MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Toggle switches - 3 position types

What is the difference between the 3 position toggle switch for a 2 speed wiper (off/slow/fast) - part number BHA 4786 and a 3 position light switch (off/side/main) - part number BCA 4294?

Is it just that the flat on the switch body is on the opposite side?
Daniel Thirteen-Twelve

A 3-position light switch has an input spade (brown) which isn't connected to either of the other two spades (red and blue) with the switch off. In the first 'on' position the brown is connected to the red (parking lights) but not the blue (headlights). In the 2nd 'on' position the brown is connected to both the red and the blue.

A 3-position 2-speed wiper switch is different in that it must switch the voltage *between* the two output spades when switching between slow and fast. If you leave power connected to the slow speed brush when in the fast position i.e. power also connected to the high-speed brush, some of the windings of the motor will be short-circuit. I've never tried it, but I doubt it would be good for the motor.

However if the switch has multiple (i.e. more than three) spades, as early toggle switches did, you should be able to juggle the connections to achieve the correct conditions in both cases. For example an early light switch had brown on terminal 4, red on either 1, 3, 5, or 7, and blue on 8. As a two-speed wiper switch brown should be on 4, slow on 6 and fast on 8. This is in theory, check it first! Make sure brown is only connected to one *or* other of the output wires in either of the 'on' positions, and neither of them in the off *before* connecting it to the motor i.e. use a test-lamp or meter first.
Paul Hunt 2

Paul,
Why stop there? Explain to us how the hazards switch for a 1972 midget works? I ALWAYS have to have a sheet of paper in front of me to figure it out ;-) To say nothing of needing to close it back up quickly before I forget how it went... Or were those only for the US market?
BTW, I am not sure about the BHA4786 switch, but I was just analysing the park function on the 2-speed wipers as used in a 72 Midget last night and there is also a connection that has to be made when "off" in order to park the wipers.
David "wiped out" Lieb
David Lieb

As David says, the wiper park circuit runs through the switch in the off position.
Dave O'Neill 2

The only way I can ever figure those switches out is to measure them and draw it all out.
Simon Jansen

Up to 67 the wiper system didn't use a normally closed contact in the wiper switch but just used the park contact on the motor itself to complete the circuit. The later system (68 and later, and early GTs with single speed wipers) *did* park via a normally closed contact in the switch as described, but the switch I described can be used to accomplish that function as well, as long as it has the multiple spades and not just three. Around 1967 the picture is very confused, with possibly four different motors (some round, some square) as well as the two switching systems. Only by looking at the number of wires on the switch, the motor and running between them can you say which switch connections and motor you should have.

AFAIK the hazard system is the same on all MGBs and Midgets. The hazard switch has two functions - one is to disconnect the turn signals and the other is to activate the hazards. The turn signals must be disconnected when the hazards are active otherwise power can go backwards from the turn signals to the fuel pump and the ignition, even when the ignition is off and the key is in your hand, which is obviously a safety hazard especially in the event of an impact and subsequent damage.

The green (fused ignition) supply to the turn flasher goes via a normally closed contact in the hazard switch. When the hazards are turned on this contact opens and disconnects the turn flasher, so preventing any power coming 'backwards', as it were, from the turn signals to the fuel pump and ignition. These two terminals are totally isolated from the other three or four in the hazard switch. The other three or four are also isolated from each other when the hazard switch is off. When the hazard switch is operated these three or four are all connected together. One comes from the hazard flasher, two of the others go to the turn lamp wires, and the third (if provided) is to yet another tell-tale on the console. The hazard flasher unit has 12v connected to it all the time via an in-line fuse (albeit brown both sides) and when the switch connects it to the lamps the circuit is completed and the hazard flasher unit starts to flash on-off-on-off after a short pause to 'warm up'.
Paul Hunt 2

I would have liked hazard lights on my Midget yesterday when it first decided to die upon entering a roundabout, then again when starting to enter roadworks from temporary traffic lights, then finally when entering the same roundabout on the way home, having decided that it wasn't a good idea to continue my journey.

Having abandoned it in the garage with my (still) inactive 'B', I had to use the Passat to attend the Wythall classic car meet.

I recently thought that it would be a good idea to replace the almost useless panel light switch with a hazard warning switch and associated wiring. I now think it is an EXCELLENT idea!
Dave O'Neill 2

Paul,
I just spent a big chunk of time this weekend analysing the wiring of the 2-speed wiper motor as used in the 1974 Midget. You are over-simplifying the park switch just a tad. When in the non-parked position, the switch does indeed connect the NLG lead to the GK power lead, but when parked, it connects that NLG lead to the black ground lead. I imagine it does this for the braking effect (not that Midget wipers generally need any help stopping...), but it certainly does do it. I pulled the switch out and tested it very thoroughly.
David "thinking about 2-speed wipers on my Bugeye" Lieb
David Lieb

David - I can't speak specifically for the 74 Midget, but the 78-80 Midget is the same as the 68-on MGB. I'd be surprised if the Midget had an interim arrangement for so long after the MGB changed, but anything is possible. On the 68-on MGB system the wiper switch doesn't have a ground, just a green 12v supply and three other wires going to the motor (which also has a green 12v supply and the only ground). When switched off the brown/light-green wire is connected to the red/light-green slow speed wire. When the wipers are *not* parked a changeover switch connects 12v to the brown/light-green wire, which goes to the red/light-green wire in the switch to continue to run the motor. When the motor reaches its park position the changeover switch disconnects 12v from the park wire and connects ground to it instead. This effectively shorts out the slow speed winding, which with the motor still spinning at the instant it happens, is generating a back emf i.e. is acting as a dynamo. The short tries to draw a lot of current from the motor, which puts a huge load on it, which stops it very rapidly. This gives a much more consistent stopping position of the blades than the earlier 'just disconenct the voltage and allow the motor to spin down' method which depending on how wet the screen was i.e. how much drag there was from the blades gave varying stopping positions.
Paul Hunt 2

BTW, this http://www.geocities.com/sprite-midget/wipers.htm and other sites quoting the same part numbers for MGB and Midget switches for the early 70s (at least) on tends to confirm the 74 Midget is the same as the 68-on MGB. Specifically the following paragraph:

"What I found was that the auto park wire on the wiper motor could only be connected when the slow and fast speed wires were off. When you supplied power to either slow or fast, you had to disconnect the auto park lead otherwise there was a certain point in the wiper motors cycle where the auto park lead temporarily shorted to ground."
Paul Hunt 2

Paul,
Yes, that was what I surmised was the reason for the short to ground. You had not mentioned it, so I wanted to clarify for those who have not spent a glorious afternoon sorting out the wiring. I, too, assume that it is common to a lot of the 2-speed wiper motors of the era, but, since I was specifically dealing with the wiper motor from a 1974 Midget, I could only vouch for that particular one, hence the specificity.
David "yes, anything is possible..." Lieb
David Lieb

This thread was discussed between 09/03/2008 and 11/03/2008

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.