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MG MGB Technical - Turn Signal Switch

I have a 1971 mgb roadster. One of the arms used to click off the turn signal switch has broken off, and I am attempting to modify a turn signal switch from another year B.

The Lucas number on the switch from my car, that has the broken arm that clicks the switch off after the turn is made, is 39552A. The Lucas number on the switch that I am attempting to modify is 35873B. The difference between the two is that the older 35873B has the horn included in its' functions. The 39522A, which I believe is newer, does not.

The older switch has a purple/black wire and black ground wire that goes directly into the switch. The newer switch has only the purple/black wire, and it is attached to a copper strip, seperate from the switch, that contacts the steering wheel and is used to activate the horn from the center of the steering wheel, not through the turn signal switch.

One more thing. The older switch with the two good arms to click off the switch after the turn is made has a different wiring harness connector than the one with the broken arm on my car. The only difference between the connectors again are the two wires (purple/black and black ground). I would also have to switch connectors to make this all work.

Other than the two wires, everything else between the two switches is identical. Does anyone think my idea will work? Does anyone understand the dribble I just wrote?

A couple years ago I exchanged emails with someone who had a very thorough knowledge of the various turnsignal switches on MGBs, and the Lucas numbers that are stamped on them. I would appreciate hearing from that person again.

Thanks for any help that might be out there. Curt
Curt

Curt, I understand what you are saying. Do you have a steering wheel hub with a horn contact slip ring and a center horn button? That would be the easy way to have to get the horns to work. The other way is to add a horn button. I have one of the later switches and the plug pins should come out with the use of two small screwdrivers to depress the retaining lugs. Does the older plug use the same type pins? If the pins are the same, remove the pins and transfer the wires from the good switch to the correct plug. If they aren't the same you will have to cut the plugs off the switches and transfer the correct plug to the good switch and splice the wires. Solder the connectors and use heat shrink to insulate them. The other choice is to use bullet pins and connectors. If you use the later type horn setup the ground isn't needed.

It will work but be sure the wiring is right, it's easy to burn up your wiring harness if you make a mistake. Good luck

Clifton
Clifton Gordon

Yes, it will work fine. Clifton is dead on with his soldering and heat shrink tubing advice. You can drill out rivets and replace with small screws and nuts if need be. The stalk you are attempting to convert is likely from a 1970 MGB which did have the horn at the end of the stalk. The Moss motors catalog has a nice write-up (by John Twist, I believe) which denotes all the functions of all the wire colors. and a wiring diagram for your car will also aid you.

I like to test the function of each part as I rebuild them with a vold meter set for resistance. Incidentally, the failure mode for the dip switch is to fail so as not to light the brights. Under normal conditions, the brights work, or the dims work depending on switch position. The only time both filiments work is when you pull the switch to momentarily flash the brights. Do that too much and you can overtax your wiring.

good luck,
dave
Dave Braun

The description of the switch being converted sounds like the one on my 74 B. I believe the the horn push was moved to the steering wheel in 73 models.

Clifton
Clifton Gordon

Dave and Clifton,

Thanks for your waluable input. I believe I am on the right track now to make the earlier switch work.

Curt
Curt

Once you get it fixed, think about converting your headlights circuits to use relays. The reason many have done this is to preserve the integrity of the switch since its contacts are only just thin brass and have minimal contact area compared to the current they are asked to carry for halogen lamps, which most folks convert to for road use. This will prevent the contacts themselves from frying and heating up the plastic they are attached to.

You can get a good set of instructions on converting to relays from Daniel Stern's site, http://www.danielsternlighting.com/tech/relays/relays.html , and they can be wired/concealed below the front hood/bonnet catch plate for that OE look.
Bob Muenchausen

You're welcome Curt. Let us know what you find when you are finished. Cool idea Bob, I think I will investigate relays for a winter project.

dave
Dave Braun

I added relays to the headlights in my 74 roadster several years ago. The lights are brighter with relays. The system worked OK so I didn't check the voltage from the dip switch to relay spade 86 on the high beam relay. A few weeks ago I noticed the high beam indicator light was very dim when the lights were on high beam. The indicator had normal brightness when the lights were in the momentary flash position. I turned on the high beam lights and measured the voltage at the high beam relay on spade 86 and it was only around 9.5 volts. I tied the momentary flash to the on position and checked the voltage at spade 86 and it was 12 volts.

Looking at the wiring diagram I decided the dip switch contacts must be the problem. I removed the combination switch and examined the high beam dip switch contact, it was badly pitted. I used a small file to refinish the contact and that solved the low voltage and indicator dim light problems.

The point I want to make is that even though the relay had only a little over 9 volts the it always worked and the headlights were bright. Apparently the relay will operate on considerably less than 12 volts.

Clifton
Clifton Gordon

I wish we could edit our posts, first sentence in last paragraph, should read "The point I want to make is that even though the high beam relay had only a little over 9 volts on spade 86 from the dip switch the relay always worked and the headlights were bright."

Clifton
Clifton Gordon

Well, guys, I completed the work last night and it works like a charm. Thanks for the suggestion about the relays, Bob. I did that last year when I was converting to halogen headlights and sorting out the wiring mess from the previous owner. The additional brightness is really noticable and welcome. Thanks to all of your for the expert advice and guidance. My car had sat since 1987 when I bought it in 2001, it fuinally started again in 2004, and it runs quite well now. Its inspected, and now its ready for some good old fashioned bodywork.

Curt
Curt

This thread was discussed between 13/07/2006 and 15/07/2006

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