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Blown fuse leading to radiator fan. Please help.
December 11, 2010 9:29 AM   Subscribe

My radiator fan won't kick on when the car gets hot. I replaced the fusible link and let the car idle for awhile. After 15 minutes or so, the fusible link blew... Need specific advice on how to find the short circuit.

I had a problem awhile back where my 1995 Ford Escort wagon went completely dead and wouldn't start. It had a likely short circuit. I took it to the dealer and they replaced the ignition switch and the neutral safety switch. I got the car back and moved to another city in another state (Charlotte, NC). Now I drive in a bit more traffic and I noticed that the car would get too close to overheating in really slow traffic. Yesterday, I had some time to work on it and I flushed the radiator with a product from Prestone. I flushed it three times and got lots of gunk out. I was quite happy and left the car to idle for awhile to mix the coolant and water as suggested. After twenty minutes, I check on the car and the temperature gauge was dangerously close to hot again (pretty much 4/5s of the way to the end of the gauge).

This morning I looked in the engine fuse box and the fusible link for the fan was clearly fried. I got a new one at Autozone and put it in hoping to solve the problem. When the temperature gauge started creeping up I opened the fuse box in time to watch the fuse glow bright red and then "Pop!"

I have a multimeter so I could try to check for a short circuit. I have been reading all morning on how best to do this, but I am intimidated as I have generally only worked on mechanical bits with cars. I really want to learn how to fix this (the dealer charged me $1000 for the work they did, which really, really smarted), but I am also cautious about electricity.

I have the wiring diagram for the car in a Haynes manual, but I don't have any experience reading the diagram, so I am a bit overwhelmed by that, too. It looks like I would need to check the wires to the starter, to the ignition relay, to the fan itself, to the engine electronic control module, etc.

From what I've read, I need to

1) Disconnect the positive terminal for safety.

2) Cover the positive terminal to prevent accidental contact (any good recommendations for what to do this with?)

3) Trace wires from the fan itself to the ECU to find bad wires.
3a) Unwrap a bit of wire and hook part of my voltmeter up to the wire and another part to the chassis to make a ground. Look for 12v readings. Should I use a particular lead on the wire versus the ground?

3b) Do I start with the fan or trace backwards from the fuse box to the ECU? Does the blown fuse indicate a particular direction to look for shorts?

3c) Do I need to check wiring in other components? The wiring diagram shows that the starter, ignition relay, ECU and fan are all on the circuit...

3d) If the fan is the only electrical thing not working and the only blown fusible link, does that narrow down what I am looking for?

Sorry for such a long question! Thanks in advance for any leads!
posted by Slothrop to Travel & Transportation (3 answers total)
 
You should disconnect the negative rather than the positive. Technically it doesn't matter, but that is convention. (I think it is because the negative terminal is the source of electrons.) Ah! Now I remember why, at least partially. If you disconnect the positive first, almost everything in the engine compartment is a potential path back to ground. Which makes it much easier to accidentally short yourself or your tools. So, at least, always negative first.

Car electricity is different from house electricity. It isn't particularly dangerous the way 110v AC is. 12 volts isn't a heck of a lot of potential, so accidentally grabbing a positive while touching a ground probably won't do much of anything to you. Where it is dangerous is if you short un-fused battery power. If, for example, you accidentally short the terminals with a piece of metal like a wrench, the battery WILL happily send hundreds of amps through that. Creating melting metal and blinding arcs. I like to take the battery out, just to be safe. Because I'm a clutz.

Anyhow, to solve your problem.

0- I remember that Ford had problems with its engine wiring bundles around that time. There was a recall for some cars, not sure if it is yours. You might try and research it and see if it still applies. It is possible it still does and you can get the dealer to solve the problem for you.

1- Was the fan spinning while the fusible link was connected and getting hot? (Not entirely relevant, just curious.)

2- The fan circuit is likely through a relay. The switch or the ECU just sends a low powered signal to the relay, which switches the power to the fan on or off. So your trouble is almost for sure beyond that. The circuit path is battery > fusible link > fuse? > relay > wiring harness > fan > ground.

3- Two things are happening. Either, the fan is faulty and drawing a TON of current through the wires, or the positive wire in the above circuit path is shorted against something.

3.5- Set the meter to resistance and test the two pins in the fan. You should get some non-zero number of ohms. If it is zero, your short is in the fan. If it isn't, spin the fan. Make sure it isn't zero anywhere in the rotation of the blades.

4- So, first, just visually inspect the wire that comes out of the fan. Follow it as it goes to (likely) a larger bundle, and then follow that bundle as it goes back toward the battery area of the car. Look for any areas where it is chafing against something, or perhaps, where it runs along the chassis of the car and there is a spot where there is corrosion on the bundle and the metal of the chassis. It would probably look like rust with chalky white, or even greenish, gunk on it. Might be wet, too. If you see that, carefully open up the bundle and you'll probably see a ton of corrosion and one or more open wires.

5- Failing that, get your volt meter and set it to continuity, or one of the resistance settings. Touch one lead to the metal chassis (or negative battery cable), and the other to one of the pins in the fan connector. It should show (almost) zero resistance. If it doesn't try the other one. One of them should be almost zero to ground. If not, you have a bad ground somewhere. If the BOTH do, you definitely have a short to ground.

6- If you see a short to ground, you need to trace down where that short might be. Figure out where the fan relay is. It might be in the owner's manual where the fuse locations are listed, or possibly in some kind of underhood electrical center. Remove the relay. If you can, figure out which two legs are the load (switching) side, and which two are the coil (control).

Anyway. By removing the relay, you cut the circuit into two halves. The circuit between the relay and the fusible link and then the battery, and the circuit between the relay and the fan. Figure out which side of the fusible link goes toward the battery, and connect one of your meter's leads to the other side of the fusible link. (A set of alligator clips might be helpful in all of this.) Then probe the connector for the relay until you get continuity. Now you know that leg of the relay connector goes from the fusible link to the fan. Remove the lead from the fusible link and touch it to ground. You should NOT get continuity. If you do, you have found where the short is: between the relay connector and the fusible link.

7- If you haven't found the short yet, move down the line. Here is where it gets complicated. You need to figure out which connector on the relay goes down to the fan. Look at the relay, and note the leg of the relay that goes into the hole in the connector that you identified above. If the relay has four legs coming out of it, set the meter to a low resistance setting. Test the other three legs until you find the two of them that show some resistance. Might even show 0 resistance. Those two are the coil (control) side of the relay. The fourth leg is the one that feeds the fan. Look back at the connector the relay goes into and identify the hole it plugs into- this is the circuit to test next.

(If the relay has 5 legs, it gets more complicated. You need to test all combinations- two of them will show zero resistance, two other ones will SOME resistance. You need to find the one that does not have any connection at all to any of the others. That's the one that goes to the fan.)

8- Same as above. One end of the meter in the relay connector, the other probes the connector to the fan. You should get (almost) zero resistance for one of the connectors on the fan, and shouldn't get any continuity on the other one. If you do, your short is between the relay connector and the fan connector.

So, when you find where the short is, you will need to open up the loom of wires and visually inspect them. If you are lucky, you will see worn or cracked insulation, which you can repair with electrical tape. Done. If the wires are in really bad shape, you might have to cut the bad ones out and replace them. Lots of soldering and shrinkwrap.

My brain is now melted, it is your turn!
posted by gjc at 11:01 AM on December 11, 2010


GJC advice is really good and how to correct the problem, but it is very time consuming. What i would do is buy a new (used) fan at a junkyard. I bet ford used this same fan for a bunch of cars. Hook it up and see if that fixes the problem. While you are getting the fan out of the junkyard also grab a relay (usually stuff like that is free if you are buying something already and always free if it a pull it yourself place). If either one of these parts don't fix it than you need to start doing the process described by GJC, or if the parts cost more than about 50 bucks for me, but you have to decide on the value of your time, and hell sometimes I just do it for the experience or to avoid chores i DON'T enjoy.

This one time, when my car was a beater Subaru brat, when i had the exact same problem, and the fan was good, I just clipped the fan wires at both ends of the harness, and hardwired the fan to the battery then to a toggle switch I mounted in the dash of the brat. I just had to watch the temperature gauge closely. This is not necessarily a good idea, but it was cheap and I couldn't find the damn short for several hours of looking. If you aren't the type to be able to keep track of the temperature when also driving in heavy traffic this is definitely a BAD idea as an overheated engine can be totally destroyed.
posted by bartonlong at 12:17 PM on December 11, 2010


GJC,
Thanks a heap! That was completely awesome!
Could you MeMail or Email me (both are in the profile)? I'd like to be able to shoot you a quick question or two if I need to... If you'd rather not do that, that's fine, but if you're up for it, I might like to pick your brain (I'm a teacher, so I know questions/answers can take a long time, so I would try to minimize it).
Bartonlong, thanks also! I do have a local pull-a-part, which has several Ford Escorts in stock, so I might check out what I can get there as needed.
posted by Slothrop at 2:54 PM on December 11, 2010


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