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[Solved] Tracing purple horn wire question

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[Solved] Tracing purple horn wire question
#1
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orion Avatar
orion Frank Nowak
Bristol, RI, USA   USA
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1970 MG MGB
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1974 MGB horns just sort of click or blip when button pushed. Applied 12 volt to horns and they work fine. Due to low voltage at the purple wire spade connectors, re-did them which were corroded, and checked fuse box, all ok. Horn button seems to work ok and I shorted the contact ring to the hub and got the same quiet blip from the horns. I now have full voltage at the purple wires at the horns. So, where the heck do I trace the purple back from the fuse box to solve this? Thanks, beep, beep.

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Re: Tracing purple horn wire question
#2
OldDuffer Avatar
OldDuffer Silver Member John S
Eugene, OR, USA   USA
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1970 MG MGB "Ella B (totaled And Parted)"
1971 MG MGB GT "Ashley"
1979 MG MGB "Mae B *SOLD*"
My ‘70 had the horn button at the end of the indicator stalk. Are you dealing with an aftermarket steering wheel?

Edit: I see now that the car is not the one in your profile. My bad.


I was addicted to the hokey pokey but I turned myself around.

prop-a-gan-da: When a British person takes a close look at something



Edited 1 time(s). Last edit at 2024-04-26 01:58 PM by OldDuffer.

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Re: Tracing purple horn wire question
#3
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MG14611 Robert P
NY, NY, USA   USA
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1963 MG MGB MkI "Born To Run"
1963 MG MGB MkI "Potential"
The horn related parts at the steering wheel and horn push are actually closing the horns to ground when the horn center is pushed.

The horns are connected to 12v , fused, at the fuse block.
But ground is held open until the horn button is depressed.

So sometimes the steering column loses a good ground contact to the chassis.
A hard wire jumper attached to the steering rack and to the body side of a motor mount nearby helps.
If you connect a battery jumper cable on an exposed part of the steering column and the car body, as an experiment, try the steering wheel horn push again.

If the horns work properly, do the wire jumper addition.

See the horn circuit in the diagram for your model year!


Attachments:
! Wiring diagrams MGB.pdf    1.49 MB

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Re: Tracing purple horn wire question
#4
ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
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1972 MG MGB
Frank,
On most year models including the 70, the horns have power all the time on the purple circuit, the horns are switched on the ground side, check the purpleBlackTracer circuit and switch for continuity. The switching requires the steering column assembly to be properly grounded. This was original accomplished with a copper band around the upper column bearing but with age it can become an issue.

As a test, make a good ground anywhere on the upper steering shaft and try the horn, if the horn works will need to deal with the column grounding.

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Re: Tracing purple horn wire question
#5
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jjonns74 Gold Member John Dailey
Indianapolis, IN, USA   USA
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1974 MG MGB "Athena"
1974 MG MGB "Apollo"
Per Sheet #17 of previously provided wiring diagram:

1974 horns have 12.6 vdc (via Purple wire) constantly applied while Key/Engine is OFF.
Nothing to trace, as wire is direct from fuse to horn = no connections.

Horns are activated by Ground via the center button of the steering wheel - the weakest link of the system.

Disconnect the Purple wire from terminal of horn on Passenger side.
With Multi Meter, confirm 12.6 vdc at the Purple wire end.
If present, reconnect P wire, then:
Disconnect the Purple/black wire from the other terminal of that horn.
Confirm Zero ohms from P/b wire end to Ground while horn button is depressed.

Horns will sound Loudly once you apply a Good Ground to the Purple/black terminal of the horn.
(ideally, from horn button -- certainly from a jumper wire connected to Ground)

JohnD

In reply to # 4789506 by orion 1974 MGB horns just sort of click or blip when button pushed. Applied 12 volt to horns and they work fine. Due to low voltage at the purple wire spade connectors, re-did them which were corroded, and checked fuse box, all ok. Horn button seems to work ok and I shorted the contact ring to the hub and got the same quiet blip from the horns. I now have full voltage at the purple wires at the horns. So, where the heck do I trace the purple back from the fuse box to solve this? Thanks, beep, beep.

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Re: Tracing purple horn wire question
#6
orion Avatar
orion Frank Nowak
Bristol, RI, USA   USA
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1970 MG MGB
I inserted a phillips screwdriver into the plunger hole and the horns worked. Guess I need a new plunger? But, when I checked the little bugger for continuity, it have full ohms reading. Must be the plunger.

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Re: Tracing purple horn wire question
#7
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MG14611 Robert P
NY, NY, USA   USA
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1963 MG MGB MkI "Born To Run"
1963 MG MGB MkI "Potential"
Try squirting some lubricant into it. The plunger might be a little sticky!

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Re: Tracing purple horn wire question
#8
teroth Avatar
teroth Tim Roth
Las Cruces, New Mexico (NM), USA   USA
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1975 MG MGB "L'il Red Car"
In reply to # 4789506 by orion 1974 MGB horns just sort of click or blip when button pushed. Applied 12 volt to horns and they work fine. Due to low voltage at the purple wire spade connectors, re-did them which were corroded, and checked fuse box, all ok. Horn button seems to work ok and I shorted the contact ring to the hub and got the same quiet blip from the horns. I now have full voltage at the purple wires at the horns. So, where the heck do I trace the purple back from the fuse box to solve this? Thanks, beep, beep.

So Frank, you state that the problem is solved. Please tell us what you did to solve it.

Thanks.

Tim

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Re: Tracing purple horn wire question
#9
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red68mgb Silver Member Jim Milavec
Murrells Inlet, SC, USA   USA
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1968 MG MGB "Redginald"
If needed, the horns can be disassembled and cleaned. I did this last year after a search here on MGE.


I'd rather wear out than rust out.

"If everyone is thinking alike, then somebody isn't thinking." General George Smith Patton

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Re: Tracing purple horn wire question
#10
orion Avatar
orion Frank Nowak
Bristol, RI, USA   USA
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1970 MG MGB
Tim, after placing the screw driver into the hole and got the horns to work, I figured the plunger is the culprit. When the new one arrives, I will confirm.

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Re: Tracing purple horn wire question
#11
teroth Avatar
teroth Tim Roth
Las Cruces, New Mexico (NM), USA   USA
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1975 MG MGB "L'il Red Car"
Oh, that's excellent Frank. Those little brushes can be problematic.

Thanks for keeping us informed and I hope that replacing the faulty brush
resolves your problem.

Best wishes.

Tim

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Re: Tracing purple horn wire question
#12
MG14611 Avatar
MG14611 Robert P
NY, NY, USA   USA
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1963 MG MGB MkI "Born To Run"
1963 MG MGB MkI "Potential"
If the horn worked when you applied 12v across the terminals, then the plunger is okay.

The plunger moves inside the electromagnetic coil inside the horn. If the magnetic field is less than the nominal level due to a lesser voltage applied, the plunger doesn’t move fully enough to interrupt the points inside.
A poor wire connection can cause the lesser voltage at the horn.

Iirc the plunger is tacked onto the metallic diaphragm disk that produces the sound. I’m not aware of replacement plungers or diaphragm s being sold new.



Edited 1 time(s). Last edit at 2024-04-26 03:47 PM by MG14611.

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Re: Tracing purple horn wire question
#13
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riley1489 Gold Member Bruce H
Great White North, QC, Canada   CAN
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1953 Jaguar XK120
1959 Riley 1.5 "King George"
1973 MG MGB
In reply to # 4789624 by orion Tim, after placing the screw driver into the hole and got the horns to work, I figured the plunger is the culprit. When the new one arrives, I will confirm.

the installation of a relay will mitigate this reocurring.

B


Life's most persistent and urgent question is, "What are you doing for others?"

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Re: Tracing purple horn wire question
#14
OldDuffer Avatar
OldDuffer Silver Member John S
Eugene, OR, USA   USA
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1970 MG MGB "Ella B (totaled And Parted)"
1971 MG MGB GT "Ashley"
1979 MG MGB "Mae B *SOLD*"
I know of a chap in the Great White North that can help you out with a horn relay. grinning smiley


I was addicted to the hokey pokey but I turned myself around.

prop-a-gan-da: When a British person takes a close look at something

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