Kind of a bodge , but you could always feed the horn relay with an in-line fuse. I think you probably need to trace the headlight stuff first; maybe the harness around the black fuse block has a rub through or there is something around the beam select relay.

 

Thanks again RGK.

So yours is different than mine I guess, I definitely only have 5 in my box. #1 is six fan and then I believe the other four are for headlamps: dip and main beams. I wonder if yours came after the 1988 1/2 changes. I’ll have to look at my green book to compare since mine has the wiring diagrams for before and after those changes.

 

My diagrams show that brown 12v wire going directly to the "+" junction block on the firewall


Cheers
DD

 

Looks like from the above diagram provided by Doug that you should verify that you have battery voltage at pin 81A of the headlight relay; use a test light rather than a volt meter because a high resistance in that circuit will still show voltage with a multimeter but not light a test light. If ok, then trace the circuit from 31B, which goes to the column switch.
This of course is if the headlamp relay doesn’t click when the column switch is actuated (I didn’t see where you replied about this, may have missed it). If it does click, trace wires from 56A and 56B to the black fuse block.

 

And I mean a real test light, one with an incandescent bulb- those high tech LED with multi colors won’t load the circuit enough to identify a corroded or high resistance circuit.

 

Perhaps it's already been mentioned but the fuse box and all the connections in that area are very subject to corrosion

Cheers
DD

 

Thanks for the replies Doug & RGK, I'll report back with my findings (and hopefully resolution!). Did not know about the test light difference. I think my dad has one that would definitely precede LED's.

 

Question: if we assume that the lack of power to the number one fuse in the engine bay is the root cause of this problem, can I apply 12 V to that fuse directly from the battery? If I do so, and the horn works, I assume that means the issue is on the positive power side. However, if the issue is actually a bad ground somewhere, what would the results be? Would nothing happen?

 

If you do that, put an inline fuse in the jumper. Good way to isolate where the problem lies; usually it is an open if no power with no blown fuses. But you have that #6 fuse in the under the dash fuse block….

 

Update: I jumped the positive battery terminal to the #1 fuse feeding the aux fan circuit (and confirmed 12v was getting through the brown wires to both the aux fan and horn relays). No difference. Horn relay still clicked but nothing came through. Double-checked the positive and ground connections at the horns themselves - still good.

Bad ground, then? If so I'm stumped. There's a loom of 3 or 4 black ground wires in the very near vicinity of the left wing relays and I have it jumped to an engine ground strap (brand new) just to be sure the ground side is good. So maybe the ground for the horn relay is happening elsewhere? I know the horn circuit grounds not only at the horns themselves but at the steering column. But if the latter were bad, why would the relay still click? And I've tried 3 different relays, all click but no horns. So maybe the ground for the aux fan circuit is messing this up? But again I would think that it grounds where the aforementioned loom is - I'll check that tomorrow to be sure, easy enough.

 

I am most confused! Could you please clarify:

1. Have you got a permanent 12v feed to the power side of the relay, ignition on (not the trigger circuit, the power-in side) of the relay?
2. You say the relay is "clicking", this would seem to indicate the relay is switching. The question is "does the relay have a power input to switch TO the horn?" Yes or no?
3. If no, diagnose the power feed.
4. If yes, and the horn is not working, ask a helper to push the horn (ignition on, mind) and place your test light on the 12v wire at the horn.
5. If this is not live, horn pushed, the wire from the relay to the horn is your problem.
6. If this is live, horn pushed, then clean up the connectors, the horn earth wire and earth connection and retry.
7. If this does not work, test the horn off the car with a solid 12v from a battery.

 

The plot thickens.

I get 12V at the pink/black and green/black wires at the horn relay, but nothing at the solid brown as stated before. However, when I directly apply 12V to the brown wire (15A inline fuse between that and the battery), the horns still don't work. When the horn is held down under these conditions, I get about 7 volts at the purple/yellow wire that powers the horns. And I gave the horns a solid extra ground to make sure that side of the circuit is good. The 7 volts is very confusing because I can confirm with different relays that the solid brown wire is getting 12V at the relay, and I believe that is the wire that powers the purple/yellow wire leading to the horns with the button is pressed. With the 7 volts applied, they sort squeak for a moment then stop making noise. Why the partial voltage if the relay is good?

Though there's still the matter of tracing the brown wire to find why it's not getting any juice.

When I power the horns directly from the battery they work.

 

Partial voltage is always high resistance, nearly always corrosion. Riveted connections (such as where the fuse clips attach to the wiring terminals) are especially prone to high resistance (Doug Dwyer mentioned this), but you can have resistance where the terminals are crimped to the wire, the terminals themselves can corrode and have high resistance, and internal harness splices can corrode. Don’t overlook the wires themselves; sometimes water can wick under the insulation, or the insulation may be knicked and has corroded as a result . Look for swollen insulation, and green or black copper which will indicate corrosion.

 

Thanks RGK. I’m still a little puzzled. If I’m applying 12V directly to the brown wire at the relay (which should circumvent any issues on the supply side) but only getting 7V on the purple/yellow wire (again at the relay), and we know the relay is good, I would think the issue is with the ground on the horns themselves, but I am certain I have given them as good of a ground as possible.

Of course, I still need to figure out why the solid brown wire isn’t getting 12V.

 

I might've narrowed things down (or rather confirmed our suspicions about a short). I cut the solid brown wire running to the #1 engine bay fuse (Aux fan) and put a 15A blade fuse in the line in order to totally remove any doubts about the reliability of the glass fuse and associated copper prongs. That didn't change anything, but then I ran a wire (with another 15A inline fuse) to the left terminal. I removed the fuse in the new #1 fuse connection so that both female connectors were exposed. I then touched the left terminal wire to one female connector at a time (one leading to the relays, and one coming from the left terminal). When I touched the wire to the connector going to the relays, nothing happened (without the horn button pressed, fyi). However, when I touched the hot wire to the side of the fuse coming from the battery, I got sparks. I'm obviously not skilled in electronics, but I assume this means the short is located in the solid brown wire somewhere between the #1 engine bay fuse and the left terminal. I see no tears or frays in the loom running from the fuse to the firewall, so I think the short is somewhere under the dash.

It is tempting to just run a wire from the left terminal directly to the #1 fuse (while obviously eliminating the existing "feed" side which has the short), but since I simultaneously have an issue with the headlights and the #6 fuse blowing, I need to fix this the right way.

Now I'm removing the dash to get a better look.

I guess this is my life now.

 

I am an idiot.

Didn't bother taking a picture, but there was an insulated wire splitting off from the loom running into the cabin in the vicinity of the brake booster. The other end was a bare eyelet and was attached to the body. The insulation was black and completely covered the wire. Before taking the gauge cluster out to get a better look, I decided on a whim to cut into that insulation. Behold: two solid brown wires inside the black insulation, and they literally could not have been grounded better. Now with them cut and hanging loose, I tried the test mentioned in my prior post and what do you know, no sparks. That was the wire intended for the left terminal post powering the #1 fuse. It eluded my eyes for so long because of its outside color - I think my mind just saw another well-grounded black wire. Why was this attached as so? Because I did a 6-speed manual swap earlier this year and clearly thought I was dealing with another ground wire, and treated it as such when reinstalling the pedal box and brake booster. Foolish!

I'm now reattaching it to the left terminal post and will report back, but I expect everything to work.

Sorry for the hassle, gentlemen.

 

Can confirm that was the problem. Reconnected those two brown wires to the left terminal post and now the horns and the headlights (low and high beams) work perfectly.

This thread is a little embarrassing, but I hope someone learns from this: a small wire or two out of place can mess up more than one thing.

 

We have all done it! Good for you for tracking it down.