Faulty airhorns

[ericbushby]

Hi All.
My twin air horns have stopped working.
They are FIAMM Serie TA with a compressor FIAMM Type CRD
Do any of you know the air pressure that is required from the compressor to sound the horns.

On bench testing at 13 volts the compressor draws 9 Amps and produces 3 PSI.
When I test the horns on a different air supply they sound correctly and as I reduce the pressure they stop sounding at 5 PSI. They each stop sounding at about the same pressure, which would usually suggest that they are OK.
So far this would suggest that the air pressure is insufficient.
I have stripped the compressor which is a vane pump type and it seems to be in excellent condition with no signs of wear. The air intake is clear,
I have stripped the motor section and cleaned the commutator. I checked the wiring to the segments and soldered over the crimped connections. It was very clean and looked near to new, with no wear showing on the carbon brushes.
It seems like the compressor is the problem but I cannot fault it.
If I knew what air pressure is normal It would help.
Any comments or suggestions welcome,
Eric in Burnley
1967 S3SE DHC

[alan.barker]

Eric,
have you been putting a few drop of light Oil in the little hole under the Plug on the Top of the Pump Maybe it's been running dry
Alan

[ericbushby]

Thanks Alan. I looked for lubrication holes, but there are none.
There are only two small holes for pins used to hold the brushes clear during assembly, but you would not want oil in there. The motor is sweet and clean and smooth running.
Eric

[Grizzly]

I'm not familiar with the FIAMM compressors but i do know other types i've seen in the past do tend to stick and sporadically need taking apart, cleaning and resembling. As above the standard compressors require a few drops of oil (i do it as part of the annual service) and i know the standard horns can develop issues if they are faced forward towards the grill (they pick up lots of crud and rain water)

[oldelanman]

I had a similar issue when trying to bench test my compressor (not Fiamm but they are probably all much the same), it would run fine but not drive the horns .. turned out to be just an inadequate bench power supply and it worked fine when back on the car. According to the Fiamm website the compressor current draw is 17 amps so your 9 amps looks a bit low and it's possibly not running at full speed.

[ericbushby]

Thanks Roger. I will look into that.
Eric

[ericbushby]

Roger, You have got it.
Although I was bench testing, the 12 volt supply was from the starter solenoid terminal in the car to the bench, as my bench power supply is limited to 5 Amps.
I wasn`t expecting 17 Amps, and my leads were too small producing volt drop.
What I didn`t do was measure the voltage at the compressor, which would have shown the problem.
I am not going to check the current as my meter is 10 Amps max, but the pressure is now 7 psi and the horns sound correctly.
So, the horns have had a service and I will now look for the original problem on the car wiring.
Thanks all, what would we do without you.
Eric in Burnley
1967 S3SE DHC

[Matt Elan]

I replaced my knackered air horns with an eBay set at a tad over 16 quid Search for ‘dual air horns’ and just replace the compressor with the new one. My compressor had rotted out and just stopped working the tubing and horns were fine.

[ericbushby]

Thanks Matt, but I said that my horns were in good condition and working correctly. The compressor looks as new inside, without any signs of wear on the brushes.
I am still amazed at how much current they draw. That is what caught me out.
Eric in Burnley
1967 S3SE DHC

[Bigbaldybloke]

Just for info, my air horns didn’t work and it was as mentioned, crud had got into the horn part and corroded through the diaphragm in the horn that makes the noise. I found that I could split the horn body and get brass shim sheet of the right thickness to replace them and they have been fine ever since.