Is it ok to just run the high speed all the time
and not the low and then high?

 

You'll want to double check what amperage is being pulled on high speed. While these fans are excellent air movers, they also require some high amperage relays to control them. There's a massive amperage spike (something above 40 amps) between when switching from low to high speed. If not properly accounted for, going directly to high speed might fry a few things in your electrical system.... but I'm also sure it can be done lol.

 

As Aaron said, it's not good to jump to high speed. It can make your engine stumble because the ignition system has no juice for a second when the fan turns on. An easy and inexpensive way to control a two speed fan is to use a controller module out of a Volvo. They cost $15 at the J/Y, Here is one on ebay: https://www.ebay.com/itm/VOLVO-COOLI...EAAOSw32lYrjPQ
There are other less expensive on ebay, but you want one with the wires to splice on to. This will control when the low speed begins. If needed it will shut off the low speed and actuate the high speed. I have two. Here is a thread that tells all about it: https://blueovalfanatics.com/forum/s...ad.php?t=22073

You'll also need a BMW two speed thermo switch which can be installed in you thermostat housing, or in the return line (top) to your radiator.

 

I'm using a 20/40 amp relay is that big enough or do I need to use a bigger one or use 2 of those in line just for high speed

 

I think Ford actually used 30 or maybe 40 amp relays for cooling fans in the '90s. So that's what I used on my Mustang which has an SN-95 rad fan, and it hasn't burnt up a relay yet in several years of being on there. I got my relay out of a Ford in the junk yard, so it's probably an OEM piece, and able to stand up to the demands of the fan.

 

Did you read the thread I linked to? Send a few minutes reading. All the answers are there. The Mk8 fan is notorious for frying relays and wires.

 

I read your reply and was going to read the link in my lunch hr
thanks

 

The actual MK8 fans are very rare. Having wide blades and a large motor does not guarantee it's a mk8. Ford made many fans that look like that for Mustangs, Cougars, Sable, Lincolns continentals, etc. If you want to know what it is, take the serial # and run it on RockAuto dot com. It is a great fan, whatever it is.

 

I've done the volvo speed controller and BMW sensor combo. It works good. A couple things to keep in mind with this is that the sensor has a metric thread, you can buy an adapter or a metric tap. I went the tap route. The sensor is available in a couple different temp high and low ranges. RockAuto has both, you'll need to decide what works best for your set-up.

Jess

 

Maybe this is helpful?

 

I am using this circuit and it is working well. Note that this circuit makes sure both fan inputs are not on at the same time which is important. Also note that the high speed input is using two relays in parallel which means each relay will handle roughly half the load current. That arrangement should put a lot less stress on the high speed relays so it should not die as easily as single relay solutions.

 

I use a Mark VIII fan using the Volvo relay setup.. pulled the parts myself. The hi/lo speed is fully programmable using Edelbrock's E-Street EFI..

In all its days, my 70 Bump (360FE with a TKO) has never energized the high speed relay.