Warning horn not getting ground

[Jeanric]

I have a 1994 OMC Cobra Ford 5.0 Engine. The warning horn does not work, whether when I turn the ignition on, or when I simulate a failure by grounding the oil pressure switch.
It gets +12V through the purple wire, but not -12V through the tan wire. Where should I look next?

 

[jimmbo]

Try grounding the Wire that Goes to the Pressure Switch, with the Ignition on.
Of course the Horn, might for be an Overheat

 

[Scott Danforth]

make sure you are grounding the correct wire. If I recall, there are 2 connectors between the temp switch and the main harness. make sure you have continuity.

 

[Jeanric]

Try grounding the Wire that Goes to the Pressure Switch, with the Ignition on.
Of course the Horn, might for be an Overheat

That's exactly what I did in the first place. But ground does not arrive at the horn.

 

[Jeanric]

make sure you are grounding the correct wire. If I recall, there are 2 connectors between the temp switch and the main harness. make sure you have continuity.

Can you share more about these connectors? It's impossible to follow the wires through the harnesses.

 

[Scott Danforth]

Start at the sensor, and worth towards the big cannon plug. Use the schematic as the guide

 

[jimmbo]

That's exactly what I did in the first place. But ground does not arrive at the horn.

Reading your Initial Post, you said you grounded the Sensor, not the Wire going to it

 

[Jeanric]

Reading your Initial Post, you said you grounded the Sensor, not the Wire going to it

Well i grounded them both, the sensor still attached to the wire. But even just grounding the wire does not activate the ground at the helm. Any advice?

 

[Jeanric]

Start at the sensor, and worth towards the big cannon plug. Use the schematic as the guide

Everything is inside harnesses that I'm reluctant to open up. I can't find a schematic detailed enough to help. Do you have one?

 

[Scott Danforth]

I do not have one, however I googled it and found one on a different iboats thread

OMC Cobra 305/350 Coil wiring

Good day! I have a question about the ignition coil wiring on my OMC Cobra 305. On the positive terminal, one lead goes to the starter solenoid and the other to the resistor line then to the alternator. on the ground side, one lead goes to the distributor and the other to the tach. My...

forums.iboats.com

 

[jimmbo]

Some Basic Troubleshooting for open/corroded Circuits. Have you verified that the Horn works, when supplied power directly from the Battery, using 2 wires?

 

[Jeanric]

I do not have one, however I googled it and found one on a different iboats thread

OMC Cobra 305/350 Coil wiring

Good day! I have a question about the ignition coil wiring on my OMC Cobra 305. On the positive terminal, one lead goes to the starter solenoid and the other to the resistor line then to the alternator. on the ground side, one lead goes to the distributor and the other to the tach. My...

forums.iboats.com

Thank you, but it doesn't look relevant.

 

[Jeanric]

Some Basic Troubleshooting for open/corroded Circuits. Have you verified that the Horn works, when supplied power directly from the Battery, using 2 wires?

yes of course. The issue is that the path from the sensor wire to the dashboard is all hidden in harnesses. I can't figure out how to access a midpoint. There's probably one at the ECU though. If I manage to find it, I might be able to tell if the lack of continuity is before, after, or in the ECU itself. I'm not sure how much that will help.

 

[jimmbo]

Might have to run a New Wire, taping it to the Harness

 

[Scott Danforth]

Thank you, but it doesn't look relevant.

What part of the schematic isn't relevant?

 

[Jeanric]

Might have to run a New Wire, taping it to the Harness

Sure, but first I need to know where it goes. The signals coming from the oil pressure switch, and from the engine temperature switch, are supposed to go through the ECU which in turn activates the horn. So far I'm not able to figure out which ECU pins are involved.

 

[Jeanric]

What part of the schematic isn't relevant?

It doesn't tell me anything I'd want to know. For instance, which pins of the ECU receive the oil pressure and water temperature signals, and which one activates the ground on the horn.

 

[muc]

Sure, but first I need to know where it goes. The signals coming from the oil pressure switch, and from the engine temperature switch, are supposed to go through the ECU which in turn activates the horn. So far I'm not able to figure out which ECU pins are involved.

Hi @Jeanric
Is this something you're still working on? If so I might be able to help.
The Ford ECA (what you are calling ECU) doesn't work the way you think.
What symptom caused you to look at this S.L.O.W. system?

 

[Horigan]

Along those lines, I don’t get a warning horn when the oil or water switches are activated, but the SLOW system does engage.

 

[Jeanric]

Hi @Jeanric
Is this something you're still working on? If so I might be able to help.
The Ford ECA (what you are calling ECU) doesn't work the way you think.
What symptom caused you to look at this S.L.O.W. system?

Hi, the email notifications don't work for me, so I'm only seeing your message now. Thank you for your offer.

I started looking at this because I saw somewhere that the horn is supposed to come on for 1 or 2 seconds when you turn the ignition on, which it doesn't. grounding the wire connecting the pressure or temperature switches doesn't produce a sound either.

Further investigation showed that there is simply no wire in the harness that would connect the horn (in the dashboard) to the engine bay! I'm thinking of putting one in, but I'd need to know where to connect it on the engine side.