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clever electric fan override


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39 replies to this topic

#26 86subaru

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Posted 15 February 2004 - 12:02 PM

this is why i do not post questions anymore , it is better to ask questions before , doing something , not all of us aleast me knows everthing , i work 12-13 hr days so i do not have all day to work on subaru's ,also skip you are a moderator , next some one will tell me how to drive a semi ,

#27 junkyardgabe

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Posted 15 February 2004 - 12:14 PM

to make this even easier just buy this fan relay it's easy to install fairly cheap and you can run daul fan with different temp setting this is what i use on my v-8 s-10 blazer and works great
fan relay

#28 thealleyboy

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Posted 15 February 2004 - 01:13 PM

pretty deep stuff...

Good thing auto electronics are so easy - else a lot of us would be in deep Sh*t.

True, "over-engineeering" has many positive and useful benefits, but unless I'm mistaken, we are talking about a "go"-"no go" type circuit here. Many of the components involved are already "overengineered" to begin with. Measuring the arcane aspects of a circuit is not as important as whether a fan turns on or not. If you need to convince yourself of the stability of a circuit, buy yourself a 12v power supply and do your testing on a bench first.

Bottom line - there is often more than one way to design a circuit; one that works, and that may work quite well.

Like many people that have spoken up here, I dabble in electronics (mainly guitar-related projects). I know enough to get by, but don't pretend to be an expert. I'll leave the heavy engineering work to the scholars, and focus on the application end of things. That means reading schematics and modifying existing circuits.

I do respect and appreciate the knowledge that is shared by those who have paid their dues in their professions. But this does not have to be a competition. I know of some "uneducated" mechanics that can work circles around the best of them (and certainly novices like myself).

We are talking 12v negetive ground electronics fellas...

John

#29 asavage

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Posted 15 February 2004 - 01:16 PM

Originally posted by Rooinater by lowering resistance you automatically increase the amperage. which can increase the heat on the switch. the more resistance in the wires the less amps that will travel through the switch. so matching the wires is a real good idea if not using a relay.

If you increase the distance of the original circuit, you will want to go to a larger gauge wire, because even a small increase of resistance at 13.5v is going to make a relatively large difference in voltage drop. This isn't household wiring, where you can get away with a 5v drop from the panel to the refer.

as for making an indicator that's easy. find the 12v bulb you want to use. or colored indicator from radio shack. on the gound side of the switch splice in the indicator. that way when the circuit is completed you'll create a parrallel circuit and the light bulb will light up. when the switch is off no power will travel to the indicator


You may want to think real hard about that one. For example, in your parallel circuit, which path to ground has the least resistance, the strap from the switch to ground, or the 10 ohm filiament from the switch to ground? IOW, just when do you think the filament will light up?

Getting back to the real reason that you want to keep the wiring large is one that Skip just mentioned: Counter ElectroMotive Force (CEMF). Without getting technical, what it means for a DC motor with brushes is that when the motor is forced to run below design RPM, its effective resistance goes down.

Put another way: if you have a bad bearing in the fan motor, and it drags from the extra load, the fan draws more current than normal. Or, at least, it can; this is not a universal constant, and some dual-speed fan designs do utilize a series dropping resistor to obtain the lower fan speed, but I digress. The extra current it not all converted to work (moving more air), it's converted to heat, which doesn't do the fan motor much good either.

If you add length to the OEM wiring, the total circuit resistance goes up. Let's say that you add 8', and that the extra 2-5 ohms makes a difference to the fan (it might or might not, I'm not going to go find out right now), and the fan runs slower but begins to draw more current, but still within the fuse's rating. Now you have a lot longer wire operating in what the FHI engineers figure was close to max rating for the circuit, and you see that the wire gauge is now a limiting factor in the circuit. You can get poor fan performance and decreased fan motor life.

In this particular circuit, it may be no big deal. Still, I personally wouldn't run a wire extension on this type of circuit that was smaller than 12ga. And as usual, make damned sure to use wire ties and a real grommet when it goes through the firewall. But you're right: if the wire shorted, it would merely smoke a little and run the fan continuously. Might not start a fire, but you can start a fire using less than 15A.

My personal preference for this kind of circuit is to add an accessory relay with contact rating of 30A or higher. Why so high? Inductive devices (ie a motor or A/C clutch) have, as Skip said, "back EMF", and when you open the circuit you get a high-voltage spike in the circuit which arcs the contacts and (re)moves metal. Hence you need larger-current-rating contacts in an inductive circuit switch . . . unless . . .

. . . you add a diode of suitable capacity across the contacts, wired "in reverse" to the circuit. Because the inductive voltage spike on the contacts is in the opposite direction from "normal", the diode shunts the spike to the wiring at a much lower voltage. It's like FM, and it makes the contacts last a LOT longer.

I was adding diodes to A/C clutch cycling switches 20 years ago, to keep from having to replace them so often. And on one system I'd heavily modified, I'd added a dash-mount 12V LED to let me know when the A/C clutch was engaged -- for engineering/test purposes I needed to know the statefullness of various control circuits. Upon disengagement, the LED would momentarily flash brighter. This was a custom-engraved-bezel LED, it cost me quite a bit to have made, and I wasn't going to have EMF kill it. Enter another diode, no more problem.

I figgur'd Skip would appreciate that bit.

(Oh, and how to do a "Fan On" indicator for this manual-fan-ground circuit without adding a relay? Well, I still like the idea of adding a relay, but if you really want to cheap out, wire the 12V indicator between B+ (battery hot, fused of course) and the ground side of the fan. Fan off = 12V on both sides of indicator = indicator off. Fan on = indicator gets hot from (fused) battery feed, ground from fan ground circuit. I do this sort of thing all the time in add-in buzzers for a "headlight ON" warning circuit: in that case, wire a series buzzer/diode combo between ign hot and headlight hot. Headlight OFF & Key ON = diode prevents buzzer from grounding through headlights. Key OFF & headlights ON = buzzer grounds through any ign-switched load, low enough current that nothing else runs. It's the same setup the aftermarket buzzer kits use -- except I had to come up with it myself one afternoon 25 years ago.)

#30 bushbasher

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Posted 15 February 2004 - 01:29 PM

Thanks asavage, this all makes sense to me... a good cap to last nights mess of posts :D

#31 asavage

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Posted 15 February 2004 - 01:48 PM

Originally posted by thealleyboy
Good thing auto electronics are so easy - else a lot of us would be in deep Sh*t.

Auto electronics is NOT so easy:
See my thread on ECU repair .

True, "over-engineeering" has many positive and useful benefits, but unless I'm mistaken, we are talking about a "go"-"no go" type circuit here. Many of the components involved are already "overengineered" to begin with. Measuring the arcane aspects of a circuit is not as important as whether a fan turns on or not.

It is never wise to ask somebody to to figure out how to do a potentially dangerous activity without also giving them the knowledge that can save their life.

I know I'd feel really bad if somebody caught their car on fire and I didn't even try to tell them beforehand that there's a better way.

Bottom line - there is often more than one way to design a circuit; one that works, and that may work quite well.

"Works" and "UnSafe" are not mutually exclusive. We're so used to installing aftermarket tunes and lights that sometimes we forget that this stuff can kill you . . . because statistically it really IS pretty safe, even if you do something completely boneheaded. Fusible links will save your rump roast. But if you wire around those links with just the right gauge of wire, you can become overcome in smoke before you have time to pull over to the side of the road.

I once charged a friend (in high school) $80 to rewire his electric fuel pump, stereo, and what was left of his ignition switch, on an old (not so old, then) '70 Firebird. Seems the former owner had run a line to the rear-mounted electric fuel pump using speaker wire and ran it inside the car -- through the rear seat springs.

Imagine what happened when my friend got three football players into the back seat, and then turned on the key? Yup, smokeshow. No, it wasn't fused.

Why didn't I cut the friend a better deal (at the time, $80 was a small pile of money)? Because I felt that that repair had to be at least a little painful -- and it *did* take me seven hours to do.

Like many people that have spoken up here, I dabble in electronics (mainly guitar-related projects). I know enough to get by, but don't pretend to be an expert.

If you wire an IC up with the 5V to the wrong pin, the worst that will likely happen is some smoke gets out of the wires, or an expensive circuit is scrapped via cascading failure. But in a car (or airplane) your life is on the line. Extra thought and planning is warranted for circuits that have the potential to risk your (or my) life.

We are talking 12v negetive ground electronics fellas...

Just because 12V automotive electrical systems have been in use for 60 years does not imply that people who work with them actually understand a significant fraction of How They Work. Most people know enough to do Black Box repair, ie, "If it cranks slow, I'll try charging the battery, and if that doesn't work, I'll buy a starter, or clean the terminals, or maybe I'll take the alternator off and take it to (Schucks/Kragen/Chief/Autozone/NAPA) and have it tested."

IOW, they don't have a clue as to how it works, why it works, or the engineering that makes it work. It's all "replace A, then replace B" Black Box repair.

Yes, empirically you can learn a lot. And you can go far (in industry, in hobbies, etc.) using this empirical knowledge, gained the hard way by trying this and that and seeing what happens.

My point is that there is more to the Why and Wherefore of this stuff than can be gleaned easily from parts swapping and the adding of swithes to a dashboard. I'd bet that fewer than 5% of folks reading this thread had heard of CEMF or know that a starter motor running at 150 RPM can draw five times the current of the same starter running at 400 RPM, or why that would have an effect on marginal battery or cables, but a I'd bet a larger percentage could tell me that if the starter is running slow, it might be time to replace it, and maybe the battery would last longer if the starter was replaced sooner rather than later.

Get my meaning? Knowledge is good, and knowing that you don't know something is a good place to start knowing more. Sometimes knowing more can save you (life, limb, money, aggravation), but other times you just have to take it on faith because in modern times, nobody can be a Renaissance Man (sp?) and know a lot about everything -- there just aren't enough hours in a life. Hence, I tap experts for their knowledge, and if an expert -- who has qualified herself to me, that is, proved that she knows whereof she speaks -- can shorten my path to a solution, I grab it.

#32 thealleyboy

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Posted 15 February 2004 - 02:58 PM

Al:

If you had read my post carefully, I think you would realize that we are basically in agreement on most of these issues.

Again, if it works on the bench, it works safely, and it does what is intended, it will work in the car. End of story.

No one is implying doing anything unsafe, stupid etc. In fact, the people that are most likely to get themselves in trouble are those that read into theory to deeply and interpret written text too literally. They forget about something called common sense.

I believe knowledge is a good thing, and would never discourage anyone from delving deeply into a topic of interest. And having done this myself, I can say with confidence that automotive electrical work is something most hobbyists can grasp without too much difficulty.

No disrespect to anyone with expertise in this area. If anything, it's a credit to the engineeers that do such a great job of designing safe stable circuits that can tolerate a high level of abuse.

John

#33 raverjames21

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Posted 15 February 2004 - 03:00 PM

Originally posted by Snowman
Are you talking about I=E/R?



What is "E"? Are you talking about V=IR? If I had an ESM I could probably help. I have my EE306 book sitting right next to me.

#34 torxxx

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Posted 15 February 2004 - 03:07 PM

Miles - props for finding something new.

If someone makes something work, and it works for them, dont talk trash about it.

I've been following Miles's Ej22 project and he seems to know what hes talking about.

Yes you can burn out a toggle switch. I've done it. The only reason why it burned out is because it was only a 15 amp switch.. I changed it to a 30 amp and I'm running 200 watts of halogen bulbs through that one switch and I use those lights every night when I drive and have had no problems.

I'm no electrical genuis, but a little common sense is all you need

Hook it up, if it works and doesnt catch on fire Use it

If it burns up, get bigger wires, better fuses and try that

#35 Hondasucks

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Posted 15 February 2004 - 04:01 PM

Well, I wired my fan in my EA81 wagon this way, and you know what happened? The wiring melted and stunk up my car after the fan had been on for abotu 20 minutes so I had to rewire the whole shindig....

#36 myossfeece

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Posted 15 February 2004 - 06:27 PM

yeah I had 3 100watt floodlights hooked up to a 20 amp toggle switch. I would not recommend useing them for long periods of time but for intermittent use It has been functional for over a year.

I too have melted switches as I'm sure a few people have done.

The fan circuit wired to my rx is working fine, and it's not like I am gonna turn the fan on and forget it about for an hour. Come common sense, I have a working clutch fan, the auxillary is just that auxillary, it is only used intermittenly like idleing in a traffic jam in the summer, or flogging my ride out in the mud, cornfields, beanfields, and woods.

Come on people think, I am no electronics/electricity guneuis but I can wire sh!t up good enough to get by.

Worst case scenario the switch melts and pops my fuse then the fan will rely on the stock thermo sensor to kick it on as though there was never a switch there. I dont see my car burning down, or exploding, or killing any body, suff like that happens in hollywood,
I was failry on the board but have shyed away because when you try to post or let poeple know what your up to, they either don't take it seriously, or it starts a "who has the biggest shlong war" It's not a competition, we are just trying enlighten people with our own experience, and let them know what we are up to that's all.!!!!!!!!

#37 Snowman

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Posted 15 February 2004 - 07:39 PM

E= voltage (electromotive force)
I= amperage
R= resistance

I'm referring to Ohm's law. There are different arrangements, I=E/R is just the one I use the most in class, so that's what came to mind first.

#38 raverjames21

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Posted 15 February 2004 - 08:53 PM

Originally posted by Snowman
E= voltage (electromotive force)
I= amperage
R= resistance

I'm referring to Ohm's law. There are different arrangements, I=E/R is just the one I use the most in class, so that's what came to mind first.



Where in the hell have you found E=voltage. V = voltage in every book I have. You classes must not be engineering classes because out of 3 books I have, not one has E as voltage. That is why I was confused. E is usually used as energy or in strain/deformation equations. V=IR is a good equation but needs to be modified if something is in parallel.

#39 Snowman

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Posted 15 February 2004 - 09:05 PM

Yeah, I always thought we should just use V to represent voltage. Both of those equations are the same, regardless of what letters are used to represent the values or what permutation of the equation is used.

i.e.: V=IR, R=V/I, I=V/R


That equation need not be changed for parallel vs. series circuits though. Only the formulae used to add up resistances must change.

Series circuits: R1+R2+R3+.... = RT

Parallel circuits: R1 x R2
---------------- = RT
R1 + R2

#40 rallyruss

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Posted 15 February 2004 - 10:21 PM

:banana: I bought the wrong bananas
http://www.rathergood.com/bananas/




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