Electric Fan Problem

The Mark VIII will require more than a 25 amp fuse. It draws just over 42 amps at just over 14.0 Volts. If you cannot get a slow-burn fuse to work, you might need to use a fusible link.

The interior fuse that sometimes blows is fuse 13? There should be no connection between it and the low coolant sensor. I would source a new control circuit for the relay. I would use a relatively unimportant donor circuit since you are having issues. Something like the dead-side of the HVAC blower motor fuse would work. Be sure to fuse this new wire. A one or two amp fuse should be sufficient (depending upon what relay you are using).

Just to be sure, on your relay, 86 goes to the low-coolant wiring?
85 goes to the output from your controller/grounded switch?
87 to the fan?
30:fused power from the battery?

Here's Monte's Monte's write-up

If you find another source for the switch, please let me know. I went so far as to email a few top tier companies and they were not too enthusiastic or helpful (because I was not searching for something for an OEM application).

Good luck.

Sounds like a wiring problem somewhere.
1. Trace and verify all wiring has good tight connections at the grounds & power, also verify there are no shorts or burn spots.
2. Make sure the relay is wired correctly.
3. As a testing point I would connect the relay's switched +12V to the battery with ~3A fuse. Ideally you want it switched, but this is just for testing, or you could leave it that way and the fan will continue to run even with the car is off until the correct temp is met (I don't like tht idea). Then it will shut off.. See if it blows any fuses.
4. What kind of a temp controller are you using? Has it been shorted and possibly blown the diode out?
5. Like already mentioned a 25A is not a big enough fuse, which also makes me think the wire is not big enough as well.

If you don't have a high current alternator, you can forget about using an electric fan. The stock 65 amp alternator isn't big enough to run the fan and the rest of the car.

Switching a fan on and off manually is a bad idea. Too many guys have been distracted (hot girl kissing on their neck, too much to drink, dog tired and not thinking clearly) and cooked things because they forgot to flip the switch. An equal number have forgotten to turn the switch off for the same reasons and run down their battery.

The best fan controller available today is a DC Control unit. Now serving. Cost is about $110. Be prepared to wait 4 weeks or more to receive your controller once you have sent in your payment. The controllers are custom made in small lots and lead times can stretch out.

Next best is a SPAL controller - $70-$90 See http://www.spalusa.com/fans/automated/accessories/fanpwm.html. Ebay will have the controllers for the $70: do a google search and see what you find.

At the bottom are the Hayden or Imperial controllers available through Advance Discount Auto Parts and AutoZone. The non adjustable one is about $30 ( Hayden 226206) and the adjustable one is about $60 (Hayden 226204). I recommend you do a Google search on Hayden and the part number for more info.

Do not use a simple on/off switch without using a relay. The current load can burn up the typical cheap automotive switch very quickly. The fan draws 30+amps and you need to use #10 wire on the fan power and ground wiring.

If you are good with electrical stuff (90% of the people here aren't), build your own controller. The numbers on the diagram (#86, #87, etc) refer to the numbers on the bottom of a typical automotive relay.



Note that the thermostat switch in the diagram isn’t really suited for the job. You’ll have to do a Google search and find your own.

This version of the fan controller will continue to run the fan after the ignition switch is turned off, just like most new cars.
To allow the ignition switch to control the fan so that it does not run when the ignition is off, connect the relay contact #86 to the red/green wire on the ignition coil or to the red/yellow wire on the coolant level sensor.

If you are an experienced electronics tech or electrical engineer, email me and I will send you the prototype drawings of a fan controller that is probably as good as the DC Control unit. It is a build it, troubleshoot it yourself item. I will not build or troubleshoot units, so it is not suitable for anyone who isn't really good with electronics.

Alternate placement for a temp gauge sender or temp switch/temp sensor for an electric fan. Use the heater feed that comes off the intake manifold. Cut the rubber hose that connects the manifold water feed to the heater and splice in a tee adapter for the temp gauge sender. Be sure to use the same water feed line as the ECT sensor. That way you will get the most accurate temp readings.

Tee adapter info:
Make a pilgrimage to your local hardware or home supply center and get some copper pipe and a tee that fits the temp gauge sender. Solder two pieces of copper pipe onto a copper pipe tee with threads in the tee part. Find the correct brass fitting to match the temp sender threads to the tee fitting.

jrichker said:

If you don't have a high current alternator, you can forget about using an electric fan. The stock 65 amp alternator isn't big enough to run the fan and the rest of the car.

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Good point.

jrichker said:

Switching a fan on and off manually is a bad idea. Too many guys have been distracted (hot girl kissing on their neck, too much to drink, dog tired and not thinking clearly) and cooked things because they forgot to flip the switch. An equal number have forgotten to turn the switch off for the same reasons and run down their battery.

Click to expand...

You may have misunderstood what I was saying. I didn't mean put an actual switch inline on the 12V source that goes to pin 86 on the relay. I was talking about the relays input that requires 12V to "turn the relay on" more or less just labeling that input as switched 12V, I was calling that a switched input. If it is hooked up to a non-switched or constant 12V all the time the relay would be "on all the time" but the fan would only work when the controller told it to keep running. I also stated that as a test to verify or eliminate the circiut problems he was having.

bubba-dough said:

Good point.



You may have misunderstood what I was saying. I didn't mean put an actual switch inline on the 12V source that goes to pin 86 on the relay. I was talking about the relays input that requires 12V to "turn the relay on" more or less just labeling that input as switched 12V, I was calling that a switched input. If it is hooked up to a non-switched or constant 12V all the time the relay would be "on all the time" but the fan would only work when the controller told it to keep running. I also stated that as a test to verify or eliminate the circuit problems he was having.

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My post was a tech note that made a general statement, and was not in reference to anything you may have said. It was not my intention to detract from any of the test & troubleshooting suggestions you made.

I think the relay i have now is just a 30a, but all the parts stores i go to supply no bigger, any recommendations?

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Use the Bosch relay noted in Monte's write-up.

What do you recommend as far as hookin up the 12v key on power? Do you splice into the wire, run it from the fuse box, or use a connector that snaps onto the existing wire?

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I'd just use a fuse-tap. Getting to the back of the fusebox and soldering a Tee is beyond most folks' abilities. Use the dead side of the donor fuse, and add your own fuse on the new wire you add.

Just be sure this is only for triggering a relay (requiring a fraction of an amp) instead of powering the fan itself.




Good luck.

Anyone want my advice on this subject???