MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Horn Test with Battery

The PO wired an air horn into my 76B. Cute, but irritating. Plus, my wife doesn't like it - the real reason for this post.

I want to go back to my original horns, which are still in the car. Before I start tracing what the PO did to the wiring, I'd like to test the horns. Based on what I've read in the archives, I can just use jumpers from the positive and negative terminals on the batteries to each of the two spade terminals on the horn, and it should blow.

Am I correct? Does it matter which terminal gets positive and negative?

Thank you in advance for your help.

John
John English

John. I have used two jumper wires from the battery in the past test horns. When I did it, I went to Radio Shack and picked up a couple of large alligator clips (similar to the ones on a battery charger) along with a couple of connectors and a roll of wire. I do not remember the horns being polerity sensitive, but it has been a while.

I do not remember if the later horns had a tuning screw or not. The early horns had a screw with a lock nut which allowed you to apply power to the horn and adjust the screw for best (loudest) sound. I found these to be out of adjustment in many cases. Often to the point the horns did not work properly. I have probably checked out 10 or so malfunctioning horns over the years and can remember only one which could not be made to work by adjusting the tuning screw.

Perhaps others will have more recent knowledge. Les
Les Bengtson

John,

The horns are still grounded to the chassis, if they are still installed. Run only the plus side from the battery, because if you don't observe polarity at the horn, and run the +12 VDC to ground, you'll get a nasty spark.

It will be more convenient, however, to jump a single wire from the fuse block (brown circuit) to each horn. A momentary touch will suffice to test.
Fred Doyen

Fred, I was actually planning on removing the horns first. It looks like the engine compartment was painted, so I think the horn may be insulated from ground by the paint, which I guess may or may not be part of my problem.

If the horn is removed, does it matter which spade connector goes to postive and which goes to negative?

John
John English

John,

Just run a crocadile clip from the horn body to a good earth (simulating the correct earthing), and then run a wire from the fuse box as Fred suggests. Suggest holding the live wire in insulated plier as you touch it to the horn, just in case!

And let us know what happens!!

Iain
I D Cameron

Hi all.

A horn is an inductive device (like a coil) and is capable of producing a back-emf which can give an unpleasant shock if you are holding the horn side of the wiring when you break the circuit, so the advice to use insulated pliers is sound.

Don
Don

Iain

Grounding the case works fine IF it is a single terminal horn. The two terminal horn requires grounding one of the terminals.

FWIW

Larry

Larry Hallanger

John, Are your OE horns plastic with a single terminal or die-cast with two terminals? The type you have determines how the horn should be tested. Single terminal horns from around 75 are grounded through the chassis. A purple wire with 12 volts goes to the horn push and changes to a purple/black at the push. They can be tested by applying 12 volts to the single terminal. Your OE horns should have been single wire horns.

If you have die-cast horns with two terminals one terminal has a purple wire with 12 volts, ground is supplied through the horn push by a purple/black wire from the push to the horns. Two terminal horns don't have any polarity or numeric markings. They are like a relay coil and polarity doesn't matter. Easiest way to check them is to verify 12 volts on the purple wires, apply a ground clip to the other other terminal (purple/black wire). There is no need to remove them for testing. I've been able to tune these honrn if they make a garbled noise but no luck on dead horns.


Clifton
Clifton Gordon

Clifton, these horns have two terminals. I looked at the Bentley wiring diagram, and it indicates I should have two terminal horns. I think these are OE, but they do have two terminals. I unplugged the wires from the air horn, cleaned all the horn terminals (they look like they had black paint on them) and then plugged the wires onto the horn terminals.

Viola! Both blow! But, the high side sounds like it could use some adjustment. They have the adjusting screws. I'm going to play with that some. I've read the Bentley editor's note on adjusting them. I'll let ya'll know what happens.

John
John English

John, It looks like the Moss catalog information wasn't correct, however, to their credit they did say approximate.

Clifton
Clifton Gordon

SUCCESS!!

Thank you all for your help! I ended up taking both horns out. I sprayed the inside with electrical contact cleaner - not sure it did anything, but I figured it couldn't hurt.

The PO had painted the horns, and the adjustment screws were frozen - PB Blaster and some vice grip pliers fixed that (after I removed the locking nuts).

I adjusted the tones to the point where my wife said, "That's exactly like your other one sounded when we were dating." (That was 31 years ago, and it was a 74 MGB - bought new from Hix Green Buick in Atlanta).

So, she's happy, I'm happy, and the MG's happy because it sounds like its suppose to sound!

Thanks again!

John
John English

This thread was discussed between 04/08/2005 and 05/08/2005

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.