Relays and Fuses in Automotive Electronics - Page 1
Forum section: electronics / beginners

author: Dumpsterholic
author_rank: Regular Contributor
timestamp: July 02, 2022, 06:46:17 pm
content: A couple of years ago, I got a good deal on a used pickup truck. Like me, it has some cosmetic issues (and a good many miles on it), but is basically a working unit. I've replaced the starter and radiator, done a full brake job, and driven the bejeezus out of it, but there remain a few minor defects which I've never got around to fixing. One issue which has me perplexed is that the horn doesn't sound, even after I replaced the fuse and the relay which are devoted to that one circuit.My query for you people is this: Why does there need to be BOTH a relay AND a fuse, and can I safely bypass them? My idea is to pull out the radio, which I never listen to anyway, and install a big momentary push button switch in the empty cavity. Since the radio does work, I know that it has power, so all I'd have to do is tie into that circuit, then run a pair of wires from a switch directly to the horn. I should add that the existing horn switch is inside the steering column, which at one time also housed an air bag, and it seems to me that messing around in there could turn into a prolonged hassle. Hence my idea to bypass the existing horn circuit altogether.Any advice, hints, warnings or opinions would be most appreciated. Since many drivers where I live have earbuds on while driving and are generally oblivious to outside stimuli, my screaming and gesticulation rarely seem to reach them when they're sitting at a green light staring at their dumbass smartphones....

author: floobydust
author_rank: Super Contributor
timestamp: July 02, 2022, 06:55:23 pm
content: Horns are very high current draw, more than than the steering column switch/slip-ring can handle so a relay is used.One leg of the horn relay connects to battery(+) and a fuse is necessary, especially if a collision occurs you don't want an electrical fire.Toyota has the wimpiest horns known to mankind, so I went to a wrecking yard and shopped around.Absolute biggest is in a 80's Buick Park Avenue, four horns, two in each fender, all tuned to a massive chord. I settled on a pair from a 1964 Pontiac lol. You can bring a 12V gel-cell to the wrecking yard lol.I had tried horns using an air compressor and they were OK.

author: Stray Electron
author_rank: Super Contributor
timestamp: July 02, 2022, 07:25:04 pm
content: Geez. How hard can it be to troubleshoot a horn circuit? It's usually just a switch that grounds one side of the horn relay and the other side of the relay runs off of battery voltage via a circuit breaker or a fuse. One side of the NO relay contacts will connect to the horn and the other side will connect back to the battery viaanotherfuse or circuit breaker. That's why you'll have two circuit breakers or fuses, one powers the horn and one powers the relay. Apply power directly to the horn to see if it work, then check the power into and coming out of each of the fuses or circuit breaker. If they're all good then the most likely culprit is the "clock spring" in the steering column that maintains electrical connection while the steering wheel is being rotated. Check the contact on the horn relay to see if it's getting the proper ground when the horn button is pressed, if not then the contact in the horn button is bad or more likely the clock spring is bad.My 2010 Ford has two relays and one feeds the other but otherwise the principle is the same and most vehicles just use a single relay.

author: Stray Electron
author_rank: Super Contributor
timestamp: July 02, 2022, 07:31:00 pm
content: Quote from: floobydust on July 02, 2022, 06:55:23 pmIf you want a set of horns that are LOUD but not as obnoxious as air horns or a train horn then get a set of the old Cadillac horns from around 1980. They had as many of five horns in a set, each with a slightly different pitch. You can check them out on You-tube.Ditto on the high current draw. Even with a single horn you need to use a relay!

author: Benta
author_rank: Super Contributor
timestamp: July 02, 2022, 07:50:08 pm
content: @DumpsterholicYou can be certain that there is not a single part or device in a car that's not needed. We're talking highly competitive mass production here. If even a simple washer can be omitted, it will be.Trying to outsmart automotive engineering is a fool's errand.

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 02, 2022, 09:25:51 pm
content: circuit with both relay and fuses or contactor and circuit breaker are common because the relay or contactor is to switch high power load with low power switch and the fuse or circuit breaker is to protect the circuit from over current or short circuit.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 02, 2022, 09:27:10 pm
content: I concur with the others, buy a $5 12V test light, find a wiring diagram (Google or service manual), and figure it out.Working around the airbag and "clock spring", if necessary, is not complicated. I by no means recommend doing as follows, but I have seen some flat rate mechanics just unplug them without disarming the system and casually toss the bomb in the parts pile.I too enjoy a strong, functional, horn. Currently run an 80s Mercedes S class high tone I found in a junkyard, greatly out performs the stock Toyota buzzer. I initially thought about building a harness and going to the GM four note setup, but that disc horn was good enough for Detroit traffic (very fast and dangerous highways).

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 03, 2022, 12:39:46 am
content: As for troubleshooting the horn, simply measure the voltage at the horn. If the voltage is there and the horn doesn't sound then it's the bad horn. You should be able to determine what is wrong without too much problem.

author: floobydust
author_rank: Super Contributor
timestamp: July 03, 2022, 04:07:14 am
content: Another issue with car horns is the inductive spikes they generate can be pretty nasty. Since you can't really get inside and add a capacitor across the sounder switch, I think a reverse-diode (across horn power) with small capacitor help to lessen EMI.

author: Dumpsterholic
author_rank: Regular Contributor
timestamp: July 03, 2022, 12:37:19 pm
content: Thanks everybody!In the interest of brevity, I failed to mention that the electrical system of my 2000 F-150 was already FUBAR when I got it, presumably by a previous owner (or mechanic?), unless it's true that Satan still rules the lower world.... This makes troubleshooting exponentially more, uh, troublesome, especially as I'm working alone and hence have no one to depress the horn switch as I probe out the circuit. On reflection, it did occur to me that I can just jam a 2X4 between the driver's seat and the pad on the steering wheel to close the switch.Quote from: Stray Electron on July 02, 2022, 07:25:04 pmQuite.Quote from: Benta on July 02, 2022, 07:50:08 pmYep.Quote from: floobydust on July 02, 2022, 06:55:23 pmTHIS drills down into the core issue of WHY the circuit was designed as it was. Based on all of the input I've received, engineering a work-around doesn't seem like the proper course of action; rather, it makes more sense to trace out the circuit as it is. I know from bitter experience that tearing into a steering column can make a bad situation even worse, hence my initial hesitation to do so.Quote from: BrokenYugo on July 02, 2022, 09:27:10 pmBut, but, wouldn't that void the manufacturer's warranty? Oh....

author: Siwastaja
author_rank: Super Contributor
timestamp: July 03, 2022, 05:13:28 pm
content: Relay turns on the horn. If you bypass it, you have endless honk.Fuse is to protect against fire. Short circuits can happen; fuse blows before wires get so hot their plastic insulation melts or smokes.To troubleshoot, ask someone to activate the horn, and using a multimeter, measure for the presence of voltage right at the horn. If you have your battery voltage present there, relay and wiring is working and horn is defective. If no voltage, work your way up the wires to find where the gap is.

author: chinoy
author_rank: Regular Contributor
timestamp: July 03, 2022, 05:26:09 pm
content: Its either the horn or the switch.I would test those two first. Testing the horn just unplug it. Run a wire direct from your bat + and _ to the horn.Old horns rust and need to be re-built / cleaned out every 10 years or so.If your horn works then check the switch with a multimeter. If you cant get to the switch. See which wires going into the stearing colum connect to horn. Peal the wire back with a blade and short them. To test.You have already replaced the fuse and the relay so cant be that. Still chk if the fuse is intact.Also se if the relay clicks when you press the horn.If you came to my workshop with this problem this is how I would diagnose it. Direct power to Horn. Next press horn button and listen for relay click. 30 seconds we would have your culprit.

author: james_s
author_rank: Super Contributor
timestamp: July 03, 2022, 07:09:54 pm
content: Relays and fuses serve two completely different purposes, I'm confused as to why you are surprised there are both. The fuse is there to protect the wiring and prevent it from catching fire in the event of a short circuit. The relay is there to avoid having to pass the large current required by the horn through the clockspring or slip ring in the steering column and the horn switch in the wheel. Why would you want to bypass either one? The problem is most likely in the horn switch itself or the wiring in the steering column. If the fuse is good then it isn't that, and it's unlikely to be the relay but you can test that easily. If you lack the skills to diagnose a basic electrical fault then I would suggest taking the truck to a mechanic or automotive electrician, they will be able to figure it out quickly.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 03, 2022, 08:20:10 pm
content: Quote from: Dumpsterholic on July 03, 2022, 12:37:19 pmNo need to do that, let's start fresh.Find the horn relay, find it's pinout if it isn't obvious (e.g. two big terminals and two small ones, or printed on the case), briefly short out the relay contacts (B+ and HORN) with a chunk of wire, does the horn sound?If the horn doesn't sound you go towards the horn/power side until you find the fault, check the horn's ground, check the B+ feed to the relay socket, inject 12V right at the horn and see if it even works, and to rule out trouble with the wire from relay to horn.If the horn does sound, you go towards the control half of the circuit. Check relay coil continuity and relay function (connect 12V and ground with jumpers to the coil and see if it clicks and shows continuity on the contacts), then work back to the column and switch. I don't know how Ford usually wires stuff, but if I had to guess the relay circuit is probably a 12V feed on one side of the coil and the button just grounds the other side out locally to complete the circuit and close the relay.That's pretty much the entire "traditional automotive horn circuit troubleshooting flowchart". On something this old and neglected you may find multiple faults, but again, it's a super simple circuit easily diagnosed with $10 in specialty tools (a test light and some jumper leads), and worst case, repaired with stuff you can rent from most parts stores.Bad grounds to chassis and corroded connections in general are very common, always be looking, I've fixed stuff before by merely flooding grungy looking fuse and really sockets with WD40 and working the component in an out for a minute.Another trick, if you have multiple identical relays in the box, you can just swap them between working and non working circuits to rule out the relay.New parts are not to be trusted if you have not personally tested them.If you have any working pro mechanic friends/family they'll probably provide connector pinouts, locations, and wiring diagrams if you ask, some local libraries also hold Alldata subscriptions you can use (same as the mechanic), Haynes/Chilton's will do worst case, you can probably easily find a scan for such a common truck.The following is about all you'll need to figure this out quickly, note you can also connect the test light to 12V and use it as a ground tester. It also finds blown fuses as fast as you can probe the two probe points on top of each. I don't like using a DMM to check for power, 10 megaohms isn't enough load, I only really use one for ECM signals and references, ohming out connections, checking drop on cables, etc. You can't beat the test light for go/no go testing.https://www.harborfreight.com/36-inch-low-voltage-test-leads-66712.htmlhttps://www.harborfreight.com/612v-circuit-tester-with-5-ft-lead-63603.html

author: Dumpsterholic
author_rank: Regular Contributor
timestamp: July 03, 2022, 09:23:08 pm
content: Wrap up:After following standard test procedures (extra-special thanks to BrokenYugo!), I was able to determine that the steering-wheel mounted switch mechanism was not delivering power to the horn relay, so naturally there was no voltage present at the horn assembly itself. Further, I removed both horns and applied power directly to their contacts and got no sound; they are dead, which is not surprising given their age and the wet climate they have been used in over the past twenty-some years. After perusing a half-dozen YouTube videos, I am almost certain that the problem lies with what is called the "clock spring". In order to verify this, I would need to disassemble the steering column, which I am loth to do, as I know from past experience that repairs like this can take a long time and will render the vehicle inoperable until the job is completed.A new clock spring at local retail auto-parts stores costs upwards of $150, though I found the same part (supposedly) online for about a third of that price. A bird in the hand....Another YouTuber who had the same problem with (almost) the same vehicle took matters into his own hands and cobbled together a complete circuit comprised of a switch, a relay and an inline fuse. I have a bit of experience tracing out cruddy grounds and corroded connectors on old trucks, and a good deal of experience pulling wires and gripping cables (plus a half-mile or so of heat-shrink tubingin seven colors...), so I think I'm going the "roll your own" route as has long been my preference. Thanks again to everybody who tuned in to my admittedly rather feeble-minded thread. Sometimes a problem can be resolved by simply sharing it with outsiders, all part of what I call the "rock tumbler of life".

author: Benta
author_rank: Super Contributor
timestamp: July 03, 2022, 09:44:40 pm
content: IMO, you've made the right decision. The clock spring is a bitch.Heck, you could even go the way of 1930s/40s cars and place the horn button in the foot well.Cheers.

author: james_s
author_rank: Super Contributor
timestamp: July 04, 2022, 06:00:39 pm
content: You don't need to disassemble the steering column to test he clock spring, you just need to disassemble the wheel to to the horn switch, and then find the wiring at the other end of the column and you can check continuity through the clock spring. I've never worked on one in a vehicle that new but the clock spring that was in my Volvo to connect the airbag was easy to get to, just one bolt to remove the steering wheel and a few screws that held the clock spring on. In my case I took it out to install the steering wheel from the non-airbag model I had previously but replacing it would have been easy too.If it's just a beater I suppose you could just install a pushbutton for the horn in one of the empty switch locations on the dash. Or you could put an air horn on top of the cab with a pull cord like semis have

author: Benta
author_rank: Super Contributor
timestamp: July 04, 2022, 06:33:20 pm
content: Quote from: james_s on July 04, 2022, 06:00:39 pmThat's one area of a car that I wouldn't touch with a ten-foot pole. There's explosives under the horn switch (airbag).

author: james_s
author_rank: Super Contributor
timestamp: July 04, 2022, 11:09:30 pm
content: Quote from: Benta on July 04, 2022, 06:33:20 pmI do it all the time, it's no big deal at all. The service manuals will all say to disconnect the airbag(s) and the cables for them are always clearly marked and color codedorangeyellow. Modern cars have so many airbags you pretty much have to learn to work with them if you're going to work on a car. The airbag itself is quite safe, they have to be, the part you have to really be careful of is the sensors that trigger them.

author: jfiresto
author_rank: Frequent Contributor
timestamp: July 05, 2022, 07:28:57 am
content: After studying the various safety precautions for working on my car's airbag system, I was startled by how the local dealership ignored all of them. They told me that has never been a problem: they just clear the electronic fault codes afterwards.Airbags are designed to be hard to accidentally fire. Still, I might be a little nervous spot welding around them.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 05, 2022, 05:11:22 pm
content: I figure if one is intelligent enough to not crush themselves doing under car work, they can probably handle bare airbags safely enough. Like I said, mechanics throw them around with no more respect than they give other interior bits in the way of whatever they're getting paid to replace. AFAIK they don't go off short of putting power across the pins in the tiny recessed connector probably designed to discourage that. You're way ahead of the curve if you think such a thing should be handled with caution.james_s makes a good point about the sensors. I'd guess you're probably more likely to accidentally detonate an airbag from under the hood than in the cabin. EDIT: Unless it's newer car with side curtains and whatnot, IIRC it'll blow them all if someone unbolts the airbag computer and simulates a rollover.

author: Dumpsterholic
author_rank: Regular Contributor
timestamp: July 21, 2022, 01:11:41 am
content: That's one area of a car that I wouldn't touch with a ten-foot pole. There's explosives under the horn switch (airbag).So, I'm still working on this problem.... I have received some good feedback (and some well-deserved ribbing) which will allow me to complete this task. Since this is not an automotive forum (there are plenty of those), my goal in creating this thread is to generate some valuable clues for anybody else down the road who may encounter similar problems when modding their car's electrical system and has a sincere interest in understanding the "why" in addition to the "how".Apart from the hassle of tearing into the steering column (which may not be necessary after all), and the very real threat of exploding airbags (though there is a safe way to disable that circuit), one more issue I've encountered is the presence of "smart electronics". Virtually every car or truck on the road these days has a computer called an "Engine Control Module" or some such thing; it is there ostensibly to increase the vehicle's performance by monitoring a host of sensors which report back on issues with the fuel mixture, the braking system and who knows what else (the driver's level of intoxication?)I finally broke down and bought a factory service manual, which unfortunately contained no wiring diagrams, so I dropped another $30 on a complete set of those. What I eventually determined is that the horn circuit ties into the anti-theft circuit, which kinda makes sense but adds another level of confusion for me. The upshot is that I intend to bypass the regular horn circuit altogether. So, just to briefly recapitulate the original query, an automotive horn circuit can draw a lot of current, so a relay is necessary in order to prolong the life of the switch, and a fuse is needed in case of an abnormally large current surge as may happen should there be a short circuit.What I will end up with is a very basic circuit that draws power from the battery which is routed through a 20A fuse and a 40A relay. Even I can visualize this.... And that leads me to a further goal which I've had all along but didn't mention in the earlier posts: I'd like to add some fog lights and a searchlight, probably employing high-wattage LED's. Once I get the horn working, adding these will simply be more of the same: More wires, more relays, more fuses. Thanks again to all who chimed in with helpful feedback.