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MG Midget and Sprite Technical - Lazy brake lights

I have lazy brake lights. They do work, and are bright. It is just that they only work when pressing quite firmly on the pedal. A light foot, as one might choose to do for progressive braking on a slippery road, and they don't come on.

It has the hydraulic pressure switch. I am wondering if the switch itself is at fault (about 2 years old) or maybe air is collecting in the short branch of the 4-way junction. Any thoughts? Would I be better off abandoning that switch and fitting a mechanical one at the pedal box?

Guy
Guy Oneandahalf Sprites

Guy, the hydraulic type switch does take a minimum amount of pressure to activate. Changing to a new switch may help, but if you want lights that are "instant on" when you step on the brake pedal you'll have to change to the mechanical switch which will change with even a minute movement in the pedal if properly adjusted.
Bill Young

Guy

Mine's mechanical so I can't comment on how much pressure you should need to active yours. If it's worse now than it was, change the switch. As for air in the circuit, as the switch is operated by a diaphragm, theory says that regardless of any air in the switch the pressure being exerted on the diaphragm will be the same, hence shouldn't be an issue. My guess is the contacts in the switch are suspect or the switch is leaking internally and there's brake fluid in the switch. Either way replacement is the answer.

Bob
Bob Davis

Hmm, sort of what I suspected. A bit loathe to simply buy another switch if this one has lasted rather less than 2 years. It implies a poor design of switch. I think I will look at a mechanical one as I think it is important that following traffic has adequate and timely information!
Guy Oneandahalf Sprites

Guy

Maybe not as much poor design as poor manufacturing quality.

It's getting harder & harder to find replacement parts (at least here in the States) that aren't made in China.

I've had Chinese switches, relays, etc. fail astonishingly quick.
Dave Rhine ('78 1500)

Recently saw 'fast reaction' replacement bulbs in Halfords which sounds like a good idea.


A link to some research into reaction times to differing bulb types which may be of interest:

http://deepblue.lib.umich.edu/bitstream/2027.42/64045/1/84696.pdf
Doug Plumb

I'm afraid it's the usual buy at least two to get one working

I think I'm on my second replacement now on my midget but it is working at reasonable pedal pressure

I had the same problem with my Rover only there they stopped working altogether one was even NOS from the 70s and stopped working
Nigel Atkins

Your right. I meant poor manufacture rather than specifically poor design - although "design" to me includes materials so it could well be both. Whatever, the one I replaced 2 years ago was, I think, the original from 1971 so it had done acceptable service. It failed because one of the spade terminals eventually corroded through. The pressure activated switch was still working fine after 37 years.

The issue now is do the current mechanical switches have any better a life span? I might look at using a M/cycle one from a manufacturer with a good reliability record.
Guy Oneandahalf Sprites

This is why I now use the hydraulic switch simply to turn on the relay

The contacts inside modern switches reach this point quite quickly as I found out when fitting the third one to Lara since 1999, I think they "burn" across the faces and this causes the lecktrickery to have to fight the resistance before it can get to the bulbs

The lower power requirements to switch the relay may even work fine on the old switch

Even if I were fitting a mechanical switch nowadays I'd run the things off the relay

Low switching current and high load supply straight from the battery power supply gives a better supply at the bulbs

The lights still cannot turn on unless the ignition powered circuit switches them on via the relay



rather scrappy diagram shows how I wired mine


Bill 1

Bill

Good idea...I've been incorporating relays into the system on my 1500 for the very same reason...to take the load off of the switches.

That's a pretty good diagram...self explanatory.
Dave Rhine ('78 1500)

I could revive my switch by applying my full power (I am 93 kg) on the brake pedal for 10 times. It worked for another year. If I use the sprite as a daily car there is no problem. It is after the winter stop that the switch fails.

Flip
Flip Brühl 948 frog 59

Bill,
I had assumed that the diaphragm inside the switch was being sluggish to move. But I take it that you are saying that the diaphragm is moving but initially is resulting in only a light connection of, possibly low quality and corroded, contacts. The corrosion provides sufficient resistance to reduce the current to the brake light bulbs, at least until the diaphragm movement increases significantly to make a firmer contact, reducing that resistance.

Sounds very likely to me. And the solution of adding a relay to the circuit is dead simple. I like it! Thanks for that!

Whilst we are on with electrical modifications, I will now start another thread....

Guy
Guy Oneandahalf Sprites

Guy, these 'reproduction' switches are junk. They don't last at all well. Theres nothing wrong with the design or the original concept. If you can get an O/E switch then you won't have any problems. I bought a few a couple of years ago, but wouldn't have a clue where they were stored when we left...

Mark.
M T Boldry

I have a NOS brake light switch for earlier cars - Frogeyes, presumably - with screw terminals rather than spades.
Dave O'Neill 2

Guy - As the others have stated, the problem is sh---y replacement switches (one lasted me a whole two weeks before dying). Dave O., if you have a true red box Lucas NOS switch is it should last you many, many years (you might want to consider removing th eswitch every night and keeping it in a safe at night - They are very desirable). Since the good switches are no longer available Guy, you have two choices - 1) order a heavy duty switch (P/N SW-34) from Ron Francis Wiring http://www.ronfrancis.com/ These switches are rather on the spendy side (plus shipping from the US), but from reports that I have received, they do last very well or 2) purchase one of the cheap sh---y switches and make and install a relay/arc suppression circuit at the same time. As Dave R. (as on the T series BBS, there are too many Daves on this site) states, the relay takes the load off the switch and the arc suppression circuit will insure that any arcing at the switch is held to the absolute minimum. For information on making and installing the relay/arc suppression circuit, see the article, Brake Light Relay in the Other Tech Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/ Cheers - Dave D. (told you there are too many Daves on this site)
David DuBois

This thread was discussed between 26/11/2010 and 29/11/2010

MG Midget and Sprite Technical index

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