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1970 Opel GT
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I am just finishing up installing new headlight wiring harness. I am looking to install better /louder horns, & driving/fog lights in the place of the lower can/signal lights and I will be installing relays.

Where is everyone placing their relays for ease of installation/servicing?
 

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Unhappy with what I was getting in the way of horns, I installed a fairly inexpensive but quite effective set of air horns that I purchased from Rimmer Brothers in England. Rimmer is one of the go-to vendors for LBC (Little British Cars) including Triumphs and MGs. Whether you go this route or not, it is best to make a small wiring modification using a standard off-the-shelf horn relay (that comes with the Rimmer kit). The OEM wiring setup for Opel horns has a fairly high level of current across the horn button, that leads to deterioration that will not occur with a relay in the circuit. I needed to fabricate a bracket for the horns and the air pump, which was not too hard, and I mounted the relay up next to the horns behind the grill.

If need be, I can send you my schematic for making the wiring changes.
 

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Opel Key Master
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Michael, the horn button is the Control for the ground, the power comes directly off the fuse box. Yes, you can get a spark at the contacts of the ground, but the system is designed to allow adequate ground signal to the horns. But you don’t really get voltage through to the ground…or you would short out the circuit. They make loud horns that will hook up just like the factory ones, and do not require a relay. I think a horn relay would be overkill, but do what you want. If you plan on constantly using the horn, maybe you need it? There is no specific location you need to put the relays, but I would mount them on the fuse box studs next to the other relays, so they are not seen
 

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Can't see my relay either -- it's tucked up inside the space up front, next to the horns (this makes the wiring modification far simpler. Relay is accessed by removing the grill mesh.

You are suggesting that newer horns available do not have the current draw that the OEM horns had, that routinely destroyed the horn button connection over time. This may be so, but, as I mentioned, I was not satisfied with what I found available, even with some of the "Top Names" in the field.

If you go my route and swap in the air horns (which have a wonderful high-pitched European note to them), you will need a relay, as the compressor does draw a relatively higher current. The OEM horns on the GT are wired so that the hot wires (Black-Yellow) run right to the horns up front; the horns are not grounded directly -- they are grounded via a brown wire that runs to the horn button on the steering wheel. Thus the relatively high current draw.

The swap involves the following:
1. Disconnect the 14-g Black-Yellow wires from the horn and connect one to the #86 terminal of the relay and the other to the #30 terminal.
2. Disconnect the brown wires from the horns. Clip off the one that runs between the two horns and connect the one that runs to the horn button to the #85 terminal of the relay.
3. Run a new 14-g wire between the #87 terminal of the relay and the positive terminal of the horn compressor.
4. Run a new ground wire from the negative terminal of the compressor to any ground.

This circuitry change can be used to implement a relay in a conventional horn circuit, except that the connection from #87 would run to the positive terminal of the horns and the horns would be permanently grounded

The air horns work from tubing connecting the compressor to the horns and the recommendation is the tubing should be as short as possible to maximize efficiency.

There is one small error in the attached diagram. The switch between the #30 and #87 terminals of the relay is labeled "AO -- Always Open". This should be "UO -- Usually Open"
 

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Opeler
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I am just finishing up installing new headlight wiring harness. I am looking to install better /louder horns, & driving/fog lights in the place of the lower can/signal lights and I will be installing relays.

Where is everyone placing their relays for ease of installation/servicing?
Did you install new Opel style headlight wiring harness or aftermarket harness?
 
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