Expert: Ernest (Ernie) Kenward - 6/7/2008

Question
QUESTION: I have a original restored 42 olds coupe. It has a trumpet horn that i cannot get to make even a "peep" or squeak. I am looking for help in repairing the horn or at least determining if it is even repairable. I have removed the horn but have not tried to disassemble an of it except for removing the cover. What I do know is this: I have good power source. Contact points look good (not all burnt away). They seem to be separated Ok. What i have realized is that the "plunger" is seized fast in the coil. I have pried a little on it to see if it would loosen up, but it didn't and i didn't want to force it any further.  I know from working on industrial solenoid coils that the plunger will seize fast when the coil fails and there is an internal meltdown.  This coil doesn't appear to have been in meltdown.  Can you give me direction on trying to get this horn to operate?  thanks in advance for your help. P.S. I have been looking/shopping for another horn(s) for almost 2 years and havent been able
to find one - so i thought i would once again make an attempt to repair this one.

regards

tweed

Because You're in a pretty tough spot. A '42 olds is about as rare as you can get!



I don't know any way to free up the armature if it's stuck - maybe penetrating oil will help - something like PB Blaster.

This above answer ws given by Dick Benjmin. Today i decided to tear into  the horn again and see if i could figure aythiing out obout it.
What if did find it that when i loosened th nut on the front of the diaphram it allowed the armature some freedom of movement. So now i know the armature is not stuck. It works freely and moving it up and down makes and breaks the points. What i have discovered now is that when i apply 6 or even 12 Volts to it - all i get is a one-time little scratching noise.  For some reason the coil doesnt seem to have enough "oomph" to it to pull the armature down enough to break the points. I dont have an ohm meter to check draw across the coil but i do know that i have continuity everywhere!!  when i attach continuity tester to the power lead, I get continuity everywhere i touch from both sides of points to both sides of coil to any point on the body?
This shouldnt be? should it? But then maybe it should? If i connect ground to anywhere on the body ant then apply voltage to the input line -  again all i get is that one-time little scratch" as the power goes to ground. I would dearly love to make this horn work or find a replacement. I have searched over a year to no avail on finding a replacement. I only have the one -- it is a "Low" - dont hae a companion 'High" at all.
Any help you can offer will be appreaciated. Also if you coud even lead me to someone that is able to fix/restore these i would appreaciate that also.
thanks in advance.
Tweed




ANSWER: You realise that, once you have solved this problem, you should become the point person for any similar future questions!

That six-volt horn coil would be a very low-resistance coil, as it would need to draw a significant amount of current (especially at 6V) in order to develop the acoustical power one would desire if an auto horn is to be useful as a warning device.  Note, of course, that this high current would be drawn only for very brief periods and low duty cycle.  

The point is that the current being drawn goes THROUGH the coil, and does not bypass it - ie: short to vehicle ground - through any breaks in the insulation.  Therefore, while measuring current draw would be important, being able to measure coil end-to-end resistance would be even more important.  To do this with this type of coil, you would need a meter with a very LOW ohms range, ie: 0.1 or 0.01 bottom end of range and resolution.  This is beyond the specification of most general-purpose multimeters, but is within the capability of an instrument-grade Wheatstone bridge detector or of a specialty instrument such as Fluke's model 1520 MegOhmMeter; while this is primarily an instrument for insulation testing up to 4 Giga-ohms (an electronic version of a "Megger"), it also has a nullable low-ohms range.

My thought is that a finely calibrated reading of the coil resistance would probably be useful, in order to verify that all the current drawn is, in fact, coil current, and not significantly short-circuit current.  

My instinct is that, over years of both use and in storage, heat, humidity and corrosion have probably conspired to break down the insulation, to the extent that very little of the current that flows actually flows through the coil so as to produce enough flux to be concentrated and move the armature.

The good news - if I am right - is that your eventual fix will probably involve your rewinding the coil of you original horn.

* * *

Thanks for a very good question.  I, too, have old horns kicking around, and you have given me pause to think about them and their possible problems!

Do let me know how this works out.

Cheers ... EGK



---------- FOLLOW-UP ----------

QUESTION: Will be glad to let you know how it turns out. You make excellent point that the  fix will be to re-wind the coil since it appears that the flow of juice is being dissapated prior to reaching the outlet side of the coil thus rendering it without enought strength to pull the armature!
Thanks again for your help.  Just yesterday i found a similar horn from an early chevy truck that blows great. I am going to test the resistance on its coil and then compare it to the one that doesnt work -- If there is a significant difference then i will presume that re-winding the coil is, indeed, the repair solution!!
Thanks again.
Tweed

Answer
The greatest kick of all when reworking vintage hardware, whether it be classic cars, vintage locomotives, ancient engines or whatever, is being able to breathe new life and new reliability into the original components.  If one is able to do a hidden upgrade (better magnetic and insulation materials, for example), then one does even more to celebrate the original design by renewing its relevance in a modern world!

All the best ... Ernie Kenward