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MG TD TF 1500 - Mystery Switch

Can anyone help me figure out what the mystery switch in attached photo is on my '50 TD? It's the one switch hanging by the red wire just above the date stamp "2008". The photo is taken from under the dash, right side, looking up. The red wire from the switch goes up to the ignition switch, I think to either the "H" or "IG" terminals on p. N.22 in the shop manual??
The blue/greenish wire coming off the other end of the switch goes thru the firewall and back under the car following the brake lines to the back of the car (I think-awful dirty down there!). The switch itself pushes in/out. It does not turn like a rheostat would. Using the switch seems to do absolutely nothing by the way!
I'll post another photo to clarify.

efh Ed

Second photo...

efh Ed

Some previously installed backup lights perhaps? Not a OEM installed switch. Cheers - Dave
David DuBois

That has to be the thingamabob that does the job.Bibbity-bobbety-boo.
Merry Christmas to all.
conrad sanders

IMHO..I would look for signs of 2 things:
1: previously installed backup lights (like David D said...look for signs that there was a back-up light installed at one time ...hole in the apron ect.) This was a popular aftermarket add on.
2: Any chance at some time PO had installed a solid state replacement fuel pump toward the back of the car? I have seen a few TD where PO "gutted" a non-working fuel pump but left the old one in-line for looks. this could be a cut-off switch they ran to a new pump.
I run both on my TF ...but installed an "aux" panel under the dash for switches.

The hidden fuel pump cut off is something I have put on many of my cars. My theroy is simple there. i.e. : If they are going to steal the car these autos are so easy to hot wire let them get it started, get it out in the street and then the carbs run dry! Good chance from there they will abandon ship & run away. It has worked for me in the past & my insurance Company agrees!
Cheers & Best Regards,
David 55 TF1500 #7427
David Sheward

Isn't that the lighting side of the ignition sw? Would mean that it is powered only when the lights are on?


Ed -
Does the switch rotate? Could it be the fog light switch without the knob? Part number 141-000, item number 40, page 46.
Roy Challberg

No rotate, just push-seems to be spring loaded. The fog light switch is intact and even works on the fog light PO installed.
I find no trace of an alternate fuel pump.
If anyone can tell for sure if it's on the headlight, Taillight or IGnition side of the switch that would help me.
efh Ed

Could it have been somehow attached to what seems to be an empty shell mounted in the dash next to the choke cable? What does that switch look like from the drivers side of the dash?
Jim Merz

No Jim, it's connected to the real ignition switch which works fine for key ignition and all lights.
efh Ed

Ed, one possibility is a cut off switch for the brake lights. Teenagers used to do this so the brake lights wouldn't come on if they were parking with their dates, or if "they had to run from the law" ;^)
If the switch was for this and is still connected, the brake lights would only work in one position.

D C Congleton

Wow, that's a good one. But I just tried it and the switch has no effect on anything.
efh Ed

Follow the blue wire, that's the only wat to tell what it was hooked to.
Heavan knows what a PDO may have done, there is no way for us to tell.

So follow the wire to its end.

Don Harmer

Hi Dallas,

Only "teenagers" !!?


Ed - If the mystery switch is connected to the H terminal of the ignition/headlight switch, then it is only energized whenever the lights are turned on. If it is connected to the IG terminal it is energized anytime the ignition is turned on. Cheer - Dave
David DuBois

Ihad a similar switch to a reverse lamp prior to my restoration, hence the hole in the rear apron. That apron is still on my parts shelf (with 2 student parking stickers, Franlin and Marshall College, PO, and Western Conn. State,mine). Is the switch a push on/push off? Bob
R.AF. Robert Finucane

Bob, it appears to be a push on/off but since it doesn't do anything I can't tell for sure. By a "reverse lamp" do you mean a white light that goes on when the trany is in reverse (like modern cars have)?
I think maybe I should just disconnect the entirely and see what "doesn't work". I've tried to trace the blue wire but it's a dirty job down there. Maybe I should just start the total restoration now. This would be a good excuse to spend another $20k at Moss!
efh Ed

A switch added to our circuits can be for just about anything....if you can't follow the wire, you are pretty well out of luck in tracing anything...

I assume you have:

turned on the key...switched the switch on/off
turned on lights... turn switch on off
checked your dash lights in the dark ... turn switch on/off
with key brake switch ... turn etc etc
put in reverse ... etc etc

Replacing the wiring is not a 'giant' job... a friend just did it and it was a day or two....


Ed, does the switch stay in when pushed? Could it be a starter push button? Did the car come from the UK and you have a rear fog light? Sorta messy wiring under that dash! Barc
Barc Cunningham

Barc, No, the switch is spring loaded and does not stay in and doesn't attempt to start the car. Not from UK either. Yea, the wiring is a real mess! PO had an old air horn that never worked and a front fog light that doesn't match anything. A new harness is on my "wish list" as my confidence builds. I hope to do a wood veneer dash shortly and this would be a good excuse to redo that mess as well!
efh Ed

Tracing the wire shouldn't be that hard to do.

The PO of my car replaced EVERY wire with a yellow or blue one, and it was pretty trivial to trace the wires even then and figure out what they did. And, that included additional wires for aftermarket turn signals and an electric starter switch.

The electrical system on these cars is extremely simple.


If it is 'spring loaded and doesn't stay in' and there is an 'air horn that never worked'... i wonder if that might be the switch for it?
Take out the seat bottom, put down a flat pillow...wiggle into the foot space on your back with a good light source and just look at where all the wires go...find where that one goes through the firewall and have somone watch while you wiggle it...that should get you the end of it?

Ed, I've replaced many wiring harnesses in British cars and its really a must do kind of thing to maintain your sanity and well being if you plan on enjoying your car and its reliability. Its relatively a simple job, especially a TD. Unless you have 58 years of oil/grease/grunge everywhere. Then, cleaning takes more time than wiring.
I'm still in agreement that its for a reverse lamp (backup light) if its feed is from the the light switch, cause you only need it when its dark! Bob
R.AF. Robert Finucane

Wish you were closer....see if you can find a local electrician or somebody that does "system" wiring that owns a "FOX & HOUND" test set. You attach one unit to end of the wire and the other gives an audible "beep" when you get close to the other end!
My money still on "Aftermarket reversing lamp". IMHO ...if don't know where it's going I would certainly disconnect it as it is at the very least a potential for a short and possible fire!
Cheers & Best Regards,
David 55 TF1500 #7427
David Sheward


If the switch clicks on and off I vote for back up lights

If the the switch is on only when you hold it in, I vote for Air Horns

Safety Fast
T. L. Manion Thomas

Ed ,
The scary thing about your "switch"...back in "the day" (50's)The aftermarket switches sold by Lucas and others could handle higher amperage. I have an old Lucas switch sold for reversing lamps. That puppy looks like it could handle 20~30 AMPS with no problem. Thus the use of a "relay" was no big deal.
The switch in your picture looks to be a more modern "micro" switch. (Maybe 2 AMPS max) If it is connected to the lighting side of "T" series car you could have 15/20 AMPS of unfused DC just waiting to short out! Please tell us you have disconnected it at the ign switch...some of us are very concerned for you!
(see attached)
Cheers, Best Regards & Merry Christmas!
David 55 TF 1500 #7427

David Sheward

That will buff right out Dave.

This thread was discussed between 12/12/2008 and 18/12/2008

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