Horn Relay

[KVH]

I thought I'd buy a spare horn relay for my TR4A. So I did. It didn't work, so I sent it back.

I have another, and it doesn't work.

Is there something I'm not doing? Unlike the OE, the one sent from MOSS is cylindrical, but I can tell which wires go where, or at least I think I can.

Has anyone had experience with the new MOSS relays?

 

[tdskip]

Hi there - have you tested the relay separate from the rest of your wiring? Seems like it might be a good idea to make sure it is in fact the relay(s) that is the issue.

 

[DougF]

Did you ever think the day would come that we'd be wishing for Lucas parts for their dependability?

 

[Geo Hahn]

...I can tell which wires go where, or at least I think I can...

Is the relay labelled? Perhaps using the standard 85/30/86/87 codes? If so, pretty easy to confirm the installation.

I'v never had a relay fail out of the box -- possibly because they are such a simple device -- 2 in a row makes me suspect something else is wrong.

 

[KVH]

I'll check the codes again, and try one more time. It might help if I knew what the relay does? What is the purpose of a relay?

If the horn button makes the connection, and if the hot wire is fused, what would the relay being doing?

Not too proud to ask.

Merry Christmas Everyone!

 

[Rhodyspit75]

The purpose of a relay is to switch a heavy load. The horns require more current then say a light. If you were to try to pass the horn current thru the contacts for your horn button you would eventually fry the button. So pushing the horn button sends power to the coil of the relay which in turn closes the heavy contacts of the relay. I am not sure if the relay you are using has a ground wire or if it is grounded thru the case. In either instance make sure you have a good connection to ground.

 

[Geo Hahn]

If the TR4A (like the 3 & 4) honks the hooters by providing a ground then the wiring may be a bit counterituive as we're more used to relays providing a source of power rather than a ground.

TR4s did not use a horn relay though I recently added one to mine. The horn button doesn't actually get fried, more a matter of just not quite able to honk both horns thanks to that long tortuous route the ground path must take to get onto the steering rack, around the 2 rubber donuts, thru the horn button etc.

Couldn't find a TR4A wiring diagram and the TR250 & TR6 diagrams...

https://www.advanceautowire.com/tr2506.pdf

...do not show a horn relay which is odd, so I can't offer a specific description though generally terminal 30 on a relay is grounded and when power is offered to terminal 85 then the circuit between terminals 86 & 87 is completed. But as noted, the grounding used for horns can make this a bit strange.

<span style="font-style: italic">Okay, had to scroll down to a '72 diagram and I see a relay. Looks like W1 & C2 get power (purple), W2 gets the purple/black switched ground from the horn button and C1 is the purple/yellow to the horns.

If your replacement relay has numbers (German) instead of letters (British) then you'll need a translation.</span>

 

[poolboy]

To add to what Rhodyspit75 said. when the relay contacts, be they electromagnetically or electronically closed, that completes a new circuit; whereby a heavier gauge wire capable of carrying a heavy current (amps) thru the new circuit to the device such as horns, auxillary lights and even a starter.
In other words, the devices' switch only deals with enough amperage (heat) to activate the relay, not the device.

 

[TR3driver]

The TR4A originally used a special "horn" relay; with one side of the coil internally connected to the same terminal as one of the contacts. That terminal was labeled C2 (according to the diagrams), and should get power through an in-line fuse (not part of the fuse block) directly from the A1 terminal on the control box.

When the horn button is pressed, it should ground terminal W1 of the relay, which should cause the relay contacts to close and connect terminal C1 to terminal C2, supplying 12v to the horns.

If your replacement relay has 4 or more terminals on it (as most modern automotive relays do), you may need to install a jumper between a coil terminal and the hot contact terminal to get it to work. But standard relays don't hold up well in this application, because of the strong inductive kickback from the horns. It's best to use a relay designed for horn service (which should be available from any auto parts store).

If you probe at the relay with a voltmeter or test light, you should find power on 2 of 3 terminals (W1 and C2). When the horn button is pressed, one of the previously hot terminals (W1) should go to ground, while the previously grounded terminal (C1) should go to 12v. If all this happens, then the problem is the wire to the horns, or the horns themselves.

Quick test for horn and power is to temporarily install a jumper (eg clip lead) at the relay from C1 to C2. That should cause the horns to sound, even with a defective relay or horn button/switch. If not, you've got a wiring problem (or again bad horns).

That should be enough to get you started; report back if that doesn't solve the problem.

 

[TR3driver]

I gotta run, but check the Advance Auto Wire website. Dan drew a TR4A schematic that shows the horn relay.

If it's not available there, I'll post it here after I deal with some honey dews.

 

[TR3driver]

Here's the relevant portion :