VEHICLE RELAYS - Operation & Diagnosis

alright here's the thing when you work with relays here are the critical points first thing is we days are at least two circuits it's a system not a circuit it's a system relays don't do relay things relays are switches the two components inside are a coil of wire which produces a magnetic field and instead of contacts that are pulled magnetically by that by that magnetic field by that coil when you're diagnosing a problem in a system that has a relay it's a good idea to know whether or not it's the coil circuit or the contact circuit that's failed the easiest way to do that is to take a relay with the cover removed pull out the relay bits and questions tick the relay in it's got the cover off and squeeze the relay and see if the horn blows or see if the lights come on see if the AC clutch moves if it does then it's not the lights it's not the horn it's not the clutch you can rule that out remember you want to rule out things that are working right the problem is going to be small the problem you're looking for is tiny I guarantee you it's going to be itty-bitty teeny-weeny tiny don't be looking for a massive problem you need to be looking for a small problem there are four segments to a relay there's power to the coil and ground of the coil there's power to the load and there's a ground side of the load so only one of those will probably have failed so when you're diagnosing these systems make sure that you remember two components in a relay coil and contacts and the first thing you want to try and do is to isolate from the coil to the contacts to see which one of those two circuits has actually failed this is not that hard it's something you can very easily do and you need to be studying these relays so that you can understand them well enough that they become part of the solution and stop being part of the problem a relay is an electromagnetic switch in other words a small amount of current flows through a magnetic coil that magnetic coil energizes and it pulls the contacts closed there are two circuits the small amount of current can switch a large amount of current like a horn a clutch or more the relay is a system okay it's not a single thing it's two things you need to think of it as two separate independent circuits inside one little black box there are two circuits inside a coil and a set of contacts when the coil energizes the contacts close so when you troubleshoot you need to be able to understand this okay so here's how it looks see there are two circuits and coils and one horns on the other they're joined together only magnetically there's an if-then scenario here if you do something something else should happen okay so here's how it works number one if you close the horn button then to the relay coil should magnetize which then three should make the relay contacts closed which then four should make the horn blow okay now when you diagnose a relay system remember it's not a circuit it's a system the question you have to ask yourself is which circuit is actually failed in the coil circuit or the contact circuit whatever you're actually energizing okay the most common relay you're going to see is a little black square relay that sometimes people call the Boche relay or the automotive relay okay this is a very typical relay it happens all the time you'll see it a lot of different places okay the pins never change these three pins that are marking red are the switch the bottom terminal is 30 top is 87 and middle is 87 a the two terminals on the flanks I'm color in green are actually the coil terminals 85 and 86 and notice on the bottom of relay they're 86 85 now there's another relay that kenworth uses and it's sometimes called the ISO relay it's very similar to the other relay but it has a different numbering scheme maybe one two three four and five instead of a I have eighty six thirty eighty seven eighty seven eight one and two the coil three is the common for is the normally closed in five is it normally open this is what it looks like okay so you're able to use this relay and the other really in exactly the same way the quickest easiest fastest way to do this is to pull the cover off of one of these things and either watch it when you push the horn button on the column or squeeze it like this and when you squeeze it that should complete the contacts close the contacts which would make the horn blow remember there's two circuits the coil in the contacts and you have to know which one is broken because only one of them will probably be broken so in asking the question what's right you want to answer that question as fast as possible and if you know that the horns working then it can't be the horn circuit if it's the relay when you push the button the horn will blow and you replace the relay if you squeeze it in the horn doesn't blow but you push the horn button that goes click then you know it's the horn so you've cut the problem down from four circuit segments to two just by taking this relay with the cover off and pushing the contacts oh here's the relay with the cover removed all right and all you have to do is use these contacts right here and that will complete the circuit but the quickest the easiest way to determine whether it's the coil of the context or the horn of the horn circuit fall circuit is to stick the reel and and this simply hit the contacts Sarris me you can also push the horn button and actually watch the horn really energized so let's review this known good working relay with the cover removed squeeze the contacts if the horn blows it's not the horn push the horn button if the relay clicks and the horn blows it's the relay if you push the horn button and the really clicks but the horn doesn't blow then you know the choral circuit is working and you know that there's a problem in the horn circuit remember two circuits coil contact your first objective should be figured out which one alright here's how this works on a schematic 1387 normally open contacts 30 goes to diffuse 87 goes the horn when you complete the circuit through the contacts 1387 the horn should blow right so you push the contacts on your uncovered relay does the horn blow or not if yes it isn't broken if no it probably is okay now we've got the coil circuit that runs from the fuse to the steering column for the horn switch okay here's a t7 a we're not using it but there it is okay so coral circuits green contact circuit red they're separate okay if I put in the known good working relay then I can either push the horn button and watch it work or close the contacts and hope that the horn blows either way I should be able to know by looking and listening which is actually broken by doing this you isolate the fault to one circuit or the other can you get used to this you'll know after a while how it works you'll figure it out okay all right well if you're curious and you don't want to play relay Roulette by pulling out relays and swapping them around just use a 9-volt battery 85 and 86 and the relay ought to go click okay here's one installed in a relay base 9-volt battery got to watch the diode but you ought to just be able to touch the relay to a nine volt battery and determine whether or not it's clicking you can listen to it and you can watch it you can mount the contacts you can do whatever you want to do but the contact should move but remember the coil may or may not be working so you know both have to work coil and contacts in order for the relay to be good okay so review here common shared number thirty eighty-seven a and eighty-seven normally close normally open 85 and 86 to the coil okay those are the switch terminals 85 and 86 of the coil contacts and the only connection between the two is a magnetic field there is no electrical connection inside the relay okay the contacts do not connect to the coil okay what if there's a diode the diet is there specifically to suppress the arc cause when the magnetic field collapse the coil is just like an ignition coil okay the coil stores energy and when it stores the energy that energy has to go somewhere okay now look at this illustration there's the diode on the Left pointing to 86 okay when you look at this diode don't try to figure out current flow don't try to understand what's going on with current flow notice that this diode is pointing to positive that means it's in reverse bias it will always be that way forever and ever and ever and ever and ever never any other way so when you read a schematic that's what you'll see okay here's how it works when you close the switch the diode is installed in reverse bias what happens is if we use positive flow current flows to the coil and the coil does its thing but the diode blocks the current that way so current cannot flow through the diode see it's flowing through the coil but not the diode the coil builds up a magnetic field and this is stored energy it's energy that has to go away again think ignition coil okay spark plug well when you open the switch what happens is that energy has to go somewhere and what it is going to do is try to go in the same way it's been going and run around the corner and come back over through positive and arc the switch but it doesn't it stays inside the diode and it essentially prevents that energy from arcing the switch all it does is stay home in the diode okay let's look at this in summary relay okay three switch terminals two coil terminals fuse goes to 30 87 goes the horn close the contacts the horn blows the horn could be 100 feet away 85 and 86 or the coil fuse from a fuse to 85 86 goes to the horn switch which is in the column which is hard to get to the horn could be 100 feet away and the fuses and the fused wires are buried in the panel so look if you understand how it really works you can test the entire system from this one square inch there are two voltage readings and there are two ohm readings you should read system voltage at 30 you should read system voltage at 85 the ohm meter should show you whatever the horn resistance is and we'll just say 10 ohms and you ought to read 86 oh L and zero when you push the horn switch you can test the entire system from this relay and you never have to go chasing and looking for anything else on the truck ever again don't forget and all the information that you just heard is available in the 200 page fundamental electrical troubleshooting shock book that I wrote it's written by mechanic for mechanics it's available on most tool trucks if you ask your tool man and you can't find it there then you can go online to our website at bright ideas calm and check out the links there for purchasing it from a couple of our different distributors you

Channel: Daniel Sullivan

Views: 947,201

Keywords: Voltmeter, Relay, leads, ohmmeter, ammeter, ampmeter, troubleshooting, electrical, electricity, diagnostics, powerprobe, multiplex, multiplexing, PLC, ECM, diode, kenworth, peterbilt, mack, freightliner, caterpillar, komatsu, testlight, test, light, TESlite, Enhanced, Leads, sterling, volvo, manitowoc, p&h, terex, cement, mixer, tanker, general, electric, locomotive, thermoking, corrosion, wiring, resistance, voltage, drop, test light, loadpro, LOADpro, ohm, ohms, law, electronic, specialties, sullivan, training, systems

Id: Mru8BbTGDwM

Length: 12min 38sec (758 seconds)

Published: Mon Nov 28 2011