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MG MGB Technical - Horns

I have a 1977 MGB. Horns didn't work, so I ordered a pair of new ones. The new horns have only one spade connection terminal, but the old horns each had two. How are the new horns supposed to be connected?
Thanks,
Randy
1977 mgb roadster
1969 Jaguar E-type 2+2
Randall Olson

If you're going to use the new horns, you'll need to fit a relay.
The original horns have a 12v supply to one terminal and the other terminal is switched to ground. A single terminal horn requires a switched live supply and the grounds through the body mounting.
Dave O'Neill 2

A 1977 *model* (you may have a 76 model not registered until 77) should have had one-wire horns anyway, you need to check the wiring and see if the second wire on each as been fitted by a PO. If so, it would have been an earth wire, so can be discarded. To confirm that test the purple/black wire with a voltmeter or test-lamp to earth and press the horn button, you should see 12v on the wire. But if you see an earth coming down the wire (connect the other side of the meter to 12v instead of earth) i.e. have purple and purple/black wires to each horn then you will need a relay to operate one-wire horns as Dave says.

It's not uncommon to need a relay anyway, because of problems with poor earth connections up the column which has to come via the UJ, rack and crossmember. In fact are you sure both horns have failed and it's simply not enough voltage reaching them? To confirm the voltages are OK connect a voltmeter between the purple and earth and you should see 12v. Sound the horn and you should still see very nearly 12v. If it drops more than a few tenths of a volt there are bad connections back through the purple-circuit fuse at the fusebox.

If that's OK connect the meter between the purple/black and earth. You should see 12v as before, but it should drop to 0v when the horn button is operated. If you still see a volt or more then there are bad connections back through the horn button and column etc. I've had to add relays to both my cars over the years.

This http://www.mgb-stuff.org.uk/horns.htm#hornrelay shows how to add a relay to 2-wire horns. For one-wire horns the purple at the horns must be securely taped up so it cannot short on anything, and instead of the relay terminal 30 going to earth it should go to a purple wire. I've installed my horn relays behind the dash, where there is access to the purple and purple/black wires going to the column switches/horn buttons.
Paul Hunt

I have 12 V at the purple wire. But there is no reading at the purple black wire.
Randall Olson

There is an open-circuit between the two then. Could be the horns, but both would have to have failed in much the same way. Possible I suppose if one failed years ago, and the second one more recently.

BTW, voltage tests should always be done with the wiring connected, not disconnected. After fiddling with electrics professionally and as a hobby for more than 60 years that's second nature to me so I never used to mention it, until someone recently reported some very weird readings from another circuit until I realised he must have been taking the wires off.

Test the spades as well as the wiring connectors, as there could be bad connections between those two as well as inside the horns.
Paul Hunt

I tested both horns, and one was good so I reconnected it. It works now, but the tone is weak...not a very good sound.
The two new single pole horns that I bought will likely be returned to the vendor. Unless I decide to install a relay.
So, next question is: How to install a horn relay in very simplistic terms.
I have a relay. I can mount it on the wheel well. But, from there I am not sure how to run the wiring. I can see the brown coming into the fuse box, and a purple leaving the box on the other side. But, I'm reluctant to start splicing into a system that works, especially now that the horn toots.
If someone can show me a simple way to connect my original two pole horn wiring system to a new one pole pair of horns I might attempt it, though.
I would need a diagram, and carefully worded step-by-step description.
Thanks for all the help!
Randy
Randall Olson

Randy,

You said, "If someone can show me a simple way to connect my original two pole horn wiring system to a new one pole pair of horns I might attempt it, though.
I would need a diagram, and carefully worded step-by-step description."

You have failed to recognize that Paul Hunt already answered that question in the last paragraph of his post on 7/17 at 14:21 when he said:

"This http://www.mgb-stuff.org.uk/horns.htm#hornrelay
shows how to add a relay to 2-wire horns. For one-wire horns the purple at the horns must be securely taped up so it cannot short on anything, and instead of the relay terminal 30 going to earth it should go to a purple wire. I've installed my horn relays behind the dash, where there is access to the purple and purple/black wires going to the column switches/horn buttons."

Charley
C R Huff

Let me see if I've got this right.

Disconnect both the P and P/B wires at each of the old two pole horns.
Tape the old P wires at each horn so they don't short.
Install the relay near the horns; easier than working under dash.
Splice into a P wire; probably use the one nearest the new relay that was taped so it didn't short.
Connect that new wire from P to the relay terminal 30.
Now what?
The wiring diagram doesn't show where the relay is, does it? It only has the Fuse Block, Horn-Push, and Horns.

Randall Olson

Randy,

Did you scroll down to the third diagram? It clearly has the relay. Then, what Paul wrote and I copied are the modifications to be made to that third diagram labeled "2-wire Horn Relay". If you find that diagram and read carefully what Paul said, you will solve it.

Charley
C R Huff

OK.
Looking at the third diagram with the relay it looks like I would do the following:

Disconnect both the P wires at each of the old two pole horns.
Tape the old P wires at each horn so they don't short.
Install the relay near the horns; easier than working under dash.
Splice into a P wire; probably use the one one that went to the passenger side horn.
Run that P wire to the 85 terminal.
Find a P/B wire, not sure where to find it except at the horns. Once found, connect it to the 86 terminal.
Find another P/B wire somewhere, again not sure where to get it; probably at the horns. Connect it to the 87 terminal.
Connect a P wire to the relay terminal 30.

So, it seems like the unknowns are where to tie into the P/B and P wires.
Am I correct that two P wires need to be connected to the relay? One at 85 and the other at 30 terminal?
Thanks for your patience. I'm obviously not an electrical engineer!
Randy
Randall Olson

First you need to determine (as desribed earlier) whether your weak horns are down to bad connections in the purple circuit, or in the earth circuit (or maybe in both).

If in the purple circuit then adding a relay will do nothing, and you should fix those problems separately, which is easier than fixing earth side problems.

If you have earth-side problems, then you can add a relay. As you seem to have two terminal horns then simply compare diagram 1 with diagram 3.

You should see that purple goes to the horns in both cases, and also to terminal 85 on the new relay. You do not take the purple wires off the horns and tape them up.

The purple/black is interrupted somewhere, the half that comes from the horn push goes to relay terminal 86, and terminal 87 goes to the horns.

A new good earth goes to terminal 30.

If you decide to install the relay near the front, then put it behind the radiator diaphragm where it is largely protected from the weather, unless you only ever drive your car in the dry. In this case the two purple blacks should be removed from the alternator-side horn will need to be extended back to relay terminal 86. The purple/black on the carb side will have to be removed from the horn and taped up. Then run a new wire from relay terminal 87 to the horn terminals that the purple/blacks came off, both horns.
Paul Hunt

Paul,

I can't swear to this, but I think Randy is trying to fit his new one-wire horns.

Charley
C R Huff

Yes, Charley is right. I have a car that originally had two-terminal horns, and I am trying to fit two single-terminal horns that were mistakenly purchased.
To answer Paul's question, I think the purple and purple/black wiring is fine. I hooked up one of the two original two-terminal horns and it works, barely. It has a very squeaky tone.
Randy
Randall Olson

Randy,

I think the problem with your previous description is that you twice say (I paraphrase) go get a P/B wire somewhere. You can't just get a P/B anywhere you find it. You have to get it in the right part of the run. I think the P/B wire that goes to 86 must be the P/B wire that connects to the horn push.

But, given your confusion, it might be best if you wait for Paul to come back. I might lead you astray.

Charley
C R Huff

19th July: "The two new single pole horns that I bought will likely be returned to the vendor."

That's why I described how to install a relay to 2-wire horns.

Whether you fit a relay or not will depend on whether the earth from the horn push is weak or not. If so, then you will need a relay whichever type you fit.

But if the earth is good, and you decide to fit the 1-wire horns anyway, then again you will need the relay. But if the earth is good there is no point in installing a relay, just put two decent 2-wire horns on the existing wiring.

"I hooked up one of the two original two-terminal horns and it works, barely."

I'll say again, until you do the voltage tests at the horns you won't know whether the problem is the horn or the wiring. If it's the wiring then the effect will be even worse driving two horns, regardless of whether they are 1-wire or 2-wire.

The purple/black always existed between the horn push and the horns. On 2-wire horns it carried an earth when the horn button was operated, and there was permanent 12v from the purple on the other horn terminal.

On 1-wire horns it carried 12v from the horn push, there was no purple at the horns, and the horns picked up an earth from their mounting brackets on the inner wing.
Paul Hunt

Since the topic is horns can I ask another question? My horn blows when I push on the steering wheel. The brass tab under the steering wheel got mangled and I bent it to the shape I thought it should. Is the tab too close to the wheel?

Kelvin
K Hawkins

How is the wheel moving? It shouldn't move in and out at all, it should be held firm by the taper seat the the column shaft and nut, and the UJ, and strictly speaking it shouldn't tilt either, but can do a little bit of the top bush in the column is worn.

What year? There were several methods of sounding the horn over the years. Assuming that it was the most common version where the wheel has the horn button and a slip-ring on the back it sounds like your brass brush is right on the edge of the slip-ring and able to touch an earthed part of the hub, or something else, when the wheel moves. Position the brush so it doesn't touch anything but the slip-ring.

Paul Hunt

Back to Randy's issue.

That squeaky horn does sound as if it is being starved of electricity, because of high resistance in the wiring. I've had horns do this.
What I suggest, to test your existing horns, is to take them and two lengths of farely heavy wire and connect them to the / a battery directly. That will tell you straight away if they are OK. You could even use the battery in your daily driver, probably more accessible.

If the horns prove OK here, then you have a problem with the wiring in the car. Even a relay won't solve your problem, as you still need a good 12V supply to the relay for the horns. Using the existing wiring won't gives you this. To find where the fault is, firstly check your fuses and the fuse holder, simplest. The fuse holders are known to develop corrosion and high resistance, underneath, where the clips are riveted to the brass strips. If so clean and solder, or replace.

Herb
Herb Adler

Paul

Thanks for the response. Been out of town. This is a 76B North American.

The contact seems to occur if the outer edge of the wheel has any pressure applied. This is happening without the horn brush installed because I some how broke that.

K
K Hawkins

"This is happening without the horn brush installed"

Do you mean the brush attached to the indicator switch mounting bracket that bears on the slip-ring on the back of the steering wheel, or the 'pencil' that goes through the wheel hub between the back of the slip-ring and the horn switch in the wheel centre (see attached pic)?

In the former case where is the purple/black wire that comes out of the switch harness and would connect to the brush? If that's waving around it could be touching anything.

In the latter case the brush or purple/black wire connected to it could still be touching something it shouldn't, or there could be a broken part of the pencil or some other metallic piece in the hole in the hub, shorting the back of the slip-ring to the body of the hub.


Paul Hunt

It is the pencil that I broke. In my Moss parts catalogue this is part 263-750 "BRUSH, horn". I pulled the purple/black wire off to stop the horn. I will check to see if there is something causing the short you mentioned.

Thanks
K Hawkins

The Moss description is misleading in that it doesn't 'brush' against anything, unlike 141-804 'Clamp and Contact Assembly' which really is the brush, as it rubs against the slip-ring on the back of the wheel as the wheel is turned.

The 'pencil' is simply compressed between the back of the slip ring and the horn push, and all three rotate as a unit, nothing rubs against anything else. As such it is a relatively complex and expensive piece of kit for what it does, a simple wire soldered between the two would do the same job, and is how Moto-Lita for example do it.
Paul Hunt

remember to put the pencil in the correct way round - see Paul's excellent web site. linked to earlier in the thread
Nigel Atkins

Hi, it's Randy back from travels.
I checked one of the originals horns by connecting directly to a 12V battery. No sound.
The other old horn has been reinstalled. It makes a weak tweet, only. May not be getting enough power. I still need to check it.
The two new horns have been returned.
Next, I will check the installed horn by connecting to a 12V battery. If it's stronger I know it needs more power.
I will check the fuses first, cleaning terminals.
This will likely be done in a a couple of weeks, as I am off to Quebec.
Thanks for the continued support!
Randy
1799 MGB
Randall Olson

Hmmm, my MGB is 1977, Tahiti Blue, Tan, twin SUs...not 1799!
Randall Olson

Popped round to a pal's house yesterday to look at the horn on his (OK I know, wash my mouth out) TR3 which is new to him and just been restored. The main problem was a very high resistance connection where the UJ yoke was clamped onto the steering column splines!

Connect a voltmeter between the spade with the purple wire still on it and earth, you should see 12v. Press the horn button, if the voltage drops significantly there is a bad connection back through the fusebox.

Now connect the voltmeter between the spade with the purple/black wire on it and earth. You should see 12v, if you don't the horn is bad.

Press the horn button and that should drop to zero volts. If you do, and the horn hasn't sounded, then the horn is bad.

If the voltage doesn't drop at all, or only drops part way to zero, then the earth circuit back through the purple/black to the horn button and the column earth is bad.

The results of the above should tell you where to direct your attention next.
Paul Hunt

Thanks, Paul. That's a very clear description of how to troubleshoot the horn.
I probably won't have time to do it for a couple of weeks, though. I'm leaving the country Saturday morning, and have lots to do to get ready.
Tonight, I'm going to a friends annual gathering of old sports cars. Everything from MGBs and GTs to Ferrari's, with a sprinkling of E-Type's, Mercedes, Healey's, and last year a 1903 Oldmobile. Can't miss that.
Thanks again,
Randy
1977 MGB
Randall Olson

Have followed Paul's advice on how to test, and can verify that all is wired correctly, but the horn doesn't work from the push. The steering column appears to be isolated from the rest of the car, because there is 12v at the pencil in the middle of the steering boss, but I cannot measure a continuity of earth between the copper slip ring and anything other than the steering shaft itself! I don't expect this to believable, it's definitely in the strange but true category....

Any suggestions on how to recreate an earth between the rotating bits and the rest of the car. The horn did work last week, and we have the state safety check on Thursday.
dominic clancy

Here is a photo of the horn push without the button. The pencil in the middle brings 12v, but the copper ring has no ground except to the steering shaft.

Pushing the engine earthing strap into contact with the steering shaft makes the horn work, so this is why it worked before. But what is the permanent fix?


dominic clancy

The pencil and the slip ring are connected to the wire from the horn so will have 12v on them until the horn push is operated.

You should be able to sound the horn by bridging the nipple in the middle of the column to the large nut that holds the wheel on. If it doesn't, measure the voltage on the nipple and the nut while you are bridging them.

They should be the same, and they should be zero volts with respect to earth. If they are the same but you see something higher than zero volts, then the earth to the steering column via the UJ, rack, crossmember and chassis rails is bad - perfectly believable as it is quite common, and is why pushing the engine earthing strap against the column makes it work.

If you find connecting an earth to the rack makes the horn button work as well, then probably the easiest fix is to run an earth wire from a rack fixing bolt to a bolt nearby on a chassis rail or body.

Paul Hunt

This thread was discussed between 17/07/2014 and 14/08/2014

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