Step 3: Electric fan - simple troubleshooting procedure

While having a shop manual with schematics for every electric circuit is extremely useful, the sort of circuits that power cooling fans are usually very simple, relying on basic, tried and true, electromechanical components.

Usually, a small control current runs through a thermal switch. When that switch reaches a set temperature, the coil of the fan relay is energized and the relay powers the fan until the contacts in the switch open again.

To test the good functioning of such a circuit, one has to find the fan thermal switch and bridge the two contacts. A relay should then be heard clicking and the fan should run. If the fan has two speed settings, it might have a three wire connection on the motor, one ground and one +12V for each speed or it will have a two wire connector feeding a variable voltage (for example, +9V for a slower speed and +12V for a higher one, plus a ground connection). These two speed systems usually work by putting a resistor in series with the fan motor to give the lower speed. On some cars, these resistances are known to crack or otherwise failing over time.

If the fan fails to run with the thermal switch contacts closed, check the wires for +12V. A fuse might be blown in the relay energizing circuit, the relay itself might have failed, etc.

It is also advisable to test the thermal switch.  A simple setup involves heating the switch with warm water and checking at what temperature the contacts close. Use a good thermometer strapped right to the switch for more accurate results.
This is otherwise very comprehensive and well written, but I have to request that you add something that could potentially be fatal to someone's car. <br> <br>NEVER MIX DIFFERENT TYPES OF COOLANTS! <br> <br>Different makes of cars take different coolant types! Mixing stock &quot;old&quot; coolant that's in your car with off the shelf coolant from the auto parts store can have disasterous consequences if they are not the same. This is particularly true for most German cars and some Japanese cars. For instance, mixing Prestone Green with pink G12 can cause it to gel and clog thermostats, sensors, water pumps and small lines while also causing heavy corrosion deterioration on aluminum parts. <br> <br>I still prefer a complete drain to empty, water fill and flush (while bleeding as you say), drain, and then new coolant. However when you are replacing your coolant with the same kind, this method is alright because new and old of the same type mixing will only cause dillution, not coagulation. But if you are replacing one kind for another kind, be sure and get every last bit you can out of the system before replacing it with the correct mixture of new. <br> <br>Case in point, my 1995 Audi 90 requires pink coolant. Can't get it in Advance Auto or AutoZone and the stuff they carry won't mix with it. Replacing my whole system for green would have been acceptable and it would have worked in my car only had I chosen to do so by eradicating all the pink first, but since pink is a &quot;lifetime&quot; fluid I chose to go with it again. I always assume lifetime to mean about 80-100K miles of average driving even though they'd have me leave it in there forever, but that is a different story. <br> <br>Like I said above, this is otherwise very comprehensive and well written. It will be a big help for someone trying to do it themselves for the first time. Remember kiddies, always get the air out. If you're running the car and see the temp climbing but can't feel hot air from the heater (that should be turned on), you have a bubble in the line and often cracking open the bleeder will get it. <br> <br> <br>** <br>You might also add to run the heater fully &quot;hot&quot; and fan speed low when flushing the system to clean it out as well, otherwise it is bypassed in many cars. Also, there are chemical kits to buy that can help clean out some silicone deposits and acid treatments that can help for calcium and limestone buildups (if you or someone used tap water a lot).
Also, if you do a complete drain like I do, you'l' have to take some extra time to get the air out before you even start your car. Wait for it to fill. The smaller the water column heights the slower it will spread water to the far side of the car, so you might want to pressurize it just a little. In my case I just use my mouth to put pressure on the tank, listen for the fluid shift, hold, and release. Usually bubbles will fly up into the tank and fluid will drain down a little each time. <br>Also, crack open the bleeder taps and pressurize it and listen for air escaping from them. When they stop you can close them again until you run the engine and do the main system bleed. Revving the engine about 1/3 total rpm can help speed up the water pump and give it a quick push that can help dislodge persistant bubbles. As can dropping it down off the ramps (if you've put it on ramps). <br>Check the level a few times each day (when the engine is cool) even after you think you're done, you might be suprised to see you need to top it off again. <br> <br>Sorrry this is so long winded. But failure to do it right here can cause very expensive repair costs. Burned head gaskets, warped heads, etc. I don't want anyone to try this thinking it's always straight forward and end up in the shop out $2500.
This is good stuff. Basic vehicle maintenance is something too many people overlook. <br><br>I can say from experience that oil in the coolant or vice versa is bad news indeed. My wife's car had an issue with this two years in a row thanks to old deteriorated gaskets. <br><br>It is also worth noting for people with pets that antifreeze is VERY poisonous, yet it tastes and smells sweet to animals. If there are any puddles of it laying around, make sure to clean it very well, and keep animals out of the area until you can be certain you have the leaks fixed.
Ever since the late 90's, they've added a chemical that makes it taste bitter.
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Nice work. Most of my thinking is done in the pub, not the cafe, hence no instructables.
I suppose you can still have your pint with the right hand and type with the left at the same time. A &quot;bloke's guide to drinking beer&quot; sounds award winning.
Very nice overview, with a good level of detail! I wonder if you might want to add links to other sites with useful detail for readers (I don't claim that I know any, just asking).
Thanks for the constructive criticism. I didn't want to go into too much detail. The purpose of this guide is to give a basic knowledge of how things work together. Further research is always encouraged but may be pointless for most purposes.<br><br>I may include some links to other sites, this guide is just something I knocked together from scratch in a caf&eacute;.

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