XK 120 Horns/relay setup

I rewired my 120 about 2 years ago, then stopped to build a house! Now I’m confused! There are 4 wires going down thru the steering column which means at some point, somebody installed indicators. Since my car is early 1952, it shouldn’t have indicators.So I plan on ignoring these wires. My horn push has one long wire coming out of the pipe attached to the horn push, which attaches to the horn relay. Below that spot on the relay are two other connections. The first is power in (coming from the fuse box) and the second is power out to the horns. On the other side of the relay is a wire returning to the control box. When I take a feed from the live connection on the relay to the connection from the horn push, my horns work. It appears that putting live to the horn push location on the relay transmits power to the horns via the other connector on the relay. So far so good! My problem is I’m not sure what the purpose is of the return from the relay to the control box and how I get power to the horn push connector to work the horns. Can someone explain please. Somehow, pushing the horn must send power thru the cable to the relay. I assume I need to attach a ground wire to the horn push?? Also, does the ignition have to be on for the horns to work?? The wiring diagram I’m following shows one cable from the center of the horn push going to the relay, and another wire going from the horn push to ground, It would make more sense to me if, when the horn is pushed, the wire from the relay to the horn push is grounded, allowing power to the horns. As anyone can see - I’m confused and need an explanation. Thanks in advance,

Since you say yours should not have signals I’ll assume it is an OTS and try this diagram. Hmm, no labels on the relay.

But the FHC diagram has labels on the relay.
image
I’ll also assume you have positive ground.
You should have a brown wire from the ammeter to the fuse box and over to the relay terminal C2, always powered regardless of the ignition switch.
horn relay terminals
You should have a brown/green wire or two going from terminal C1 to the horns.
You should have a brown/black wire from terminal W1 to the horn push button.
You should have a green wire from terminal W2 to the voltage regulator terminal A4.
Some relays have only 3 terminals, in which case C2 and W2 are the same terminal labeled W.
C means Contact and W means Windings.

How it works:
You always have negative power at C2, but it’s not connected to anything until the contacts close. When the contacts close, it connects to C1 and powers the horns, which are grounded.
To make the contacts close, you power the Windings. W2 has negative power, and you push the horn button which connects W1 to ground inside the button, and the windings become a magnet which is energized and pulls down the contacts.
Toot. :trumpet: :postal_horn:

W1 and W2 wires can be interchanged, the magnet works either way.
C1 and C2 wires can also be interchanged, the contacts work either way.

Wow. That answer was more than I anticipated!! I didn’t include diagrams in my initial post because I wasn’t sure how to do it. Everything as you depict and say. I have grounded the horn push - made a continuity check from the relay to a position on the horn push - then took a wire and grounded that point. Pushed the horn but nothing happened. I’ve now found the keys and switched on the ignition (need to disconnect the fuel pump since tank is dry) just in case the ignition has to be on for the horns to work. Did these before receiving your response. I will read over and absorb your comments and try again tomorrow. Your response is VERY much appreciated. Thanks

| Rob_Reilly
August 27 |

  • | - |

Since you say yours should not have signals I’ll assume it is an OTS and try this diagram. Hmm, no labels on the relay.

But the FHC diagram has labels on the relay.
image
I’ll also assume you have positive ground.
You should have a brown wire from the ammeter to the fuse box and over to the relay terminal C2, always powered regardless of the ignition switch.

You should have a brown/green wire or two going from terminal C1 to the horns.
You should have a brown/black wire from terminal W1 to the horn push button.
You should have a green wire from terminal W2 to the voltage regulator terminal A4.
Some relays have only 3 terminals, in which case C2 and W2 are the same terminal labeled W.
C means Contact and W means Windings.

How it works:
You always have negative power at C2, but it’s not connected to anything until the contacts close. When the contacts close, it connects to C1 and powers the horns, which are grounded.
To make the contacts close, you power the Windings. W2 has negative power, and you push the horn button which connects W1 to ground inside the button, and the windings become a magnet which is energized and pulls down the contacts.
Toot. :trumpet::postal_horn:

Rob - I’ve reread your comments and now find the following

  1. Always power to C2,
  2. No power to W2
  3. When ignition turned on, get power to W2 then
  4. When I connect a ground wire to interior of horn push or to W1, the horn sounds
    WITHOUT depressing horn push
  5. According to the wiring diagram, there should always be a ground wire from the
    interior(?) of the horn push to the steering column(?) but I see no place to attach
    this gound wire. When the horn is properly attached to the steering column, with the
    grub screws, does this automatically serve as the ground??
    ???
    HELP, Glyn

Wow. That answer was more than I anticipated!! I didn’t include diagrams in my initial post because I wasn’t sure how to do it. Everything as you depict and say. I have grounded the horn push - made a continuity check from the relay to a position on the horn push - then took a wire and grounded that point. Pushed the horn but nothing happened. I’ve now found the keys and switched on the ignition (need to disconnect the fuel pump since tank is dry) just in case the ignition has to be on for the horns to work. Did these before receiving your response. I will read over and absorb your comments and try again tomorrow. Your response is VERY much appreciated. Thanks

| Rob_Reilly
August 27 |

  • | - |

Since you say yours should not have signals I’ll assume it is an OTS and try this diagram. Hmm, no labels on the relay.

But the FHC diagram has labels on the relay.
image
I’ll also assume you have positive ground.
You should have a brown wire from the ammeter to the fuse box and over to the relay terminal C2, always powered regardless of the ignition switch.

You should have a brown/green wire or two going from terminal C1 to the horns.
You should have a brown/black wire from terminal W1 to the horn push button.
You should have a green wire from terminal W2 to the voltage regulator terminal A4.
Some relays have only 3 terminals, in which case C2 and W2 are the same terminal labeled W.
C means Contact and W means Windings.

How it works:
You always have negative power at C2, but it’s not connected to anything until the contacts close. When the contacts close, it connects to C1 and powers the horns, which are grounded.
To make the contacts close, you power the Windings. W2 has negative power, and you push the horn button which connects W1 to ground inside the button, and the windings become a magnet which is energized and pulls down the contacts.
Toot. :trumpet::postal_horn:

Yes, the horn push has a brass disc and a couple of grounding tabs under it.
IMG_20210827_103854738



You push the plastic button and the brass disc pushes against the tabs to make ground to W1.
W2 green wire from the regulator has power when the key is on.

Rob, what is your email address. I want to send some pics but cant seem to do it via Jag-lovers.
Mine is bessbarn@epix.net Thanks

robreilly120@comcast.net but you can also click on my icon and a message option icon will appear by which you can send a private message with pictures through jag-lovers.org


I am confused. The loom I got from SNG that goes from the relay around the front and back to the firewall on the left has three wires for the relay. Two brown with black that go to each horn, and one brown with green that goes back to the firewall. To add to the confusion, my relay has terminals c2 and c3. What goes where?

That’s peculiar. I think your terminal C3 is mislabeled and should be C1.

Brown/green - C1 to horns
Brown - C2 from fuse box
Brown/black - W1 from horn push button
Green - W2 from terminal A4 on the voltage regulator

isn’t there 2 different types of relays used here early cars late cars?

Terry,

That’s correct: early cars had Lucas Relay SB40/1 with part number 33116B and later cars had the same Lucas SB40/1 relay with part number 33135B. The only difference between the two is that the early one had only one W contact whereas the later one had W1 and W2. I assume that in case of the early relay the W2 function was in fact the earth/mass connection of the relay housing .
These SB40/1 relays always had C1 and C2 contacts, so I agree with Rob that the C3 marking is incorrect.

Bob K.

Lucas Relay SB40 1

So, I don’t need a wire from C2 back to the fuse box? My loom only has the two wires to the horns and the wire back to the regulator.

You have no green wire? My harness from Rhode Island Wiring has the green in the bundle with the others. You need something to power W1 and W2.

Terry, yes early relays had 3 terminals. I’m using one for my driving lights.
IMG_20210919_173846451
Strangely the cover has a C3 stamped in it, but no corresponding terminal on the opposite side.
Here’s another type with 3 terminals but no markings.

Yes I have green wires going to the horn relay - which works perfectly - and the fuel gauge, two of them bringing power in and taking it to the wiper switch. Thanks, Glyn

My harness is exactly as described in Viart, 1 brown w/black to each horn, 1 brown with green to go to the fuse box, and the brown with yellow to the horn push.

I have an old Delco Remy that came off the car, but don’t know if it was used for the horns or for the added turn signals.


None of the usual suspects seem to have the three wire relay available, are there any other suitable replacements? How could I measure to determine?

I suppose I could run another wire back to the fuse box from C2 to be able to utilize the 4 lug relay?

I’m back, after being off line for a week dealing with computer problems.
Randy, I guess yours must be an earlier version harness, and the brown/green serves negative power to both the windings and the contact at C2.
The other side of the horn windings should be grounded by a black wire exiting the horn and over to the chassis.
Then when the horn push makes a ground connection, the windings should be energized and pull the contacts together.
Your Delco-Remy relay should work until you can find a Lucas one, if you can figure out which terminals do what.
Is that 3 or 4 terminals?

Rob - You are what we call in Wales a Godsend - most knowledgeable and helpful.
Next is my heater, then the wipers, coil/distributor, and finally the bonnet or hood release
With a bit of luck, I’ll be trying to start the beast in a few weeks. Hope your computer
probs are in the past. Glyn

I guess I like to get to the bottom of these things.

The 3 terminal relay is not shown on any 120 wiring diagrams, but it is in all the Mark V wiring diagrams, and the wires are thus:
C1 = a pair of brown/blacks going to the horns
C2 = a brown/green going to the fuse box
W = a brown/yellow going to the horn push

Inside the relay, C2 branches off in two directions, one to the contact point and one to the winding.
C1 is the other contact point
W is the other end of the winding (i.e. an electro-magnet that pulls the contacts together)

image

I guess that’s the explanation, your early car is like a Mark V and does not have the green wire, but mine being later does.

As Terry & Bob said:
image
Looks like maybe they incorporated the green wire into the harness a little bit later.
image

So I think if you had a 4 terminal relay, you could put a jumper wire across C1 and W2, thus both would receive negative power from the fuse box.
C2 to horns
W1 to horn push button

Thanks, my buddy that is familiar with controls came to that same recommendation.