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Discussion Starter #1 (Edited)
Yes, it can be done! I've just modified my Amp, and the annoying beep has gone. Here's how I've done it.
I'm not planning on making a kit to do this; it needs a tiny bit of metal-case nibbling, and a very careful solder joint to be made. All this is easily doable by a competent diyer who is happy to do a bit of soldering of components onto some veroboard.

Problem is that the Horn supply starts at the 12V battery in the boot, goes into the fusebox on LHS of boot space, and in there is fuse F11 rated 15A. This fuse is live on one side, whether or not the car is powered up. The other side of the fuse goes off god-knows-where to the front, where clever stuff makes it beep at start of charge, cable-nicking attemps, and maybe burglary detection.

I've fitted a relay in series with the 15A fuse, and this relay is controlled by a voltage signal nicked from inside the DAB radio box sitting under the rear boot floor. This signal is 2.5V DC when the car is on, and 0V when the car is off. So the upshot now is that the my horn will not work under any circumstances when the car is powered off.

1) take the F11 fuse out. cut the 15A link between the 2 prongs. Using dremel tool with a dentists tooth-gouger or similar cutter, expose the 2 ends of F11 on bother sides so you can solder end-on a couple of substantial 15A or 17A bits of wire. One of these for me was an inline fuse-holder which takes the larger sized 15A blade fuse, and I added 1/4" spade connectors to that to make up a 1.2 metre long length of wire. This has to reach from back-left of boot space (fusebox) to front-right corner, where the DAB box is located.
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2) lift out boot floor, 4x M10 nuts and a longish socket to reach them. tip seats forwards & lift up the carpet-bit at front of boot floor, revealing DAB box. This unclips, and you can easily remove the big connector by rotating a lever, easy peasy when you know how! I left the green aerial connector in, didn't want to break it trying to remove it.
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The DAB box has single crosshead screw holding it closed. Remove, and unclip the sides using small screwdriver. Carefully nibble away at the lid you removed, to make a nice large recess in line with where you're going to solder the Mosfet Gate wire onto one of the 2.5V pins. Debur the notch you made, and fit a small grommet to prevent chafing of the Mosfet Gate wire. This wire takes minuscule current, so can be as thin as you like. I recycled an ancient green wire with 6mm round terminal on which I screwed down to the Earth stud I selected.
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I don't know whether all these 4 2.5V pins are commoned up or not, but not bothered as I only need one to provide the signal I need.

Finally, the circuit to do the relay control.
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I used an old relay I had that was 15A rated. As it's a 9V one, I've put a 120 Ohm resistor is series to limit the current. Probably best to use a standard car relay as per Halfords, or something like a CP1A-12V from uk.farnell.com. Again, I used a suitable Mosfet I had around. Farnell no longer stock this one, but there's a shed-load of N-type, 30V ones, and you really only need about 1/10 amp for the CP1A relay, so a 1A mosfet's fine. Make sure you get one with a low gate voltage, 2V or less is fine.
As the current through it is tiny, and a decent mosfet has tiny resistance, no heatsink is needed. The 120 ohm resistor in the gate line is there to reduce the instantaneous high-current surge that mosfets take when being used in high frequency circuits. This is very low frequency, and we don't want to upset the DAB 2.5V line in any way, so it's a good idea to put this in. And that's about it. I made up a small plastic box, with a 10mm hole in the lid, fitted a grommet, and the 4 wires went through fine. I used 17A car flex for the high-current horn wires, and added lots of solder along the verostrip traces connecting some solder 1/4" terminals to the relay.

Make sure you put the F11-adapter-connector in the correct way round! Get it back-to-front, and the relay & mosfet will never see 12V, so will never switch on, and the horn will never work, even when the car is on!

If anyone wants more pics or info, am happy to oblige. But the above should be enough to help anyone wanting to copy this workaround.
 

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Discussion Starter #4
Looking at pics of piggy-back fuseholders, they only seem to bring one wire out! But yes, you could use that as one of the wires out, and stick a single blade in with soldered-on wire to make the other wire. If there's such a thing as a 2-wire piggy-back fuse extender, that would be ideal to cut up!
 

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Flippin heck Andy....you really don't like the beeps do you! :)
Stupid really of GM not giving us the option of disabling beeps with a check box. Control of the Air Con would have been nice too.
Are there really no 12v fuses powered up when the ignition is on to control the relay? I guess you tried before finding the DAB radio voltages.
 

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Andy, getting back to your original investigations on the Mainline limiting control by digital potentiometer, is there no in J1779 a defined low limit error state. Is the basic problem that the Ampera doesn't just stop charging at this low state, the equivalent of less than 6A but just switched off completely. Apologies if you have already explained all this.
 

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Discussion Starter #7 (Edited)
Freddym, If you gradually reduce the charging amps below 6, the Amp seems to bottom-out at somewhere near 5.5A, and then it throws an error "Unable to charge" if you try to go any lower, e.g. attempting 5A charge. So you really have to drop from 6A to 0A. To do this cleanly without the Amp throwing errors, first yo should reduce the current to to 6A, give teh car 5 secs min to respond, and then the Mainpine needs to stop issuing the PWM signal on CP, the one that's saying "6A available if you want". So the CP needs to be pulled to 12V with no PWM pulsing present. That tells the car to pause charging. The resistor in the car will drop the CP line to 9V (car connected but not yet ready to charge), or 6V (car connected & now ready to charge).

Problem is, I can't see a way to get the Mainpine to stay at the "Car connected but not ready to charge" i.e. don't do any PWM stuff, as the Amp is always ready to charge after it's been plugged in (one resistor on CP line to gnd means it drops to 9V), so it switches in the 2nd resistor asking for current, this drops the CP to 6V, the Mainpine sees 6V and starts PWM! Hence my circuit described elsewhere where I'm using a DPDT-NO relay to fool the car into thinking the Mainpine's connected but not issuing any PWM, so car must wait to find out what current allowed, and to fool the Mainpine into thinking the car's connected, but not yet ready to charge.
 

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Discussion Starter #8 (Edited)
Russ, you're d*mn right I don't like the beeps! Especially not when I'm going to be pausing the charge each time a cloud goes by!
Curiously, it now feels strange not getting a beep when I plug in! But I'll get used to it! :)

Are there really no 12v fuses powered up when the ignition is on to control the relay? I guess you tried before finding the DAB radio voltages.
None that I could find! I think that fusebox in the boot is really just a breakout-box, to split the 12V from battery nearby into lots of smaller permanently-on 12Vs that can go down thinner wire to wherever.
 

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Discussion Starter #9
A few more pics for general interest.
Fuse replacement item.
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Closeup of the soldered bit on F11. Notice the original link removed.
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Fitted into F11 slot. Note the red wire goes closer to middle of fusebox.
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Details of PCB with relay, Mosfet & resistors on it
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Underside with rather messy soldering. Got to pass those 15A somehow!
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PCB in box, grounded
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DAB box with slot in lid for grommet, and Mosfet Gate wire soldered using 150W Weller Gun. Carefully!
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Relay box packed so won't rattle. DAB box also padded to stop any rattle.
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Top view of Boot space, ready to replace floor etc.
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Discussion Starter #12
@HandyAndy you may be in breach of some rules, I seem to remember that the horn as to operate regardless of car on or off, my MOT bloke checked it with the car off last time he did my ICE.
Hmm, wasn't aware of that! I've just googled "car horn mot rules" and got a shedload of hits, including
and nothing I've seen says it hs to be operative at all times. Just that it operates, sounds ok and is decently audible!

I'd be delighted if you can find something authoritative! And it would be a simple matter for me to install a bypass switch + another switch, probably using one of those piggyback things, so I can have a plain 'ole 15A fuse normally, but switch this out so I get my beep-suppressor whernever I'm plugging in to charge from solar panels, which is when I need this.
 

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Discussion Starter #13
Just found the authoritative doc. https://www.mot-testing.service.gov.uk/documents/manuals/class3457/Section-7-Other-equipment.html#section_7.7
and the entire, short, horn section reads as follows, nothing about whether car is on/off: ===>

An audible warning must be loud enough to be heard by other road users.

For vehicles first used on or after 1 August 1973, the sound emitted must be continuous or uniform. It cannot be harsh or grating.

The following cannot be used as an audible warning:
  • gongs
  • bells
  • sirens
  • anything that has more than one tone
However, on vehicles first used before 1906 the audible warning can be a gong, bell or siren.

Defect Category
  1. Audible warning:
  1. not working properly
Minor
  1. inoperative
Major
  1. Audible warning control insecure
Minor
  1. Audible warning not in accordance with requirements
Major
 

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Most MOT's are done with the ignition on anyway. I watch mine being done and they seem to leave the ignition on all the time because there isn't an engine running.
 

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... and I suppose you want the matching large, muscled strongman to bash it for you, like the J Arthur Rank films opened with? ;)
Interesting - back in the early 1970s I had the privilege of meeting Carl Dane, a 6ft 5in former circus strongman, who was the first gong-banger for the Rank organisation. He was born in Denmark in 1891, so was in his eighties when I was introduced to him. He told me that the gong used in the films didn't make a sound because it was made of papier-mâché!

I don't think a papier-mâché gong would work for me on my Ampera - I would prefer a compressed-air whistle as fitted to the early electric trains :)
 

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Interesting article. Not sure I am ready to do soldered modifications as I would be worried about insurance and guarantee. It would be a useful mod though. On Saturday we arrived and were unloading the car as quietly as possible when her indoors opened and closed a door while I was unloading the rear. We immediately got the keys left in car horn tone. The time was 4:10 am ! I don't mind the horn tones but wish they were off between midnight and 8am.
 
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