Fuse for alarm horn?

The_Laird · #1 21st January 2025, 01:41 PM

As I’ve said before, I gave up on replacing my alarm horn some time ago and just took it out. But, with on-going battery drain problems, I wonder if there’s a dedicated fuse for the alarm horn and, if there is, could that be causing the drain? The only other thing that could, perhaps, be a culprit, is the solar sun roof, which was retrofitted. Again, is there a fuse/relay that I could remove to test this?

steamship · #2 22nd January 2025, 10:06 AM

I can only find one fuse that relates to the alarm system, and that is fuse #1 in the bottom row (brown) in the drivers footwell. However, it states that it's only for the 'indicator lights for the anti-theft alarm system'. It's also used for the central locking pumps and interior lights, so not really an option to remove.

Like yourself, I replaced mine several times, and then gave up. It's been a while, but I think there may still be a failed one in there. My car sits out 24/7 and I generally use it only once a week, and that's only a 5-mile round trip, but touch wood, haven't had any battery drain issues.

As for the solar sunroof, there is both a fuse (40A) and relay (203) for it, as shown in the attached image (I never fitted mine). Both are located in the relay box hidden under the panel in the passenger footwell. I've borrowed these two images from IT's excellent retrofit thread, to show you their locations. This is the thread in question:

https://forum.a8parts.co.uk/showthread.php?t=5370

but more importantly, the pictures:

The_Laird · #3 22nd January 2025, 11:29 AM

Thanks again, Sean. I’ll try this after I’ve answered the TTS TPMS questions. Another question: should I be able to ‘see’ a parasitic drain using a multi meter connected between the positive terminal and the cable? If so, am I looking for a voltage drop or something else?

steamship · #4 22nd January 2025, 05:02 PM

The following video is the best one I've seen of dozens, that explains about testing for parasitic drains:

https://www.youtube.com/watch?v=KQxknvD59GA

He puts the meter leads across the negative terminal and cable, and sets it to amps. Anything higher than about 1 amp seems to be a drain of some sort. It's then a case of pulling fuses and checking the meter for a change. I'm assuming that you'd have to do this with relays as well if you can't find any drain, but he doesn't mention this.

The problem is you have to fool the car into thinking it's locked up. I did come across one video that suggested how to do this, and since our cars have fuses in the boot, the engine compartment and inside, you'd need to open both front doors (to access all the fuses), and then lock the mechanism on the door to fool it into thinking it's closed. You then need to do the same with the boot mechanism, and for the bonnet, there is a switch at the front of the plenum chamber, near the brake fluid reservoir. As a reminder that you've 'locked' them, put some tape over the mechanisms, so that you don't just try closing them without 'unlocking' them.

The_Laird · #5 19th February 2025, 11:14 AM

Thanks Sean. I finally braved the cold to test this and got a reading of about 1.2 amps across the negative terminal. Bit I didn’t try to fool the car into thinking it was locked - why do you need to do that? Also, when I put a meter across the terminals with the engine ticking over, what voltage reading should I expect from a fully-functioning alternator?

steamship · #6 19th February 2025, 05:02 PM

The reason for getting the car to think it's locked, is so that it shuts down the various modules. Trouble is, I'm not sure how long this takes on a D2, or even how many modules go through this process, but I'd probably try and leave it that way for about 15 minutes. If working outside like I do, you know how difficult this is to achieve, especially when you have to leave all the doors and boot lid open.

As for the voltage across the battery terminals with the engine idling, somewhere close to 14.4V.

The_Laird · #7 21st February 2025, 06:17 AM

Ah, second part of the above might indicate an alternator issue; mine’s at about 13.5 volts. And, if I’m only at 1.2 amps without the modules shut down, that doesn’t look like a big drain. What do you think?

ainarssems · #8 21st February 2025, 07:20 AM

13.5V at battery is probably OK, it is higher than battery can provide on it's own so alternator is pushing some juice in battery. The voltage will depend on what alternator is supplying, voltage drop in the wiring, charge level of battery and any additional loads like fans or lights. It's hard to say what is acceptable drain while car is still awake. Once everything goes to sleep I would expect drain to be in the 0.05-0.2A range.

The_Laird · #9 21st March 2025, 06:05 AM

Well, with the weather being better, I tried the ‘fooling the car that it’s locked’ approach and then left it for about 30 minutes. The test across the negative terminal and cable then read about 1.4 amps on initial connection, but dropped quickly (no more than a few seconds) to 0.4 amps where it stabilised. I put in a new battery and will test it again later today to see if it lost any charge overnight.

ainarssems · #10 21st March 2025, 10:02 AM

0.4A is too high. On a 90 Ah battery that would be 225 hours or 9.4 days to completely discharge battery, considering that it's not recommended to discharge lead-acid batteries below 50%, you would need to charge it or drive at least every 4-5 days.