Fan Wiring - 2 speed dual fans

I have 2 speed dual fans; lo and high.

I've heard that the proper way to wire these 2 speed fans is to have the #1 fan signal (say at 200 degrees) from the PCM trigger BOTH fans at low speed simultaneously; and then the #2 fan signal (say at 205 degrees) from the PCM trigger both fans simultaneously at high speed (rather than just trigger each fan separately).

This is obviously different than single speed fans where the #1 fan signal from the PCM triggers the relay to provide 12v to the #1 fan at 200 degrees (in my example) and the #2 fan signal from the PCM triggers the relay to provide 12v at 205 degrees (in my example).

Thanks in advance.

I'm not sure if the Gen 3 will do that (2-relay system; turn Fan 1 output OFF when Fan 2 output turns ON). Gen 4 does that easy with 0's & 1's.

You could copy the factory 3-relay system and only connect to the 'high speed' terminals of your hi/low (3-pin?) fans.

Quote:

---

Originally Posted by fastd

I have 2 speed dual fans; lo and high.

I've heard that the proper way to wire these 2 speed fans is to have the #1 fan signal (say at 200 degrees) from the PCM trigger BOTH fans at low speed simultaneously; and then the #2 fan signal (say at 205 degrees) from the PCM trigger both fans simultaneously at high speed (rather than just trigger each fan separately).

This is obviously different than single speed fans where the #1 fan signal from the PCM triggers the relay to provide 12v to the #1 fan at 200 degrees (in my example) and the #2 fan signal from the PCM triggers the relay to provide 12v at 205 degrees (in my example).

Thanks in advance.

---

What your talking about is having the low turn on the fans wired in serial so the voltage is effectively halved between them. Then on high they go to parallel. You need 3 relays to do this. You should be able to find a schematic online as most later GM vehicles with 2 fans are wired this way.

Unless you have a 1997 Ford Thunderbird fan that is a 2 speed fan that uses two separate wires-1 for low speed, and 1 for high speed, and act independently of each other.. meaning you only have to energize 1 of the 2 wires depending on which speed you want... unlike the Gm fans that require that BOTH wires be energized for the high speed fan to work. If you have a GM fan.. for information on wiring up GM fans, look here; https://lt1swap.com/wiringharness.htm

Gen 3 Pcms control low/high speed functions just as well as 1 fan on or both fans on. The difference is how its wired, 2 relays you have one speed and either one fan on or both. To get both fans on at Low and then high speeds you need 3 relays. Attached is the wiring diagram to make it work correctly.
Attachment 125806

Quote:

---

Originally Posted by Dewayne Smith

Gen 3 Pcms control low/high speed functions just as well as 1 fan on or both fans on. The difference is how its wired, 2 relays you have one speed and either one fan on or both. To get both fans on at Low and then high speeds you need 3 relays. Attached is the wiring diagram to make it work correctly.
Attachment 125806

---

That works fine for two single-speed fan motors. It doesn't for two 2-speed fans, unless you don't connect anything to the fans' low speed circuit.

Quote:

---

Originally Posted by blindsquirrel

That works fine for two single-speed fan motors. It doesn't for two 2-speed fans, unless you don't connect anything to the fans' low speed circuit.

---

My apologies, I missed the part of having 2 speed fans. I've never used 2 speed fans therefore my previous post is irrelevant.

I think we are all speculating in what the OP is actually after.

Quote:

---

Originally Posted by HATEHPTUNERS

Unless you have a 1997 Ford Thunderbird fan that is a 2 speed fan that uses two separate wires-1 for low speed, and 1 for high speed, and act independently of each other.. meaning you only have to energize 1 of the 2 wires depending on which speed you want... unlike the Gm fans that require that BOTH wires be energized for the high speed fan to work. If you have a GM fan.. for information on wiring up GM fans, look here; https://lt1swap.com/wiringharness.htm

---

This is how my fans work; I only need to energize 1 of the 2 wires- they don't both need to be energized.

However, the way that I am thinking of wiring it, they will be both energized:
I will wire both fans low speed 12v to #1 relay; the PCM triggers the #1 relay at the first fan temp (200 degrees), and so both fans will turn on at the same time at low speed.
I will wire both fans high speed 12v to #2 relay; the PCM triggers the #2 relay at the second fan temp (205 degrees). When the #2 relay is triggered, the #1 relay is still triggered. I think that that should be OK.

I was really just wondering if this sounds like a proper way to have fans turn on, i.e., having both fans come on simultaneously at low speed. That would be a pretty big amp spike when they both come on - I will have heavy gauge wire feeding the fans but that could be like 70 amps through the one relay when they turn on. That seems like a lot. Also wondering if I a missing any other potential problems wiring this way. For example, anything related to the PCM not liking it wired this way - I wouldn't think so but asking...thanks.

The PCM will be fine with it, since its outputs are just grounding the tiny current draw of the relay pull-in windings. Whether the fans are happy with having both their high & low circuits powered at the same time in high speed is the question. Everything that used the 2-speed fans turns off the low speed windings when high speed is powered.

With a Gen 4 you'd just do this, so that it'd only have one relay engaged at a time:
Attachment 125815
...but as far as I know with a Gen 3, there's no way to fiddle the temp settings to make Fan 1 output turn off while temp is still increasing (progressing from low speed to high speed).

If you decide to try this using just two relays, you need to do some testing first.

Measure the current draw of only the low speed windings, then measure current draw of only the high speed windings. Those will be your benchmarks.

Now, power both sets of windings at the same time and measure each one's current draw again. The low speed side should fall off to near nothing or possibly even negative (the high speed half turns the low speed half into a generator), while current on the high speed half goes up... by more than the amount of change on the low side, or else you just solved the 'free energy' problem. And fan speed probably takes a hit too.

The PCM should be used as the 12 volt to the coils on the relays. Use another 12V power source to run the fans. 2 Relays are needed. I would use Single Pole Double Throw (SPDT) relays (they will have 5 spade connectors not 4 spades). One relay will feed the other relay. A SPDT relay typically has the following Spade 30 in the power in, Spade 85 is coil (control) PCM supply, Spade 86 is coil ground (PCM ground) 87 is normally open (switches when energized). 87A is normally closed and opens when energized. This may sound reverse polish but it will make the fans switch properly. Power (12V) wires will start at the high speed relay (Relay 2)on spade 30 and high temp (205 Deg) signal to spade 85 Then spade 87A will be feed to low temperature Relay 1 on spade 30. Still on Relay 2 (high speed) spade 87 will be wired to the high speed fan wires. So relay 2 will have a total of 5 wires connected to the relay. Relay 1, low speed will have wire from Relay 2, spade 87a to Relay 1, spade 30 as power to Relay 1. Then use Relay 1 spade 87 to low speed wires on the fans. Also Relay 1 (low speed) PCM low speed signal is connected to spade 85 and spade 86 connected to PCM ground.
The way this works is less than 200 (PCM setting) all fans are off, PCM temp 200 low speed fans on, temp 205 fans high speed. The actual fan voltage just passes through the high speed relay (Relay 2) until the 205 temp is reached and all low speed control is done by relay 1 (low speed). At 205 deg, relay 1 power is turned off by relay 2 as it switches from deenergized and then powers high speed with relay 2. Note: nothing is connected to 87A on relay 1 (low speed). Do not use the PCM signal to power the fans, it draws too much current and will fry the PCM. Add a 40 Amp fuse to the power feed into relay 2 to protect the entire fan circuit. (note 40 amp is a guess since I am not sure of the fan current draws)

Like this?

Attachment 125838

Cool! But, um... there are two 2-speed fans, so...

Attachment 125839

That only works if the total high-speed amp draw of both fans is safe for a single relay and a single fuse. If the relays are the limiting factor, great! Just give each fan a dedicated high speed relay...

Attachment 125840

But after that if you need a second fuse for higher capacity, you end up with four relays, two lows in parallel and two highs in parallel, each bank fed by the appropriate size fuse. All to use the dual speed fan motors. If it gets to that point it's actually simpler to ignore the fan low speed windings, connect only to the high speed windings, and wire it up exactly like the factory setup with two single speed fans...

Attachment 125841

(And all this bullshit is why I love my variable speed brushless fan. One control wire, one B+, one ground. No idle sags or dimming headlights. No giant fuses, zero relays, and no melted fuse boxes.)

Why couldnt you just tie both low speed wires to fan 1 and both high speed wires to fan 2 then set your pcm temps accordingly

Edit its early and I just re-read about halving the voltage (which I would assume would half ish fan speed)

Ignore the just waking up dummy

BlindSquirrel, I agree, Most of the SPDT relays are rated for 40 amps, typically used for fog/driving lights. I no nothing about the fans but there should be something about amp draw or wattage or something and P=IE can give the expected amps. There are higher amp relays available with 12 vdc coils. I was more concerned about the functionality of the circuit first then fuse and load accordingly.

Been thinking of using these. Come in all flavors from 20amp all the way through 100 amp dc
https://www.amazon.com/SSR-60DD-3-32...s%2C522&sr=1-2

Those are single pole single throw and will require a more complex circuit, you will need 4 of them. I am not big on solid state/digital relays in automotive environments. Too many bad things can happen and troubleshooting gets more complex. As an industrial control person, the old school coil relays are more long term reliable. Just my opinion but I have been doing this a very long time.

https://www.amazon.com/Hamolar-Pack-...%2C107&sr=8-35

This is also water resistant and 60/80 amps which should handle everything.

hey blind squirrel i was wondering if u could help me with this its a 5.3 colorado 2010 i converted to electric fans, its 1 fan that works with 2 velocities. it was working well when i calibrated it and my engine temp was stable at 192 degrees and then fans wouldnt even turn on when i turned on the car 10 minutes later.
Attachment 144285
Attachment 144286
Attachment 144287