Horn relay getting crispy fried

[Joe V]

I've gone through 4 horn relays on my 84 Carrera Coupe. First time it happened, I noticed that my horn wasn't working, so I swapped relays with another round one, and a couple of days later, it was toast also. The last relay I installed barely lasted a single beep! Eventually I caught on that there was something wrong (I'm not a rocket surgeon or a brain scientist) and decided to hack one apart. Attached are autopsy pics of the carnage. Sucker just vaporized the contact. I checked for excessive resistance in the circuit and it all checked out. Pulled the horns out (stock black trumpets) and they check out fine also. I pulled the steering wheel and polished the horn ring. No difference. Could one of the horns be faulty? Winding in the horn shorting to ground?
Any gurus out there with any thoughts?




TIA

[dshepp806]

When pulling the horns and checking them, what exactly did you determine to be ""OK"?

As well, how did you check the "circuits" out?


With that many relay changeouts, something sure seems load (horn) related..

Best,

Doyle

[Joe V]

Doyle,

That's exactly where I think I'm missing something. I tested the resistance between the terminals at the horns (while disconnected). Got about 4 ohms each (+/-). Checked for continuity to ground on the power side of the circuit with the relay removed. Open circuit. Checked for resistance between the black power wire to horns (behind r/s headlight) and terminal 87 at the relay socket at the fuse box and we got a beep (good continuity). Checked resistance to ground on the ground side of the wires and we got a beep also. Plugged in a good relay (without the horns installed) and clamped down the horn pad to activate and got a solid 12.4 volts coming out of the black power wire. I also checked relay temperature (with an infrared thermometer) and it shows at room temp same as other relays. So no excessive heat without a load on the circuit.
I'm going to buy a handful of used relays and re-install the horns and see if the removal of the inop alarm the PO had installed was causing this relay meltdown. But I just wanted to ask on this wisdom filled forum if it was possible for the horns to take a dump and manifest this condition.

Regards,

[ToddM]

The only way this is happening is excessive current in the relay, period.

Whatis the voltage going into the relay when inactive, and what is the voltage comming out of the relay when active? Multiple this drop * current and you can calculate exactly how much power is going through the relay; obviously more than what it is rated for.

Based on what you have so far, I am guessing that OUTPUT side of the relay is low impedance to ground or some other circuit when active. Are you sure something else isnt wired up to this output?

Measuring the relay inactive is pointless because the problem is at the output in active mode.

[911pcars]

Todd is correct. Excessive current is flowing through the relay contacts and overheating them.

For the previous posts, remember that all things being equal, the load in the circuit, the horn in this case, determines the current flow through the circuit, and beyond that:

More circuit resistance = reduced current flow and less voltage to operate the load
Less circuit resistance = increased current flow

The suggested method to measure current flow is with an ammeter connected in series in the circuit. But then what? I don't think the factory provides current specs. for a horn.

Test each horn separately. Connect an ammeter to the power side of the relay circuit (anywhere) and compare the current flow through each horn. I think you'll find that one of the horns is faulty (drawing excessive current). A suitably-sized inline fuse might be a good idea when doing this to avoid damaging a good relay.

Sherwood

[rnln]

umm.. strange. Isn't the fusable copper wire suppose to burn first?

[dshepp806]

Originally Posted by 911pcars

Test each horn separately. Connect an ammeter to the power side of the relay circuit (anywhere) and compare the current flow through each horn. I think you'll find that one of the horns is faulty (drawing excessive current). A suitably-sized inline fuse might be a good idea when doing this to avoid damaging a good relay.

Sherwood

Isolate the horns, for sure. The inline fuse idea will be good.

If there's nothing else on the circuit , you're checks indicate a bad horn (load) You should see a big difference in current draw (between the 2)

Best,

Doyle

[911pcars]

Originally Posted by rnln

umm.. strange. Isn't the fusable copper wire suppose to burn first?

As far as I know, Porsche doesn't use fusible links. Most electrical circuits are fuse-protected. A few aren't. The ones that aren't are susceptible to damage in this way. NG.

If you rely on the copper wire conductors to protect a circuit by burning, plan to extinguish a car fire.

Sherwood

[rnln]

I might use a wrong term but what I meant when I say fusable wires is the bundle of copper wires on the left of the pic above, which in front of the spring.

[dshepp806]

Don't think that's a "fusable link" at all, my friend.

Best,

Doyle

[billybek]

Originally Posted by dshepp806

Don't think that's a "fusable link" at all, my friend.

Best,

Doyle

+1.
That wire is carrying power for the horn to the terminal that is moved when the relay is energized. It is braided to provide some flexibility for movement and vibration.

[billybek]

Do you use your horn a lot?
Does it look like the terminal has burnt while in the normally closed (no power to coil) position?
It looks like the top fixed terminal has seen some heat as well.

[Joe V]

Billybek,

LOL!
When I was a service advisor for a japanese car maker, I used to ask my customers a similar question -do you use your brakes often?- when their brake warning tab started screeching due to worn brake pads. Invariably they would answer no. And I'd let a few seconds tick by to allow them to think about their hasty response.

Thanks to everyone for your advice, and for sharing your knowledge and experience.
At this point I have a couple of relays on order and want to ask the following:

1) Where should the inline fuse be installed? Closer to the load or the relay?
2) What amperage should the fuse be? I don't know what amp rating the relay is, though I know the square mini 4 terminal relays are rated at 30A or 40A. I'm estimating that 20A should be a good start.

Thanks again!

[billybek]

I wonder if it is burning off when the relay is de-energized, and the power is being shunted to ground through (perhaps) the normally closed contact.
Have a good look at the normally closed and the normally open contact. You will probably see better evidence than I can from the pictures.
Perhaps a ground wire or rub through has occurred behind the fuse/ relay panel.
Sorry, I don't have a wiring diagram for your car to offer more help.

[billybek]

I have a diagram from Bentley for the SC and was surprised to see that the hot wire (30 red) supplying power to the relay is unfused. If I were installing a fuse, this is where I would put it.
Power from 30 is jumpered to 86 (red) through the coil on the relay to 85 (brn/wht) to the horn contact. Power to the horns comes from the norm open contact on wire 87 (blk). Horn button completes the circuit to ground, enabling the relay to pull.
Make sure that the wire to this relay is supplying only the horns and is not used as a common (+12 VDC) terminal. Otherwise (I hope this isn't too obvious) make the inlet side of the fuse the common (+12 VDC) and feed the relay with the outlet side of the fuse.
Hope I helped more than I hurt!

[911pcars]

"1) Where should the inline fuse be installed? Closer to the load or the relay?
2) What amperage should the fuse be? I don't know what amp rating the relay is, though I know the square mini 4 terminal relays are rated at 30A or 40A. I'm estimating that 20A should be a good start."

Theoretically, a fuse can be installed anywhere in a series circuit - current is the same throughout. However, a fuse close to the source minimizes the section of the circuit between source and fuse in case of a short circuit there. In addition, it's more convenient to group the fuses in a common location for ease of service. Use one of the unused fuse positions in the fuse box or piggyback onto a seldom-used circuit fuse. While the control section of the relay is low current, this too should be protected.

The fuse capacity should be less than the parts you want to protect, in this case the current capacity of the relay. 20A should provide plenty of overhead to operate typical OEM horns. Not sure about the aoooga or the air horn types.

Sherwood

[poorshe911]

The information you gotten from these guy's is correct .. put the fuse between the 12 volt source and the input to the relay contact. Your problem is definitely an excessive load after the relay. I would test the horns while disconnected form the car harness using an external 12V source.

[rnln]

I am not sure what that bundle of little wires should be called. It has low melting point. If you short your circuit, that wire will burn easily. I think the purpose is as a "fuse".

Originally Posted by dshepp806

Don't think that's a "fusable link" at all, my friend.

Best,

Doyle

[bkreigsr]

if perhaps you recently removed / replaced your steering wheel, it's posible that one of the horn wires got pinched.

[billybek]

It's melting point is apparently higher than the tab and contacts of the relay as it is intact and the tab and it's contacts have been vaporized.
It is possible that some relays are fused in this manner, but it doesn't look like this is one of them.