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MG MGB Technical - schematic for light switch

I am trying to figure out the wiring for my parking and headlight switch on my 74 1/2 rubber bumpered "B". There are 5 positions in the switch but only positions 1-3 have actual electrical lugs present. My Haynes manual schematic says that lug 1 is blue wire to hi/lo beam sw. Lug 2 is brown and should be direct to +12VDC. Lug 3 is red with green stripe to park/dash lights. On mine the red-green was connected to lug 1, brown to lug 2 and blue to lug 3. I thought that possibly just the blue and red-green were reversed BUT it is the blue wire that is connected to +12VDC on my car. I'll expose the hi/lo beam switch and start probing there but I'm confused. Does anyone have any ideas or the schematic from another manual they could email me, at tomgillett@comcast.net, to confirm the Haynes manual. I'm sure the red-green does go to the park/dash lights but not sure about the other two.
Tom Gillett

Tom,

Brown is the color for all circuits on the MGB that are hot all the time, unfused, and not controlled by the ignition switch. Both Haynes and Bentley correctly show the Brown (N) going to lug 2. The blue lead (U), as you observe, correctly connects to lug 1 and goes to the dipswitch. The red-green (RG) goes to the panel light rheostat as well as to the fuse box where it powers the two fuses leading to the parking lights. (Lugs 1 and 3 on the fuse box are bridged internally. All of the wires on the fuse box leaving those two fuses (lugs 2 & 4) should be red.

Tests:

1. are you getting power at the brown (N) lead all the time?

2. are you getting power from the blue lead even when the switch is off? The blue lead is the switched output of the switch, not the power source.

If so, something is definitely wrong. The only thing I can think of off hand is that there is a problem with the dipswitch such that the purple lead going to the headlight flasher (activated when you pull the stalk toward the steering wheel - it should spring-return when you let go) is powering the blue lead. The diagram of the dipswitch shows you how this could happen if some fault of the flasher switch caused it to be on all the time.

3. If you are getting voltage from the blue lead, with the headlight switch off, try unplugging the purple leads from lug 8 of the fuse box. If that kills the voltage on the blue lead at the headlight switch, you have found the problem.

If you're careful enough, I suspect any Lucas switch can be repaired. Otherwise, if my theory is right, you'll need a new dipswitch/flasher/turn-signal switch. I don't see otherwise how that blue lead could be hot all the time.

Do report back. You don't state what the problem is, but I'm guessing it's that your lights won't turn off, right?

FWIW,
Allen
Allen Bachelder

I forgot: is the blue lead hot, even when disconnected from the switch? If not, the problem would be in the headlight switch.

Allen
Allen Bachelder

Tom go to Advance wiring and click on stock schematics. Find MGB schematics and go to diagram 18. The wiring digrams are in color.
http://www.advanceautowire.com/

Brown should be on 2
Blue on 1
Red/green on 3

If you have 12 volts on the blue wire your headlights should be on as long as the battery is connected. Are the headlights always on? It's possible you can have 12 volts on the blue wire if the flash to pass switch is defective but the headlights should be on if that is the case. Pull the blue wire off the switch and check for 12 volts.

Clifton
Clifton Gordon

It should only be a three-position switch - off, park, and headlights (AKA dim, flicker and off).

Brown should be 12v unswitched and unfused i.e. live all the time. Red/green is parking and instrument lights. Blue is headlights via the dip-switch and should only be hot when the headlights are on.

At the one extremity of the switch movement all three connections should be isolated. In the middle position two of the contacts should be connected together, the brown and red/green go on these. It doesn't really matter which way round, although the Leyland Workshop Manual shows brown on 2, red/green on 3.

On the other extremity of switch movement all three terminals should be connected together, and blue goes on 1.

If the blue is hot at any other time, and when disconnected from the main lighting switch, then a PO has done some funnies and you are on your own. If it is only hot when *connected* to the switch, *then* the switch is faulty, or you have the wires on the wrong terminals, switch upside down in the housing, or something similar.
Paul Hunt 2

'Crossed in the post' with Clifton. The headlamp flasher thing is a good point, this will make the blue live but only when the dip-switch is in the main-beam position and the flasher pulled or its switch stuck in the closed (on) position.
Paul Hunt 2

Clifton,

Great website! Sure beats squinting at Haynes or Bentley diagrams, and in color and laid out much more logically. If that blue lead is actually live all the time while disconnected from the headlight switch, I can't see where else it would come from but a defective "flash-to-pass" switch, as you call it. That's why I suggested disconnecting all the purples from the fuse box to see if the blue remains hot.

FWIW,
Allen
Allen Bachelder

All,

Thanks for all the ideas. I've only had a little time to pursue further so far but I'll be back on it by the weekend. Let me state the original problem. New car to me and the headlights are on a separate switch that has been added that runs directly to the low beams. Wires were cut to accomodate. Also, the left direction on the dashboard is always on but none of the directinals work. Dash lights work correctly except for the dimmer. Also the hazard circuit doesn't work. Hence, I have no directinals, hazards or high beams.

I've learned a little more. The purple wire at the directional switch is not hot. Also, the blue wire at the directional is connected to a brown wire at the directional harness connector. Also the purple does not connect to a purple at the directional harness connector. I think that possibly they have connected the wrong harnesses together but have not been able to confirm yet. It looks like the same connector is used for a couple of harnesses right in the same spot. Hence, things are really screwed up but with your help this is sort of fun! I'll chime back in this weekend after I've tried your suggestions and sorted out the harness issue.
Thanks,
Tom
Tom Gillett

We'll be anxious to hear from you Tom. Just remember, the original way is the RIGHT way. The blue should NOT be connected to a brown. Trust your wiring diagram and sort the car out to fit. All will then be well.

Allen
Allen Bachelder

Ah well, if a PO *has* fiddled with it as I was beginning to suspect from what you said there is little any of us can do except what has already been done, and that is describe how the system originally worked.

With everything that you now describe, it does seem as if several of the wires are cross-connected between the main harness and the column switch harness.

With those multi-way plugs/sockets you can remove the pins out of the back of the socket (wire side) by depressing the one or two little tangs that should be visible if you peer down the length of the pin from the pin side. I've used a length of metal brake pipe in the past, swaged down to fit over the pin - you may have to experiment a little as if it is too tight a fit it may not go over the tangs to depress them. Pushing this down over the pin, and wiggling things around, and you should be able to pull the pins out. That done, you can rearrange them to suit the other half of the connector.
Paul Hunt 2

Well, I did find that two of the large 9 or 11 pin connectors under the dash were cross-connected. I should have seen it earlier, one connector was red and the other was white, hence a red and a white were incorrectly connected together. It's a wonder anything worked! Corrected this and now I have directionals, low beams, brake lights and other good things. I'm not completely out of the woods yet though. THe hi beams are not getting powered at the steering column switch when activated. I suspect this is a faulty switch and I'll have to try to repair. Any hints on how these fail and how to fix them would be appreciated.

Also, could someone tell me where the following items are physically located?
* In-line fuse in the hot wire to the hazard switch. (Hazard lights only work when the directional is activated, either left or right)
* In-line fuse to the seatbelt cluster.
* Directional flasher unit.
* In-line fuse to the wiper/heater/radio.
* In-line fuse to anti-runon valve and switch.

And finally, for now anyways, my wiring harness has what I think is the original Blue taping on it but it also has Black taping over it in many places. All of the Black tape I have removed so far has revealed some PO modifications so I'm wondering, was there any Black taping on the original harness at all? You thoughts?
Thanks again for all your help. You've made this fun. (BTW, actually took my longest ride in the car today now that the brake lights and directionals were working!)
Tom
Tom GIllett

Headlamp flasher switch is usually pretty flimsy, the contacts are exposed on some years allowing you to check whether the problem is no voltage coming in, the contacts not closing, or no power getting out.

1. I think in my 75 V8 it is very conveniently (not!) located behind the centre console by the (equally conveniently located) flasher unit. Later models had both hazard and turn flasher units side-by-side on the firewall (see below).

3. Usually pushed into a spring-clip on the right-hand firewall in the cabin. Turn them on and locate using the Mk1 ear.

4. Usually under the fusebox.

Original harnesses could be either black or blue taped depending on year and market.
Paul Hunt 2

This thread was discussed between 04/07/2007 and 08/07/2007

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