Member Since: 14 Feb 2016
Location: Lancashire
Posts: 834
300tdi alarm / horn issues
Any pointers appreciated
1998 110csw with factory alarm immobiliser. All works fine, alarms and disarms according to the dash light and indicators flashing but I have a permanent live to the horn. The purple wire with black trace is live all the time at the horn end and also at the switch end. I have disconnected the horn switch and it’s still live on the loom connector on the purple wire both black trace. I’m thinking that the only thing is the alarm unit itself on that wire that could be causing the issue.
Anyone else had anything similar ?
Cheers
Brad
29th May 2022 2:03pm
Penfold_6290
Member Since: 22 Sep 2021
Location: Dorset
Posts: 320
Hey Brad, is the 12v you are seeing at the horn a constant 12v or a pulse? My understanding is the alarm relay is controlled by the alarm ECU, when triggered the alarm ECU pulses a feed to the horn via the relay then the horn switch. It is possible the relay has failed closed, worth checking this, the relay is behind the instrument panel next to the pink headlight dim-dip relay.
Check the input side of fuse 10, if you have 12v here permanently on the orange/red wire your next thing to look at is the relay. On the Td5 (should be the same logic on late model 300Tdi) the orange/white wire from the relay goes to the alarm ECU, when the alarm is triggered this OW wire becomes an earth path allowing current to flow on the orange/red wire to the fuse box, in turn to the horn switch. In normal use the OW wire should show 12v at the alarm relay, when the alarm is triggered the ECU replaces this 12v with a route to earth, the relay is energised and current flows to the horns - a pulse, not a constant earth. This is why the horn is an on-off not a constant beep.
Screenshots from Td5 electrical library explaining how the horns are triggered (the information in the 300Tdi electrical library is poor so ignore the references to fuse numbers, they'll be different) and screenshot from 300Tdi wiring diagram.
Click image to enlarge
Click image to enlarge
30th May 2022 8:58am
Bps
Member Since: 14 Feb 2016
Location: Lancashire
Posts: 834
Cheers for that. I’ll give the relay a look at. I didn’t think to check that one as it’s got an air horn on it with a separate relay under the bonnet so I’d only gone to that one. Yes it’s on constant, no pulsing
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30th May 2022 8:58am
