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MG TD TF 1500 - Horn button Lucas SP140

Confused over why I can't get continuity to ensure operation before installation of my Lucas SP140 horn/headlight/dip switch. I have assumed that on depression I should get continuity between the cable normally connected to H on the ignition switch and that one connected to the horn? I have attached a couple of pictures with it dis-assembled. I assume that on depression the inner copper/brass coloured plate makes contact with the silver/chrome coloured plate. How should this make contact with the cable leading to the horn? If so have I something missing or has the internal terminal ( slotted screw holding the cable in place ) lost something or worn away. Anyone got any advice please?






JK Mazgaj

The horn wire connection does not go through the ignition switch. Current flows through the horns from the fuse block and when you depress the horn button it completes the circuit to ground. You should have continuity between the end of the lead to the body of the horn when the button is pushed. The metal center dash panel must be grounded in order for it to work.
L E D LaVerne

Here is an image of a replacement that I bought from Moss a number of years ago for the TD I had. I did not realize that it was incorrectly assembled by whoever built it until I pressed the horn button the first time and smoke started coming out from the wiring. Instantly hot enough to melt the solder from the bullet end (notice it's missing) and cause the horn button spring to loose tension.


The wires on this unit were all off one pole to the right. The blue wire is a hot feed from the ignition switch so when I pushed the horn the unfused hot was going straight to ground.

So the blue wire is the hot feed from your ignition switch and should be in the position where the white blue wire is on this photo. The blue/white wire and the blue/red wire should also be moved one terminal clockwise. The brown/black should be in the position where the blue wire is.


I was luck not to have burned the car up.



L E D LaVerne

Ye Gods, Laverne! Did you feed that info back to the supplier?? - I'm just now bench-testing my entire YA instrument set prior to installation, and yes it's confusing when an item works fine when direct fed and grounded, but then when hooked up to the rest of the dash on the bench, "appears" not to work until you remember whether it's ignition fed or otherwise dependent on something else being switched on or grounded. Keeps the old brain active!

Jan, you have probably by now made different connections and had success. Thanks for the timely posting.
John.
J P Hall

Thanks LaVerne. It was one of those headless chicken moments when focusing solely on the component I failed to remember the route of the power supply. In my defence (I've got to find an excuse Ha Ha) I must remember I'm working with POSITIVE EARTH
JK Mazgaj

This thread was discussed between 17/02/2022 and 18/02/2022

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