Cooling Fan relay setup
 

Hi all, I have a new question 0n my LS 57 pickup project. I want to run the one 16" fan I am using with 2 speeds off of fan 1 and fan 2 from the PCM. It looks like typically there is a 2 fan 3 relay setup that runs the fans on low speed by running the fans on a series circuit. I want to run the same 3 relay system with a resistor instead of a fan to slow down the one 16" fan I will be using. Does this seem like it would work? Thank You Steve

Re: Cooling Fan relay setup
 

I am going to move your question over to the LS swap area. I think you might get more views there.

Re: Cooling Fan relay setup
 

Why not just run a 2 speed fan?

Re: Cooling Fan relay setup
 

I already had the fan but will look into it. Thank You Steve

Re: Cooling Fan relay setup
 

Ok, understand, didn't know you already had one

Re: Cooling Fan relay setup
 

I don't see why it wouldn't work. You are going to need a pretty hefty resistor though. The reason GM uses the series wiring is to avoid a resistor due to the high amperage. That resistor is going to get mighty hot.

Re: Cooling Fan relay setup
 

Yes has to be rated high and will make some heat. I found a gm resistor with a fan plug on it just can't get any specs on how many ohms it is. Steve

Re: Cooling Fan relay setup
 

GM has used the same 2 speed design in the past. BMW and Mercedes have done it also. One version of the GM fan resistor is P/N 94812213. You could also use a ballast resistor from an old points system. Your truck probably had one on the firewall.

Re: Cooling Fan relay setup
 

What was that resistor used on? It is showing 17.25 watts. If I take a 16" Summit fan and look at the amperage, it is 12.9 amps. So..

Wattage = Volts x amperage
154.8 watts = 12 volts x 12.9 amps

The Mishimoto 16" fan draws 7.18 amps. So..

86 Watts = 12 volts x 7.18 amps.

Wouldn't that resistor smoke in like 5 seconds, or am I doing something wrong?

Re: Cooling Fan relay setup
 

I think you are right Joyridin.

I think the solution is to get a resistor that is equivalent(or close) in resistance to the fan. That should give you 1/2 voltage to the fan.

To find the resistance of the fan, you would divide the voltage by the amp draw.

In your 16" summit fan example, for simplicity lets say 12.9V divided by 12.9A = Fan has 1ohm resistance.

At this point I'm not sure on figuring out how many watts the resistor would need to be. I feel like a 100w resistor would work, but the math might say 154w(or 167w at 12.9v) or 200w for simplicity. Someone that remember more electronics math than me can hopefully come along and explain. I think since you are doubling the resistance your wattage is halved. But it may be quartered across that resistor. Help....
----
I'm back after some googling and at 12.9v with 2ohms of resistance, your at 1/2 the wattage so about 83.2 watts. I'm lost again at figuring out of you only need 1/2 of the 83.2 watt rating for the resistor.

A 200w is almost 4 inches long - https://www.amazon.com/dp/B08G8JGW1M...d_i=B08G8JGW1M

Here a 100w - https://www.amazon.com/Comidox-Alumi...77ZMFZM6/?th=1

Or a 50w and see if it smokes - https://www.amazon.com/LM-YN-Wirewou...dp/B0728B8XQR/ :smoke:

Re: Cooling Fan relay setup
 

Why don't you try a DRL resistor. Toyotas and Subarus used them to reduce power to their headlamps for DRL's. If you have a u-pull-it yard in your area, I'm sure it wouldn't be cost prohibitive, and most have a 2 wire connector for power in/power out applications

Re: Cooling Fan relay setup
 

Lots of great information guys. The resistance on the fan is 1.5 ohms. The specs say it draws 18.5 amps. I am looking at all types of resistors in that range now. I will get it figured out. I am a little ways away yet on that part of the project. Just looking ahead at what I will need. Thank You Steve

Re: Cooling Fan relay setup
 

Until you get it worked out or find out if two speeds are even needed.


I run a big 6.0 LS in the 1956 with stock radiator and have AC in it.

Fan is turned on when i want it to manually or comes on automatically with temp switch if its gets hot and I am not paying attention when I need it.

Running no AC and just driving and sitting for short period it barely hits 200 with all that room in truck not much gets trapped. And rarely runs.

In the summer GA August heat at 100+ humidity way up and AC running it will start to over heat without a fan.
But otherwise its off until it comes up to temp and on when it gets to 195/200 temp start temp. If its cool enough outside it will cut off when driving and sometimes off when sitting. Hwy driving it drops to 188 to 195 and has a hard time getting over 200 unless I am pushing it hard uphill.

LS motors like a higher temp (really the computer and all its calculations does) at least all my modern ones do.

Fan is really only for sitting and not forcing air when driving and on highway then even less needed.

Any way ON/OFF one speed is a option.

Clay

Re: Cooling Fan relay setup
 

Thanks for the information. The pcm has fan 1 and fan 2 outputs and thought it would be quieter. It looks like fan one turns on at 210 and fan 2 at 220. Not sure how to tell the pcm when the ac is on. There is a ac request input but not sure if is seeing a ac request or putting out a ac request. I am sure I am over thinking this but this is my process. Steve

Re: Cooling Fan relay setup
 

Going to call overthinking on this one.

Yes the PCM has a two fan option.

With the three relays it controls speed

It does this by putting the fans in series on low speed call.

As in

Positive wire ----- fan 1, wire out of fan one to fan two then wire out of fan two to ground. (in reality i think it does the grounds but either way works)
aka sharing power therefore half speed each.

When it calls for high
The relays flip and

Fan one positive direct fan one and wire out to ground

Fan two with a separate positive and wire to ground.
Both fans get full amps and run wide open.

Since you have one fan just wire the fan to the low side of fan one. Low side is always on and hot when pcm calls any fan to run.

Set your PCM to be on when you want temp wise since you are going to remove all the vats and other items anyway. and be done. It will run full speed as long as the pcm calls it to run.

Assuming you do not have a E78 ecm. it runs a mechanical fan and the pcm does not call the fan relays. even though its in the computer and the relays are in the fuse box. Ask me how I know. E38 which is the most standard is relay and pcm fan triggering worthy all day.


I doubt in a old school truck you will ever need two fans even with AC.
It needs the higher speed in modern trucks with a crammed full engine bay not letting out heat. mountains of towing power in modern trucks running full bore AC to multiple systems in some cases. This will tax engine heat etc.

Re: Cooling Fan relay setup
 

This really spells it out.

Re: Cooling Fan relay setup
 

Thank You for the information. Steve

Re: Cooling Fan relay setup
 

When I eliminated the clutched fan on the '91 GT for intercooler piping, I researched alot of junkyard parts. I realized that there is alot of confusing ways to go. Being my ECU does not have signaling outputs for e fans, I went for a single SPAL 16" fan with a variable speed controller. Has an A/C auto on input and a stand alone temp probe that fits into the radiator, being I'm running a 180* thermostsat for the supercharger. It doesn't matter what temp anything else is running, it only kicks the fan on when it needs to. Hot days when shutting it off, the fan will still continue to run for 30+ seconds after just to cool things down, then turn off. It does vary the speed... not just low/high. 15 years ago $110 for the fan and $80 for the controller/temp sensor. Love it. My 2 cents.

Re: Cooling Fan relay setup
 

Thank You for your opinion on this. My goal is to let my PCM control the fans. I am just looking for a way to let the PCM know the A/C is on which is a Vintage Air Gen IV system. I have a A/C request wire just need to figure out how to use it. Steve

Re: Cooling Fan relay setup
 

What version of PCM are you running?
Or ECM e38? or something else

and I just realized you could run a PWM (pulse modulator)
GM/crysler/generic has them for single fans to control the speed as needed.
It can wire directly to most ECM modules and do what your asking nice and clean.

search for your model of ECM add PWM and fan wiring and a lot of sites especially on LS swap sites have all the pins wires you need.
You don't even need the fancy expensive corvette model there are some basic ones that work well .

Re: Cooling Fan relay setup
 

steve look into the fan controllers by The Auto Cool Guy.

i use them when i dont want the ECM to run my stuff.

if the engine needs 20% fan thats what itll get.
if it needs 35 or 55% itll get it..

if you click on the ac itll go 100%

https://www.autocoolguy.com/

Re: Cooling Fan relay setup
 

I use one of these. It is a killer set-up. I just assumed the OP was trying to use some type of factory set-up instead of an aftermarket deal.

I have mine controlling dual 16" fans. I think they come on at 185 degrees and slowly increase as the engine heats up. When the AC kicks on with the trinary switch, they come on full blast.

Re: Cooling Fan relay setup
 

well good sir, you know what they say about those who assume?

:) :) :) :) :) :) :) :) :)

Re: Cooling Fan relay setup
 

I think is a 411 it is the stock unit used on 05-06 silverado. I am trying not to add any other controllers just let the stock pcm do what it needs. Thanks Steve

Re: Cooling Fan relay setup
 

I am trying to use the PCM as the controller as it has 2 outputs for it. Thanks Steve