Electrical troubleshooting!

[Michael999]

My fuse for horn is repeatedly blowing each time i drive the car.
I cannot find what happens that causes it, since i do not drive around beeping non stop.

starting and stopping the car does not do it, it seems to only happen after a period of driving, but so far i have been unable to find the issue.

Having a look in my Bentley manual page EWD3, i see that on the wiring diagram that fuse (15A fuse number 9) is used for "Horn Relay, Water Pump Relay, Compressor Control Relay"

In my car the sticker in the fusebox says "Horn, Aux water pump".

In the Bentley manual in the fuses section (610-4) number 9 15A fuse is for "Horn, Integrated Climate Control Regulation, Cellular telephone, Auxiliary fan"

This is really disturbing, im reading completely different things in all 3 places, the only thing in common is the horn, which works when the fuse is changed and car restarted a few times, and does blow it.

Any help in this matter is greatly appreciated, i really dont want to have to go to an auto electrician, i have a feeling it would turn into something very expensive for a horn which i only need to use a few times each year!

[DaveVoorhis]

According to the 1990 wiring diagram -- and I doubt the '89 is appreciably different -- F9 is switched via four relays, and powers the horns, auxiliary water pump, the phone alert (which just fires the right horn), and the A/C compressor clutch. I'd be inclined to suspect the A/C compressor clutch and related wiring first. Try unplugging it at the compressor. If that doesn't change the problem, then unplug the aux water pump under the heater valves on the left-hand side of the firewall. If that still doesn't do it, you've probably got a cut in the wiring harness somewhere. Then you'll either need to start tracing the wiring from F9 out or from the A/C compressor, horns, and aux pump back to F9.

[winfred]

id start with a diagram of the circuit of the fuse that's blowing (theres a few online copys of the etm floating around if you search) and that will show what is on that circuit that you can troubleshoot by unplugging and seeing if the problem goes away, if theres nothing obviously wrong with it

[DaveVoorhis]

I forgot to mention: Since the stuff powered by F9 is switched via relays, so a quick way to isolate the short circuit is to pull the relays. Pull the horn relay, the water pump relay, the phone alert relay, and the compressor control relay in turn and see where the problem disappears. Then you've found the bad circuit, which could be the powered device, the wiring, or even (rarely) the relay itself.