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MG Midget and Sprite Technical - help me find a new switch

In the moss catalog they have two brake warning switches. One for brass valve housing and one for cast iron valve housing. Mine is brass. See attached picture.
Mine is the older looking one. The one on the right is the part that Moss sells for the brass housing. I am pretty sure housing is the original one but I am not familiar enough to be sure.

Does anyone recognize my switch. Mine seems to be permanently shorted and I want to replace it. The new one that is next to it does not fit (too long). However, I can see that electrically it is different than mine (not shorted).

Does anyone know where I can get one that looks like mine. I have tried twice to get help from Moss tech support.

Thanks,
Rebecca


R Harvey

Hi Rebecca

This looks like the switch you need:
http://www.leacymg.co.uk/index.php?product_id=518560&option=Prod_detail

Leacy are a good company only a few miles from me.

HTH

Tony
A Brough

Rebecca. How do you know your switch is shorted? You do realize that this switch is a "normally on" type of switch and goes off when the plunger is depressed? Thus, if tested off the car, using either an ohm meter or a test light and power source you would expect to see zero ohms resistance or the light illuminate because you should have power flowing through it. At least that is true of the switch on the left (the new one) which is also used on the MGB brake system. Do not own a midget currently, so cannot say exactly how your system operates.

So, it would be good to know how you determined your switch to be shorted and it would be good if someone who has a similar set up could check to see if their switch is one of the ones which are normally in the on (transmit power mode) used in applications where the plunger is held to the off position unless there is a problem.

Les
Les Bengtson

If I put an ohm meter across the pins of the switch on the left I read about zero ohms normally. Then if I press the button then the ohm meter reads infinite (open circuit). I concluded that the switch on the left if you push the button the circuit opens.

With the switch on the right if I put an ohm meter across the switch pins it reads a short with or without the button pushed.

With the switch on the right, I noticed something different about the design of the pins. If you inspect the pins of the switch on the right it looks like a continuous piece of metal connects the two pins together (by design). This makes no sense to me. I might try and get a photo of this. I will also look at it with a magnifying glass.

Tony, that switch looks like it would fit my housing. According to the text on that web site it is not for my car so perhaps the part in my car is not original.

I will do some more testing before deciding to buy it.

Thanks,
Rebecca
R Harvey

Don't remember all the application details, but some of these are real switches - they turn their own two contacts off and on; and, they might come in N=ON or N=OFF configurations. Some are simple "to ground" switches - they connect one or two pins to ground, again as either N=ON or N=OFF; and the two pin types of this sort may have both pins connected together as you say.

Some may both connect/disconnect the two terminals and ground them under some condition. On Triumph these were connected to the dual function brake/oil pressure warning light, which got dimmer or brighter as the shuttle moved, when the light was lit engine not running but key ON. On MGB there are diodes and other complex circuitry on at least some cars, to do with telling you you left the handbrake ON.

Some shuttles have a high point at the normal rest position, others have a groove. The high point type neatly shear off the plunger after a failure if you don't unscrew them before bleeding the brakes!

You can be sure that any with plastic plungers are two contact switching ones, but otherwise you need to figure out from the correct wiring diagram which you need. Sorry I don't have a Spridget book.

I just looked at some MGB diagrams and they are two pin, appear to show the two connected, and also show them going to ground - but it is not clear or explained what the operational status is.

It's all a bag of snakes and many cars have them gone or disconnected. Many or most of these shuttle valves do not in fact shut off the fluid to the failed circuit, though they may restrict flow somewhat. The whole thing was a stupid attempt to warn of a leak, and has been universally replaced by a simple float operated warning light in conjunction with the divided reservoir.

FRM
Fletcher R Millmore

You say you tested the switch, but then it seems you don't actually know how it works, so the test may not be valid. Do you have any other reason to think it is bad? Usually, the only failures are that it is physically broken, or the plastic plunger ones have been sheared off, but this is a metal grounding plunger type I think. Note what I said about the MGB and I think the Spridget is the same - the one warning light indicates both hydraulic failure and handbrake ON, so deficiencies on either system/wiring may result in the light misbehaving.

FRM
Fletcher R Millmore

This thread was discussed between 10/06/2010 and 11/06/2010

MG Midget and Sprite Technical index

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