122 Horn Notes - Blow Your Horn!                                       3/09/R. Kwas

Background:  Horns are a necessary accessory on a vehicle which needs to be ready to use at any time, but since they are only called upon fairly rarely, most of the time they are just standing by, and doing their best impression of going into mechanical rigor mortis!  Add to that the fact that they, and their electrical connections, which need to handle a fairly high current load, are located right out there in the rain, and seasonally under further attack by a stream of salty mush bath, it is easy to see why they often quit functioning...if that’s not enough, add to that, a couple of built-in weaknesses, both electrical and mechanical, and it’s easy to see that preventative maintenance is called for, and quite possibly some improvements...

As a significant electrical load, one of the first things that should be assured, is all the connections are clean and tight...this is generally covered under the Gas Tight Joint and ACZP information. 

I've combined an older article and new info here, so the reader may notice some discontinuity or repetition...I may clean this up a bit further sometime in the future. 

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Weaknesses of the original design 
Function of an electromechanical horn.  An electric bugle 
Symptoms of Problems, Repairs
 
Improvements
Retrofitting a Relay to handle Horn Current

Repair of the 122 Steering Wheel Horn Switch. 

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Weaknesses of the original design:  Several electrical circuits and steering wheel control switches were used from 544 to 1800 models, and some areas of weakness exist.  In 544s, a relay is shown, in all but the last of the 122s and 1800s, no relay is shown.  This inconsistency is likely due to several factors, but because different people designed these systems, and they were apparently not governed by a "company standard practice". 

Horns of the earlier vehicles are subject to corrosion issues mentioned before and they are of course the oldest, so one can add even more old-age sluggishness to that.  Electrically, I don’t know of specific design weaknesses plaguing them.

Design Weakness of the 122 Relay-less Design:  Since horns are a fairly heavy load requiring high currents, having solid connections for the entire current path is especially important.  In addition to the push-on terminals on the horns behind the grill having been under corrosion attack for decades...and putting all of that current through the poor switch in the steering wheel may have been OK when the vehicle was new and all connections were at their best, but now, taking into account the age of the components and connections, this is just not good practice!  The relayless design of the 122 should be upgraded to include a relay.  

Recommended Improvement - Electrical Upgrade of the circuit to include a Control Relay:  High (power) sides of the horns are supplied with IGN power by way of Fuse 2, and since Fuse 1 and 2 are supplied by the IGN Switch, this means that their current is passed through the IGN Switch.  In the original wiring, the low sides of both horns are connected together and this node is routed into the steering box, up the steering column, past the isolator coupling and to the steering wheel, where a connection to chassis completes the circuit and energizes the horns.  Refer to Before circuit.  By adding a relay as shown in the After circuit the steering wheel switch is subjected to much lower relay control current only. 

The early 1800s also lack a relay in the design, and their horns are supplied by battery power (?), by way of their Lucas FuseBlocks (Oh No!)...no, I don’t believe British drivers and vehicles have a particular need to be able to sound off...while parked..."but it is what it is".  This could actually be seen as a bit of a design advantage, in that the substantial horn current does not flow through the IGN Switch... the Horn control Switch design at the steering wheel is also a much better design from a current handling standpoint, than their 122 brethren, and well suited to handle all the horn current...but still...two strikes there... 

Function of an electromechanical horn.  An electric bugle!:  A metal diaphragm is connected to the armature, around which an electrical coil, not unlike a relay, is wound.  The arrangement is set to vibrate...initially the contact is closed, but opens after the voltage applied causes current to flow which attracts the armature...this cycle repeats for as long as power is applied, and at a resonant frequency set by the mechanical characteristics of the system (were you paying attention in Physics 101?).  As the armature vibrates under the mechanical authority of the armature, it moves air, this action is not unlike a bugle...(maybe) you read it here first! 


Electromechanical trumpet automotive horn. 

Some horn designs even further amplify the sound with a tube of increasing cross-section much like a single frequency electrical bugle.  An adjustment screw is included on the breaking contact mechanism to adjust for the variations in the mechanical system.  Sometimes a non-working horn can be brought back to life by diddling this adjustment when present. 

Typically on vehicles, including vintage Volvos, a combination of low and a high frequency tuned one of these are fitted, so that in addition to their own notes, additional sum and difference tones (heterodyning) would make the final complex tone hard to miss

Symptoms of Problems, Repairs:  If for some reason (like corrosion and lack of use), the horn diaphragm has frozen in place and does not move when power is applied (little surprise there!), no current may flow at all through the corroded contact, OR the current may flow continuously instead of being constantly interrupted as the normal ON-OFF cycle of the actuating relay and mechanism does its thing.  This may result in blowing Fuse 2, as a lot more current flows that would be expected if the mechanism were working correctly. 

Improvements:

Repair of Electromechanical Horns:  By grinding away the rivet heads which hold together the case, access can be gained to the internals.  They are quite simple inside...so if not too much rust has taken over within the assembly, they may often be brought back by cleaning the breaking contact, and lubing the mechanism (I'd use synth-grease for this).  Test by connecting across battery (or beefy(!) shop power supply) in the same polarity as during installation.  WEAR HEARING PROTECTION or it may be hours before your ears stop ringing (and your shorts dry!).

Retrofitting a Relay to handle Horn Current:  Upgrading the circuit such that a relay does the heavy current handling is recommended!


Horn wiring, Before and After a relay upgrade for a 122. 

The early 1800 wiring is similar, taking power from the “Battery Power” Fuse.  An important difference to notice for both 122 and 1800 vehicles is that in the “Before” circuit, power is supplied to horns, and the low side is switched to chassis to energize, in the relay “After” circuit, the opposite is true...low side of horns is permanently tied to chassis and power is supplied to energize...switch at steering wheel supplies connection to chassis in both cases, but at a much reduced control current only in the relay design.

Note:  Since the horns are supplied with IGN power in a 122, current is passed through the IGN Switch is even after performing this relay upgrade. 

Mechanical weakness:  On steering wheels of early 122s two continuous circles of silver plated steel, separated by a foam acting as a “compression spring”, are used to control the horns.  I don’t know of what material this foam was originally made, maybe Cheese Puffs which were waaayyy past their “Best eaten by....” date, but it sure doesn’t seem to stand up to time and the temperature cycling to which it is subjected so good...deteriorated foam not being able to keep the contacts apart is typically responsible for “self-sounding” when you're going over bumps. 

LINK to BB Threads:  http://www.brickboard.com/RWD/volvo/876548/120-130/122_horn_insulator.html

Repair Procedure for the 122 Steering Wheel Horn Switch: 

I have used the procedure presented here in practice many times myself but it is presented strictly as a guide to use in conjunction with normal, cautious shop practice...your results may vary...and you are responsible for your own actions and knuckles!  WORK SAFELY!

Background:  A dense foam is used between the switch contact halves of the horn switch, to keep them separated, but still allow contact when squeezed together by the overwhelming force of leverage advantage one has when pushing on the chrome horn-ring.  This foam deteriorates after decades (those damn Volvos just don't last!), and looses its effectiveness in keeping the horn-switch contacts apart.  When this occurs, at first, the weight of the horn-ring alone is enough to momentarily cause the contacts to close when going over bumps and this results in "self-sounding".  This is the early symptom...and it never gets better by itself...creative, and less than permanent, repairs I've seen include sticking bits of cardboard or matchbooks between steering wheel and chrome horn-ring at the 6 o'clock position to prop up the horn-ring and give it additional support...cute (but less than impressive Dad!) or, one could consider a somewhat more permanent repair.  It's not so difficult!


Blow-up diagram of assembly:  Source GCP site (mystery part 32 is shown in factory exploded diagram...it looks like they intended to have some sort of dress-up ring, perhaps a Volvo logo in the middle of the steering wheel, but as I have never seen this on a production vehicle, I expect it was dropped from the design and forgotten to be removed from diagram). 

Disassembly for Repair:  Access to the switch contacts is gained by loosening the two large Phillips head (size No 3.) screws (Item 30) on backside of steering wheel, which secure the chrome ring, part of which is the switch assembly.  After the chrome ring is removed from steering wheel, the rest can be done on the bench or even in comfort of your most favorite work surface (kitchen table?).  Remove the three small circlips and washers (Items 28, 29), which will allow the mounting plate to be lifted off the locating pegs of the chrome ring (21, insulated shoulder washers, 27 can stay with assembly).  Then lift off switch contact assembly (both may require gently prying, having gotten used to their position for a long time).  Separate the switch contact plates (22, 25)...depending on state of deterioration of the foam (23) between the two halves, it may come apart easily, or require cutting with a string between the halves with a sawing motion (dental-floss-like action).  Avoid damaging the contact edges with more aggressive prying!  

 
122 Horn Switch after removal and separation showing deteriorated foam...yummy!

Preparation:  Without damaging the contact edges of the switch plates, clean away all remains of the separating foam, or whatever goo it has turned into (see above)!...an overnight soak in solvent or paint-stripper works well and spares the important contact edges from mechanical insult.  I’ve seen the foam deteriorated to varying degrees and to varying end-products...from dried and crumbling to a gooey, sticky blob...  Clean the foam area well in preparation for a new separation material, being careful not to damage the silver plated contact area at the edge (see micro-pic following).  Polish to a clean shiny surface, the thin silver plated contact area around the edge, with a pencil eraser or scotchbrite pad or similar, but nothing more abrasive (like emery cloth).


Close-up of continuous contact area, after clean-up of deteriorated foam, showing heavily compromised silver plating.  A contact ring in this sort of shape is clearly not up to the task of handling the full horn current!  If contact ring is found to be in such a condition when performing an inspection / refurbing, and upgrade to a relay control circuit is highly recommended to maintain reliable horn function! 

To replace the foam which is intended to keep the contacts separated, I recommend RTV silicon for over an original replacement (if these are even available!?) or using a computer mouse-pad (a clever solution which someone actually tried and posted in a Brickboard thread a while ago, but which I suspect will deteriorate again...after all, a desktop doesn't undergo the temperature variations a steering wheel does, and that's all a mousepad needs to endure).  RTV on the other hand, has all the properties called for:  Firm but compliant rubber, long-term highly physically stable and quite inert and impervious to attack and deterioration, and also easily adjustable in size to get the desired separation yet allow compression and contact of the plates.  

Applying the RTV:  Apply a bead of RTV, in the flat area of one of the contact plate halves, avoiding the three mounting holes, and also staying clear of the outer contact area. The entire bead should be proud of the contact area by about 0.250", but it should be trimmed to this after the RTV cures, so a little more is better and not a problem (a lot more is just a waste of the RTV, which would need to be trimmed away).  Allow the RTV to cure completely for 24 hours.

When RTV is cured, trim any high spots with a razor blade or similar, such that there is a plateau of RTV, 0.250 above the level of the contact.  Trim any RTV which blocks mounting holes.  Trial fit the other, clean contact plate...the plates should be evenly separated around the entire circle by about 0.100 without compressing of the plates, and compressing the plates should allow them closer, but making contact should take a fair amount of effort.  This is to assure separation under all conditions except for when activated by driver who has additional actuation leverage when the assembly is installed.

Reassembly:  Reassemble the stack onto the chrome horn ring.  Because of the preload occurring between the contact plates, it will be necessary to compress the assembly in order to slide on the circlips.  Verify separation of the contacts after circlips are installed. Reassemble the horn switch assembly onto the steering wheel.

Checkout:  With IGNition ON, push the horn ring to check for switch action.  It should be possible to close the switch with a push at any clock position of the steering wheel to make the horn sound, but the force required should be significant and deliberate.  If the action or force required seems too slight, may be necessary to increase the separation force...disassemble and add to the RTV to increase preload. 

Upgrade to horn relay!  I highly recommend performing an upgrade of the horn electrical circuit to include a relay for handling the significant horn currents.  With it, the repaired switch plates only need to handle a small control current while the relay handles the high load current.  Without it, the switch plates must handle all of the horn current...the resulting arcing and carbonization at the contact can cause poor and intermittent contacts in the future...not so good...when you need the horn, you need it NOW!  This is covered above. 

Any comments or Improvements to this procedure are welcomed.  June 07 R. Kwas  Revised 02 09 

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The term Volvo is used here for reference only.  I am not affiliated with Volvo…although I do also Roll…  I have used the information and procedures presented here in practice many times myself but they are presented strictly as a guide...your results may vary...and you are responsible for your own actions and knuckles!  When did jungles become rainforests?  When did Personnel become HR?…

The contents of this article are Copyright © 2009 by Ronald Kwas.  You are welcome to use this article and its contents for non-commercial purposes.  But if you copy and republish it, whole or in part, without giving credit to the author, or linking back to the Sw-Em site as the source, you’re just a lazy, scum sucking plagiarist!  Go work for the Boston Globe or something!  

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