Classic cars forum & vehicle restoration.

FAB Cruiser · Joined: 03 Jul 2008 Posts: 77

Fitted my alarm today. All the features work so I was well chuffed.

Decided to treat myself to a drive around the country lanes this evening.

Started the car. Horrible whirring sound from under the bonnet. It was the siren not actually alarming, but making an irritating noise in rhythm with the turnover of the engine.

Pulled the fuse to the alarm and the noise stopped. Went for my drive!

The system involves a control unit inside the car with a separate siren behind the grill. The siren is wired +VE from the control unit and earthed via the self tapper holding it to the body.

I haven't worked out it there's real volts being fed to the siren or if it's just RF interference. Do I need some kind of filter or attenuator, screened cable, or should I move the siren to the boot.

The joys or mixing 21st and mid 20th century technology!

Suggestions welcome.

Cheers
David

Greg · Joined: 03 Dec 2007 Posts: 445 Location: Dreamland Margate

Hi David,
I don't know how complex your Alarm system is but is there a wire that has to go to an 'Ignition' live?... if there is, check that it stays 12volts whilst cranking also the same with your permenant supply, make sure it doesn't drop when cranking as well. As you say check the earth connection is satisfactory.

FAB Cruiser · Joined: 03 Jul 2008 Posts: 77

Hi Greg,

Yes there is an ignition connection which I've wired to 'ACC' on the ignition switch. It's live when turned left (e.g. to listen to the radio) and live when driving. It may drop off when cranking the starter because my CD player stops momentarily in this position and I've used the same connection, but it returns to live once the engine is running.

Thanks
David

Greg · Joined: 03 Dec 2007 Posts: 445 Location: Dreamland Margate

Hi David,
Yes the wire you explain, ACC is ,obviously,the Accessories wire, you need to connect this wire,(of the Alarm), to an Ignition wire. ie the same as your coil wire, this is usually the 2nd position of the key and stays live (12volts) whilst cranking.
1st position usually Acc.
2nd position full ignition
3rd position (momentary) is cranking or Strarter.
This may not be the cure for the siren noise but this wire needs to be connected to the right supply otherwise it can cause other problems, eg false alarming, Immobilisation problems etc.
Hope this helps,
All the best,
Greg

Dirty Habit · Joined: 26 Mar 2008 Posts: 398 Location: West Midlands, UK

My first thought reading this..........is the car positive or negative earth? If it is a modern alarm, are they all negative?
_________________
1964 International Harvester

FAB Cruiser · Joined: 03 Jul 2008 Posts: 77

OK guys, here's the latest in my alarm saga!!

Checked the earth to the siren. It's good. I've earthed it right next to the siren so only the positive wire runs through the firewall to the alarm control unit which is under the dash.

I've moved the switched positive from the Accessory position on the ignition switch to the ignition position (so it still has +12V when cranking).

I've moved the permanent +12V from the output of the voltage regulator to the live terminal of the battery itself.

And you guessed it...the siren is still wailing away when the engine's running!!

Other thoughts I've had...

Should I fit suppressed plug caps (there's an original set of 6 on ebay at the moment). However, the CD radio doesn't suffer interference.

Should I move the siren to the boot?

Should I replace any of the wires with screened cable?

I'll try the alarm company's help desk during the week but any further suggestions appreciated.

Cheers
David

Greg · Joined: 03 Dec 2007 Posts: 445 Location: Dreamland Margate

Sounds odd, a bit difficult to diagnose without seeing it as well but has the system got immobilisation circuits, if it has, are you using them?
Which make of Alarm is it?
Is it a battery back-up siren?
Have you soldered the connections?

FAB Cruiser · Joined: 03 Jul 2008 Posts: 77

The make is Scorpion SA30

It has an immobiliser circuit but I haven't wired it in yet. Want all the basics working first.

No battery back-up

No I haven't soldered. I've used crimp connectors, but I've done them carefully.

Worst case, I could put a toggle switch in the permanent 12V feed and kill it before I start the car as it remembers all the settings! Just a tedious option.

Thanks
David

FAB Cruiser · Joined: 03 Jul 2008 Posts: 77

Latest on this saga guys....

The supplier thought I had a faulty alarm 'brain' so he replaced it. Put the new one in today and it's just the same. The siren still wails when I'm driving.

I took the fan belt off to check if alternator interference was the cause. It wasn't

So basically having checked everything and tried everything, all that's left is electronic noise being generated by the ignition system causing the alarm unit to put out about three Volts to the siren.

So what I'd appreciate now is advice on suppressing the coil, plugs, leads, caps etc.

Also, if that doesn't work, I need a circuit to choke anything under about 4 volts getting to the siren so that only genuine alarm conditions at 12V make it wail.

Cheers
David

peter scott · Joined: 18 Dec 2007 Posts: 7124 Location: Edinburgh

Hi David,

If the radio was free from interference when you ran the car with fan belt off then I really think the alarm unit is faulty or badly designed.

To try to narrow down the problem you could try powering the alarm from a separate battery that is not otherwise connected to the car's wiring. If you still have problems then progressively disconnect the connections to the various sensors of the alarm. (Sorry, I'm not really familiar with car alarms so I don't know specificially how it detects intrusion.)

If the alarm is sorted by the separate battery then you really need to place something like the capacitor that I suggested you put on your alternator terminals between the alarm metal case and its power input connection, preferably keeping the leads as short as possible.

If you need to disconnect sensors to get alarm free driving then having established which wiring into the alarm is causing the problem by the progressive disconnection then go back and re-establish the normal 12 volt supply and check again.

I would suggest that for clarity you perform these investigations without the alternator running even though you still had problems without it. The alarm could be upset by both the alternator and ignition.

Peter
_________________
http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

FAB Cruiser · Joined: 03 Jul 2008 Posts: 77

Hi Peter,

All top suggestions. The supplier (biggest in the South east) says he's sold hundreds of this model without any problems. It would be a big coincidence for me to get two faulty ones in a row. I do think the design is suspect (plastic not metal case). I've already eliminated all the sensors. Basically just the permanent 12V, earth and siren cause the problem. I put the unit in a metal tin and earthed it which also made no difference. Oh well, I'll keep trying stuff.

Many thanks
David

peter scott · Joined: 18 Dec 2007 Posts: 7124 Location: Edinburgh

Hi David,

I think the best strategy might be to concentrate on getting the radio interference free before you tackle the alarm. Reason being that the radio gives you an immediate and qualitative feedback of your progress whereas the alarm maybe be spasmodic and gives you no sense of whether you are making improvements.

UJ has already mentioned good earths and I also think this is a very important area to check. Long thin earth wires will not adequately tie your radio or alarm to chassis potential. What you need for both are very short
(2" to 3" max length) connections made with heavy duty or thick braided cable. The outer (braided) conductor of coaxial aerial cable also needs to be very solidly connected to the metal of the bodywork. Rusty metal won't do.

The connection must be to shiny bright metal with grease applied to keep it that way. If your aerial is mounted on a wing check that the wing is also solidly connected to the rest of the car.

The normal approach with ignition interference is to replace all the distributor HT cables with resistive cable. If you don't like resistive cables (they aren't as reliable) then try to find modern spark plugs that include a resistor in the plug. I think your car is meant to use Champion N8B so change to XN8 or some other equivalent that has an "R" in the number.
The resistive plugs should be more effective than the cables.

You might also benefit from a capacitor connected between the body of the coil and SW terminal. (i.e. Not the one that connects to the contact breaker side connection on the distributor)

Peter
_________________
http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

FAB Cruiser · Joined: 03 Jul 2008 Posts: 77

Peter,

Would that be exactly the same type of capacitor you use for the distributor?

I seem to remember putting one on a coil years ago but the ones Halfords sell now only mention the distributor.

Thanks again.
David

peter scott · Joined: 18 Dec 2007 Posts: 7124 Location: Edinburgh

Hi David,

Yes, the capacitor that I suggested for the alternator is likely to be very similar in value but is possibly shaped specifically for easy mouting on that device. I haven't actually had the £3.99 Halford part in my hands but I would
think it just has a conveniently shaped earth tag.

The value is not critical. The most critical thing is to have short connecting leads and having the capacitors in the correct places.

If you can find some of those little cylindrical caps with a couple of metal tags designed to straddle a screw to earth and with a flying lead for the other connection then that would be fine in all the places we have discussed.
These used to be very common. I just haven't spotted a current supplier.
You could just use a few normal distributor condensers such as:
http://cgi.ebay.co.uk/Fiat-124-127-131-132-241-900-Panda-Ignition-Condenser_W0QQitemZ270004537213QQcmdZViewItem?hash=item270004537213&_trkparms=72%3A985|39%3A1|66%3A4|65%3A12|240%3A1318&_trksid=p3286.c0.m14

Peter
_________________
http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon