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Re: 73 B horns

Subject: Re: 73 B horns
Date: Mon, 17 Aug 1998 15:21:16 EDT
Dan Ray wrote: 

"Just a sanity check to ensure I'm reading my 73 B's wiring diagram
Can someone out there confirm that my horn wires should be as follows?:
>From Left to Right, as if you're sitting in the car, P=Purple, PB=

Left Horn                Right Horn
P      PB                 P  P     PB PB

The Left horn (High note) leaves two connectors empty, or am I missing
If I'm correct, I've got some trouble shooting to do! (I do anyway, since I
just burnt a fuse guessing :{ )"


That is correct. The purple wire goes from the fuse to the right horn, and
another purple wire goes from the right horn to the left horn -thus the two
post connectors on each side, one of each pair which is unused on the left
horn. Actually, the horns are not polarity sensitive, you can reverse the P
and the P/B wires with no problem. The P/B wire is routed in the same manner.

Below is a copy of a post I made to the Triumph list awhile back. You may find
it of use.
First, a little explaination about how horns work. Internally, a horn has
three primary components - a solenoid, a diaphram, and a switch. The switch is
wired in series with the solenoid, and the diaphram is fastend to the solenoid
plunger. When the solenoid is energized, the plunger is pulled into the
solenoid, and the diaphram moves forward. Attached to the plunger is an
actuating arm that mechanically operates the switch. When the plunger reaches
the end of its travel, the arm opens the switch and current to the solenoid is
cut off. The plunger/diaphram then retracts, the switch re-closes, and the
cycle starts over again. The design of the assembly is such that the in/out
movement is rapid enough to creat the high pitched sound we hear.

The function of the adjustment screw (the large one with the lock washer) is
to limit the forward movement of the plunger. If the movement is too limited,
the plunger will not move far enough to open the switch. If it is allowed to
move too far, the frequency of the sound will be too low, and the volume too
soft. Physically, the adjustment screw goes into the core of the solenoid from
the opposite end from the plunger.

The smaller screw is a left handed thread, and physically bends the fixed arm
of the switch, increasing the rearward travel of the plunger before the switch

Since the switch operates in an ON-OFF cycle, the average current is less than
the maximum current, in the ratio of the ON time to the OFF time. I am not
sure exactly what the ON-OFF ratio is, but assuming 50-50, the average current
will be 1/2 of the max. For this reason, a horn that is misadjusted such that
the switch doesn't open, or if the switch sticks closed, may appear to be
shorted, based on the apparent excessive current draw. A 2 amp horn would draw
4 amps. If the ratio were to be 20-80, a horn that would draw 2 amps when
working would draw 10 amps if the switch remains closed.

I have a photo of a dis-assembled horn. The photo is not very good, but I plan
to re-do it, put captions on it, and post it to the VTR site, along with an
article on how they work, and how to repair them when they don't. If you would
like a copy of the photo, let me know and I will send it.
And, from another post:
There are three types of failures typical with horns: 1) the contacts corrode
together, preventing the switch from opening, 2)
corrosion builds up on the contacts, preventing current flow through the
coil, and 3) the coil wire breaks. An internal short is possible, but very

By drilling out the rivets, you can get to the contacts to clean them. It
will be self evident what needs to be done. the
success rate for this repair is fairly good. If the coil wire is broken, it
can be fixed, but finding someone who knows how
to do it is very unlikely, and probably very expensive, unless it is broken
close to one of the input terminals or the switch. 
Hope this helps.

Dan Masters,
Alcoa, TN

'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74

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