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Old 02-21-2015, 10:48 PM   #1
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Basics: Is it the Elec. Motor, Wiring, Fuse, Relay or PCM?

The implicit bargain I make with you guys and gals is that if I get excited that I have learned something new and I post what I've learned (thereby exposing my ignorance to ridicule) if and when you gurus get time, please fix my mistakes and add to the knowledge in the thread so I and others who are interested can learn more. Here goes.

Warning: I don't know what I am doing. I just did this yesterday and I posted it so others could help me learn what I am doing. Don't do what I did below without independently confirming it is the right thing to do in your situation (or even in my situation because I don't know what I am doing).

Let's say you have an Electric Motor of some sort (e.g., a fuel pump or an electric auxilary radiator fan, etc.) and you know it isn't working (e.g., your engine won't start and you can't hear the fuel pump hum when you turn the key to "on" or you can't see the elec. aux. radiator fan turn when you know it should be turning).

Is it the Electric Motor, or the Wiring, or the Fuse, or the Relay, or the PCM that's broken?

First we need to understand what these components do and what order they are placed in to complete a circuit and send electrical power to energize the Elec. Motor in question. Let's take the elect. aux. radiator fan which I diagnosed in my wife's Dodge Dakota yesterday.

I was getting a P1491 trouble code and a check engine light on her Dakota. From the internet I learned that P1491 was an "open aux. radiator fan circuit." Which could mean that anywhere in the "circuit" there was something broken. The engine was also overheating and I could see that the electric auxilary radiator fan was not turning to provide additional cooling when needed.

Reading the forums for my wife's Dakota I found that the electric auxilary radiator fan is turned on by the PCM (the vehicle's "computer") when either: 1. the engine coolent temperature exceeds 195F or 2. the airconditioning is on. The vehicle's PCM contains a software program which monitors various sensors to know what's happening in the physical world (engine rpm, ABS, engine coolant temperature, vehicle speed, etc.). For example, when the coolant temperature sensor tells the PCM that the engine is getting too hot or that the airconditioning is on which will definitely make the engine too hot, the PCM somehow turns on the elec. aux. radiator fan to provide additional engine cooling (not all vehicles have an elec. aux radiator fan, my van does not). But the PCM's low voltage/low amp electrical current is way too weak to power the elec. aux radiator fan which requires a full 12 volts and at least 10 amps to spin those big fan blades fast enough to pull cool air through the radiator to cool the water in the radiator which circulates to the engine to prevent it from overheating. Although the PCM doesn't have enough voltage or amperage to power up the aux. elec. radiator fan motor, it is strong enough to flip a switch (like the light switches in our homes).

A relay is just an electromagnetic switch. I.e., the relay contains a coil of wire inside. So, when the PCM sends its low voltage/low amperage electrical current through the coil inside the relay, that coil becomes magnetic, which attracts a metal swing which pivots on a post which is hooked to a wire which goes to the electric motor (in our case the elec. aux. radiator fan) to touch another post which is hooked to a wire which comes from the Fuse which comes from the vehicle's 12v Battery.



The PCM's low voltage/low amp electrical current goes through the coil of wire causing it to magnetize which attracts the metal swing on post #87 to touch post #30 which sends the big 12volt 10 amp current through post #87 to the elec. aux. radiator fan which turns the fan a spinning. When the PCM stops sending the low voltage/low amp current to the relay, the coil loses its magnetism and the metal swing swings back "open" and breaks the circuit and the 12v 10 amp current from post #30 can no longer cross to post #87 and the fan loses electricity and stops.

The completed "circuit" goes like this:

12v car Battery -> Wiring -> Fuse -> Wiring -> Relay -> Wiring -> Elec. Motor (e.g., fan, fuel pump, etc.) -> Ground (either on 12v Battery or to vehicle chassis)

Any interruption in that circuit and your Elec. Motor won't run.

Here's the troubleshooting approach that I took yesterday to find out whether it was the elec. aux. radiator fan, or the Wiring, or the Fuse, or the Relay, or the PCM that was broken in my wife's Dakota. Look in your manual (I own the complete shop manual set purchased on ebay for each of the vehicles we own)* or the internet and:

1. Locate the Fuse which protects the Elect. Motor you are diagnosing and make sure that the Fuse is not blown. I just looked at the Fuse but it is better to test it with a battery powered test light probe or a multimeter. If the Fuse is ok, go on.

2. Locate the Relay which switches the Elec. Motor you are diagnosing. The Dakota's looks like this:



Take a 16 gauge wire, put one end in the relay terminal receptical for post #30 (or whatever receptical in the terminal which is coming from the Fuse from the 12v Battery) and the other end in the terminal for post #87 (or whatever receptical in the terminal which goes to the Elec. Motor you are diagnosing). If your Elect. Motor runs (e.g., your fans spins or your fuel pump pumps) then you know that the Wiring from the 12v Battery to the Fuse to the Relay Terminal post #30 and the Wiring from the Relay Terminal post #87 to the Elect. Motor and the Elect. Motor itself are not broken. When I jumped Relay Terminal post #30 to Relay Terminal post #87 the elec. aux. radiator fan started spinning. (So, apparently, if the engine was overheating out in the boonies, we could just jump the relay terminals to power the fan and drive home.)

Comment from Dakota-Durango.com: The receptacle corresponding to Pin 30 on the relay is always hot and connects the relay to the fuse & battery. The receptacle corresponding to pin 87 on the relay goes to the electrical motor. While jumping pin 30 & pin 87 is a valid test, I wouldnít recommend doing this unless you are absolutely sure that you know which receptacles correspond to which pins in the relay. If you get it wrong you could potentially send a ton of current down to the PCM and blow a fuse if youíre lucky or fry the PCM if youíre unlucky. Or you could send full current to ground and produce a nice big spark/shock. So be careful.

3. Buy a new Relay (they are under $20) and/or take a known good relay from somewhere else in the power distribution block and stick it into the Relay Terminal which you are diagnosing and see if your Elec. Motor works when it should (e.g., the elec. aux. radiator fan motor spins when the engine coolant exceeds 195F or the airconditioning is on, or the fuel pumps hums when you've turned the key to "on"). If the Elec. Motor now works, it was your Relay which was bad. If the Elec. Motor doesn't work, it is your low voltage/low amp Wiring from the Relay Terminal to the PCM or the PCM itself which is bad. Wiring doesn't often fail.

Comment from Dakota-Durango.com: That is incorrect. Wiring actually fails more often then the components they attach to in my experience. Think about it. Wires are fairly thin and it doesnít take much to break or damage them. As the old saying goes, mechanics go to the part while electricians go to the wires. A lot of times the wires fail by inadvertently rubbing against something due to the normal vibrations a car produces. Simply puncturing the plastic coating around a wire creates a vector for moisture to get in and corrode the wire from the inside out. Corrosion on a wire causes excessive resistance which will cause all sorts of problems. Not to mention bad grounds

You hadnít rules out the wiring between the coil side of the relay and PCM or ground. A series of voltage drop tests would answer that question fairly quickly.


In my case it was the aftermarket PCM which failed so we are sending it back for replacement under warranty.

[My guess is that the isolator, separator thingys which you guys sometimes put between your house batteries and your starting batteries are actually super heavy duty relays which function like this (but independent of the PCM) and my guess is that you could shock yourself or kill yourself if you were to test them like I tested the relatively low amperage relay I tested above.]


* The secret to having old vehicles is to have a number of them, because they usually don't go down all at the same time. Except when they do...
Attached Thumbnails
Dakota Fan Relay.jpg   Relay 30-87 500 pixels.jpg  
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Old 02-22-2015, 05:24 AM   #2
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Re: Basics: Is it the Elec. Motor, Wiring, Fuse, Relay or P

Quote:
Originally Posted by E350


* The secret to having old vehicles is to have a number of them, becuase they usually don't go down all at the same time. Except when they do...
Absolutely the 100% correct truth!
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Old 02-22-2015, 06:12 AM   #3
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Re: Basics: Is it the Elec. Motor, Wiring, Fuse, Relay or P

E350, that was a nice piece of trouble shooting. If you have a Spec sheet of the relay they will usually tell you what the resistance of the coil should be, and you can test by measuring that across the coil when it is out of the circuit, the normally open tab to common should read in Meg Ohms, most likely at your highest resistance setting it would be 1, the normally closed contact to common should read almost 0 ohms.

You are correct in that many of the separators have, a high current relay of some sort.

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Old 02-22-2015, 10:55 AM   #4
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Re: Basics: Is it the Elec. Motor, Wiring, Fuse, Relay or P

Thanks Scalff77! I am really proud of myself* Question: Is the number nomenclature assigned to relays always the same across all manufacturers and all relays. In other words, will a 5-post (i.e., 5-"plug") relay always have post #30 as the hot/positive feed and post #87 will be the "normally open" and post #87A will always be the "normally closed"?

* So no doubt here comes the fall...
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Old 02-23-2015, 05:22 AM   #5
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Re: Basics: Is it the Elec. Motor, Wiring, Fuse, Relay or P

Quote:
Originally Posted by E350
Thanks Scalff77! I am really proud of myself* Question: Is the number nomenclature assigned to relays always the same across all manufacturers and all relays. In other words, will a 5-post (i.e., 5-"plug") relay always have post #30 as the hot/positive feed and post #87 will be the "normally open" and post #87A will always be the "normally closed"?

* So no doubt here comes the fall...
Nearly every automotive-type relay does use that same schematic, they refer to it as a "form". This bit of standardization can be seen here: http://www.accessconnect.com/car_relays.htm. Its safe to say most relays for 12 VDC are pretty much the same.

By way of the schematic representation we also see their intended function which is highly valuable info to have when designing a circuit instead of simply replacing a failed part.

Hope this helps!
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Old 02-23-2015, 10:19 AM   #6
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Re: Basics: Is it the Elec. Motor, Wiring, Fuse, Relay or P

JWA: You bet it helps. That means assuming that if the post numbers are labled on the Relay for a particular Elec. Motor in question, we should be able to jump the terminal to power the motor in any car for diagnostic purposes or for getting us down the road in a pinch.

For instance, since my wife's Dakota is a long bed and I need to make a Home Depot run, I am considering jumping the Fan Relay Terminal, taking off to Home Depot where I will pull the jumper, do my 2"x8"x20' shopping, and then replace the jumper to get the fan running again before heading home.
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Old 02-23-2015, 09:59 PM   #7
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Re: Basics: Is it the Elec. Motor, Wiring, Fuse, Relay or P

Quote:
Originally Posted by E350

You won't shock yourself with a battery separator......

[My guess is that the isolator, separator thingys which you guys sometimes put between your house batteries and your starting batteries are actually super heavy duty relays which function like this (but independent of the PCM) and my guess is that you could shock yourself or kill yourself if you were to test them like I tested the relatively low amperage relay I tested above.]
One of the inherent beauties of 12V systems is that it's next to impossible to kill yourself with this voltage. The outer surface of your body (your skin mostly) just has too much resistance to pass enough current to do any harm. That's why you can grab each of the battery terminals with each hand and feel nothing.....even when you are sweating.....and that's why your starting battery and house batteries can get soaking wet in the rain and not short out...simply not enough voltage.

OTOH, when designing devices for surgery or other procedures where electrical devices are contacting places inside the body (or just about anything that is patient connected), there are very stringent specifications regarding current leakage since it does not take very much current across the chest wall to stop your heart. Current leakage rating is one reason why medical grade electrical components cost more than garden variety stuff.

http://slpower.com/reference/An113%20Le ... urrent.pdf
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Old 02-24-2015, 04:36 AM   #8
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Re: Basics: Is it the Elec. Motor, Wiring, Fuse, Relay or P

Quote:
Originally Posted by E350
JWA: You bet it helps. That means assuming that if the post numbers are labled on the Relay for a particular Elec. Motor in question, we should be able to jump the terminal to power the motor in any car for diagnostic purposes or for getting us down the road in a pinch.
You're on the right path E350. While many aren't too interested in knowing every teeny tiny aspect of their vehicles operation a basic understanding of relays, switches and similar controlling devices goes a long way solving a small problem we might encounter roadside.

I recall my shift interlock solenoid failing one day---had stopped along the highway to have a cell phone call discussion, engine still running. When time to go again for some reason that solenoid wasn't working to design. Having a bit of knowledge as well as an always at-the-ready EVTM with accompanying fuse chart the first and natural instinct was a blown fuse. Quick test and replacement had me back in traffic in 5 minutes or less.

The typical driver might have had to resort to having the van towed to a shop for repair. That cost would have been maybe $300, not to mention downtime and inconvenience. Familiarizing oneself with their equipment saves not only dollars but time too---both in short supply many days.

For the above reason is but one why I constantly suggest a factory EVTM and spending time learning what it tells us. Along with these forums with plenty of DIY mechanin' stories we're able to mentally catalog small issues that could or have stranded too many others.

And not to divert this topic into health care however....................

Generally any voltage or current flow or whatever we're calling it at the moment that can/does flow through one hand to the other is potentially fatal---BoyWonder touches upon this very well.

A good rule to follow is never give electricity the chance to flow from hand-to-hand. While 12 VDC might not be lethal its still good practice to avoid its flow by adapting or modifying our maintenance and/or repair procedures accordingly.

Please don't become too unconcerned with safety issues when working with any live electricity. Yes fatalities from working with automotive voltages are almost unheard of that alone doesn't mean we should lapse into thinking safety isn't a concern. Working with shore power for auxiliary power would be one example because it might be present when working with those systems.

Not trying to be an alarmist but safety should prevail regardless.


Okay soap box put away, rant over----return to your regular programming now.......
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Old 02-24-2015, 07:04 AM   #9
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Re: Basics: Is it the Elec. Motor, Wiring, Fuse, Relay or P

Quote:
Originally Posted by JWA
Quote:
Originally Posted by E350

A good rule to follow is never give electricity the chance to flow from hand-to-hand. While 12 VDC might not be lethal its still good practice to avoid its flow by adapting or modifying our maintenance and/or repair procedures accordingly.
One of my other hobbies/obsessions is designing and building vacuum tube amplifiers. These regularly use up to 500VDC, at least that's the maximum I'm willing to play with. Other folks build them up to 1000VDC, but that is way to much risk for me.

The strict rules for troubleshooting these is one hand in your pocket at all times or stuffed down your backside under your belt. This keeps the juice from flowing across your chest wall in the event that you screw up, which at these voltages can easily be fatal. I use clip leads on my multimeters so that I can test completely hands-off.
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Old 02-24-2015, 07:42 AM   #10
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Re: Basics: Is it the Elec. Motor, Wiring, Fuse, Relay or P

Quote:
Originally Posted by boywonder
Quote:
Originally Posted by JWA
Quote:
Originally Posted by E350

A good rule to follow is never give electricity the chance to flow from hand-to-hand. While 12 VDC might not be lethal its still good practice to avoid its flow by adapting or modifying our maintenance and/or repair procedures accordingly.
One of my other hobbies/obsessions is designing and building vacuum tube amplifiers. These regularly use up to 500VDC, at least that's the maximum I'm willing to play with. Other folks build them up to 1000VDC, but that is way to much risk for me...
Now you have my attention. I was a fairly hard core audiophile for years (pre-wife, pre-child), and I still have a Conrad-Johnson tube pre-amp, and a professionally rebuilt Dynaco ST70 tube amp that I hope to start using again, maybe after I retire and can set up a proper man-cave.


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