TR4/4A TR4 Horn Relay

[KVH]

There's a horn relay, three pronged, near the right horn and carbs. Sometimes my horn won't work, or there's a delay, and it seems that is related to insufficient current from the generator. The problem is only on low RPMs.

So I ordered a new horn relay from Moss. It looks different, but I verified the part number, and even reordered one after the first one didn't appear to work.

I now have my old relay back in and it works the same as ever, with some issues on low RPMs.

Does anyone know why the Moss one wouldn't work? I'm certain I had the wiring correct, but I'm wondering if I have to add a ground and make some other adjustment.

thx

 

[angelfj1]

The voltage produced by DC dynamos(generators) is diretly proportional to the speed of the rotor. Theoretically there would be no limit to the voltage produced if there wasn't a limit to the speed at which it could be driven. That said, there is a minimum voltage that your dynamo should produce even at low speed, e.g. 2000 rpm or less. To maintain a proper voltage TR's (and other cars so equipped) depend on a control box with a simple relay which regulates the voltage. At lower rpm it is possible that insufficient voltage is being produced to operate the horn relay so that it's contact close and make a low resistance connection, completing the connection to the horns. There are a number of possibilities:
1. control box needs adjustment
2. bad connections resulting in low voltage at horn relay , horns or both.
3. bad connection in horn push.
4. dynamo service required, maybe brushes need replacement or commutator needs to be turned down.
5. horns are out of adjustment or contacts dirty or corroded, requiring higher than normal current

This is a very simple process of elimination. If I had to guess, I would suspect the horns, and/or a bad connection somewhere. Start by checking all of the connections including the earth connections for the battery and dynamo. If the problem is not corrected at this point check the horns and finally the control box and dynamo. You should be able to find all of the diagnostic tests in a good shop manual.

 

[TR3driver]

Personally, I wouldn't even be looking at the charging system at this point. The horns should be able to work from just the battery! Of course, it wouldn't hurt to check what your battery voltage is doing while the horns are sounding, but if it was that weak I expect you'd have trouble starting the car as well.

I'm not sure how TR4A horns are made, but the ones used on TR3/A have a pushrod through the center that operates the points. If the pushrod gets cruddy or rusty, or even if the points are slightly out of adjustment, the horn may refuse to work at lower voltage. In fact, I need to pull apart the low note horn on my project TR3 as it won't sound reliably on just battery voltage.

But I agree entirely about the bad connections being likely suspects. I fought what I thought was a bad switch for a long time on the 3A (which had no relay originally) before I finally discovered that the 4-way bullet sleeve at the LH horn was broken inside and not making good contact.

If the Moss relay had 4 terminals, then you probably need to jumper the extra terminal to the hot terminal, rather than ground. The switch grounds the terminal that it connects to, and the coil needs voltage across it to activate the relay. But most automotive relays will work reliably to down under 9 volts, so I doubt that low voltage is causing problems with the relay itself.

BTW, TR4s did not originally have relays, so I assume we are talking about your 4A ...