Triumph Repair: GT6 Horn, jumper wire, vacuum advance

Question
QUESTION: Hi Howard;
The horn in my GT6 only works when the car is wet.  The horn will fire every
time if I bypass the switch and the relay (just touch a wire connected to the
horn to the hot wire into the relay), so I think the connections to the horn
itself are fine.  When the car dries out, it lives in the driveway under a tarp,
the horn push will cause the horn relay to go clunk - but no noise from the
horn.  I think it is the relay but why would it work perfectly when it is wet
under the bonnet?

ANSWER: Hi Garry,
I agree with you diagnosis on the wires to the horns. You can use a voltmeter to diagnose where the problem is, so you don't have to just toss money at the problem in the hopes of hitting the failed item.
Or just do this with a jumper wire. When the horns are in their failed mode go to the relay and jump across pins "C-2" (purple wire) to "C-1" (purple w/yellow stripe)

If the horns blow then the horn wires are good as you already know and now the power wire from the fuse box is also good.

Next, disconnect the jumper wire and put it on a known good ground and touch it to pin "W-1" on the relay.
If the horns blow then the wire to the horn button and/or the horn button is the fault. If the relay just clicks and the horns don't blow or the horns make a single clunk, then the horn relay is at fault. You can either purchase a new relay or open this one up and sand or file the contacts.

Let me know,
Howard

---------- FOLLOW-UP ----------

QUESTION: Howard;
It worked as you described.  Sanded, realigned contacts and the horn is
working.  I switched my spark plug cables, identified one as bad and replaced
the set.  I have two more questions.

1.  If all is running well is  there a reason to test compression?  I do not have
the tool.  I might be able to borrow one but . . . do I need it?

2.  The vacuum advance on my distributor is connected to an inlet on the top
of the rear carb.  The vacuum retard is plugged but the connection for it on
the bottom of the front carb is open and sucking air.  Should I connect it to
the retard on the distributor or plug it.  

Thanks for your help.

Answer
Hi Garry,
Glad you are making headway on your car.
1. If it is running well you may not need to check compression however, the man with a hole in his roof said the same thing since it wasn't raining why bother to fix the hole?
You can get a inexpensive compression gauge at Wall Mart.

2. If you have a vacuum advance and retard unit on your distributor and you found the vacuum port and the ported vacuum port. Yes, you should connect them up and set the timing.
Let me know,
Howard