change had to be done from the outset in the UK as we never had any emissions and anti-runon systems, which is what shuts down the engine on US cars. The problem was caused by moving all the ignition loads to the relay except the ignition warning light. With the ignition off but the engine still spinning there is enough current from the alternator through the light and the relay winding to keep the relay operated. Which is why either moving the light to the relay contact, *or* putting the load of the coil back on the ignition switch solves the problem. The former by removing the leakage current through the relay winding, the latter by virtually shorting out the relay winding which pulls the voltage below that which will keep the relay operated.
Paul Hunt 2010

Paul,

I guess I'm going to have to go look at the car to be sure which of the two I did. If I understand correctly, you are saying that doing only one of the changes will correct the failure to shut down the engine. Are you also saying that doing only one of the changes will prevent the starter motor run on?

Thanks,
Charley
C R Huff

Grrrr, nearly finished typing this when I hit some key combination that closed the browser, so I've got to try and remember what I wrote.

I'd always considered that either change on its own would be effective (at shutting down the engine), even when I was only aware of both. It is only relatively recently I discovered some schematics only show the one change, which if correct proves my thoughts for that change anyway, butraises the question of why they subsequently thought the second change was required. OTOH maybe the schematic is in error and both changes were always done.

Continual cranking of the starter is completely different, and is caused by the brake test/handbrake warning light diode failing short circuit, unless you have the presence of mind to drop the handbrake. It's complicated on US cars by if the brake imbalance switch is operated then dropping the handbrake won't work, you have to remove the green circuit fuse. The circuit is completely pointless on UK cars, the only time it does anything is if you happen to crank with the handbrake off, when it cause the handbrake warning light to glow. How many times do you do that, and would you even be looking at the console anyway? On US cars it does have an additional function in that it proves *some* of the wiring and the bulb for the brake imbalance warning is functioning, but again only if the handbrake is off when you crank and you are looking at the bulb. On these cars the handbrake itself proves the bulb, but relatively little of the wiring, rendering the diode of very little use indeed. If you want to remove the possibility of this happening should the diode go short-circuit in the future, then remove it (red/white and green/orange wires) and securely tape up the two wiring spades that are left (tape them to the main harness for their full length but in opposite directions). If these two should happen to come into contact, then it will *cause* the continual cranking problem.
Paul Hunt 2010

Thanks Paul. Sorry you had to type that twice. Part of the complication in the system you are addressing has already been eliminated since my brake imbalance switch is not connected.

I'll copy and save what you have here so I can look it over when I get back to work on the car. After doing the paint work on the engine bay, it got cold, so I stopped before hooking everything back up. I also stopped at that point in case I find a deal on an overdrive gearbox.

Charley
C R Huff

This thread was discussed between 06/03/2010 and 17/03/2010

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