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MG Midget and Sprite Technical - Attaching a voltage stabiliser

My '71 car should have a voltage stabiliser, but I have run without one for years.
My question is this - I want to fit one but does the unit have to be earthed to the bodywork or can it just "float"? I have just bought one from Moss. It has arrived in two parts, but I think it is supposed to be as one. The unit has 4 terminals, connected as 2 pairs. There is a metal strip with a hole at one end loose inside the packaging. I think it is supposed to be spot welded to the unit casing - to form a mounting bracket.

If it doesn't need to be earthed I can just hold it with a zip-tie, but if it needs earthing it needs to go back to Moss.
Guy Oneandahalf Sprites

Goodness I wouldn't have bought one of the old "hot wire" voltage stabilisers. Much better to get an electronic one like this http://cgi.ebay.co.uk/Semiconductor-Voltage-Stabilizer-MG-midget-MGB-MGC-/280615200810?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4155f8282a

Still, if you have got the old type, according to the beautiful coloured wiring diagrams on AdvanceAutoWire, the unit case should be earthed.
Mike Howlett

Thanks Mike,
Actually the only reason I bought it was I was buying a new hydraulic brake light switch and the Moss site wouldn't accept an order less than 10 so this was to make up the shortfall. Didn't count on getting a faulty one! Given that it is faulty I will return it and get my money back and go for an electronic one instead.

Thanks for the advice.

Guy
Guy Oneandahalf Sprites

Hi Guy. My brake switch has packed up too. Hasn't been on long. I've read the archives and the crux seems to be to fit a relay. Bought one today and will fit soon. I'm discounting my use of silicon brake fluid as there appears to be no evidence to support that this is the cause of failure as these hydraulic switches go kapput with mineral based fluid too.
Neil
Neil K

Hi Neil,
Yes I have had 2 pack up within the last 4 years. The tale seems to be that the contacts of the switches now available aren't up to the current that they need to carry and they either burn out or build up deposits which introduces unwanted resistance. The solution, so I am advised, is to wire the hydraulic switch to operate a relay which in turn handles the heavier current flow.
I have the bits I need, and will fit them in the next day or two if it is dry.

Guy
Guy Oneandahalf Sprites

We'll swop notes then as we progress. Wrong thread title I know but there you go!
Neil K

I also made mention of it on the "Relays" thread
Guy Oneandahalf Sprites

Oh. I must read all the threads. I tend to skip a few. Some interest me, others don't I guess. Now, if I was Prop I would live, breathe, sleep and s--t this BBS!!
Neil K

This is what I thought I would do

Neil K

I was aiming for similar, but piggy backing a supply to Relay #30 from the existing 12v in to the hydraulic switch. Saves bringing in another supply line.

Guy Oneandahalf Sprites

When I've fitted relays before I've supplied the power to pin 30 direct from the battery. Pin 87 then goes to the accessory (in this case the brake lights). Pin 86 is the supply to the relay (in this case via the hydraulic brake light switch)and pin 85 goes to the earth.
Neil K

Agree,
But in this installation I don't think that the rating of the supply to the brake lights is in question, and I see no reason why the existing supply to the hydraulic switch shouldn't also supply the relay.
When I fitted relays for my headlights, the logic was different as I was looking for an increased capacity cable to supply the lights via the relay, because I was uprating the lights themselves and drawing more current.
Guy Oneandahalf Sprites

I am a bit confused on the need for a relay to the brake switch. I thought it was to reduce the amount of power going through the switch to feed the brake lights as it was this that caused the contacts in the switch to fail? By using a relay a fresh power source feeds the lights and the brake switch is used merely to activate the relay.
Neil K

Yes Neil, but unless I misunderstand things the power through the hydraulic switch is determined by the current drawn by the relay solenoid, between terminals 85 and 86 on your diagram. And this is much less than switching the brake lights themselves (2 X 21w / 12v = approx 4 Amps) - Actually I think more than this because I would bet that the voltage at the brake lights themselves will have dropped well below 12v

How the power arrives at the switch and at the relay input terminal 30 is (more or less) irrelevant to the power that the switch carries. This assumes that the feed cable is sufficient to operate the relay solenoid as well as the two brake light bulbs. I am guessing that the relay itself draws very little current as that is the whole purpose of using it. Hence I don't think it necessary to run a new cable to the relay in this instance.

Hence I will place the relay close to the switch and use the existing loom to supply it, and from it to the brake lights. If I was using it to operate a high power consumption item I would give it a separate power supply and also increase the cable size between it and the device (e.g spotlights) that it was feeding to.

However, I may be misunderstanding this - electrickery is not my strong point!
Guy Oneandahalf Sprites

I'm going to fit a mechanical switch operated by the pedal. The current crop of hydraulic switches are rubbish and last no time at all. However, a relay ought to prolong their life.
Mike Howlett

Sorry but a bit of doom now - if the mechanical brake switches are anything like those for Triumphs they self exploded apart after a short time and you'll find the white plactic (?) cap and spring in the footwell

I'm on my (third) hydualic switch but it's working fine, so far (no relay)

I'm with Mike rather than a new modern made voltage stabiliser I've got the semi-conductor

this is an alternative - http://cgi.ebay.co.uk/SOLID-STATE-VOLTAGE-STABILIZER-GT6-Spitfire-Stag-2500-/130476218290?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item1e60fd17b2
Nigel Atkins

Why not change to led brake light bulbs? Much lower power consumption and should be longer life. May not be cheaper but much easier to fit.
Just a thought.

Dave
D Brown

Well l have fitted the relay and all seems to be working fine. Lights are nice and quick to illuminate.
Stabilized has gone back to Moss for a refund so l will order an electronic one instead.
Guy Oneandahalf Sprites

Thanks Guy. Got it. Job for this evening.
Neil
Neil K

Yep, done it. Works aswell! Simples.
Neil K

This thread was discussed between 19/01/2011 and 20/01/2011

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