MS3 2 Speed Electric Fan

How have people had success setting these up? I will use the 940 Volvo fan relay. I'd like to use built in fan control for the low speed setting to allow for easy idle up, and set a programmable output for the high speed. However, in fan control there is only a fan on temp and a fan off temp. I can't set a high limit to stop the fan control output to allow me to switch to my temperature controlled programmable output. I know I shouldn't ground both high and low relay pins at the same time. Any ideas how to work around this?

Simply use a 5 pin relay on the low side fan trigger. So when the relay is off your using the center pin to control low speed, when high speed is triggered, it will ground the fan relay, and trigger the low speed trigger relay to be on, thus turning off the low trigger

Good idea. This is what you're thinking?

That's exactly what I'm thinking

I've done this under dash with odd stuff and use the mini 30amp relays to keep space down

Great idea. Thanks for the help. I'm planning on using a bussmann small fuse/relay box and I'll just stick the triggering relay in there.

Doesn't the factory two-speed relay already handle this? It looks like it does, from the factory diagrams. When the trigger for high speed occurs, it kills the signal to the low speed side, if I've read it correctly. If I'm wrong, then I've just learned something that will help me set up my MS fan control, haha .

Green is low speed, red is high.

I have a two speed Volvo fan on my car, and I don't have any special consideration for only triggering one speed at a time. AFAIK when high speed kicks on, both circuits are grounded, it doesn't seem to cause any issues.

On my car - low speed is controlled by MS, to turn the fan on about 5 degrees over the thermostat temp (in theory, on a road trip it won't turn on at all). I also used to have a relay that let the A/C system engage low speed as well, but A/C has been off my car for a while now.

I wanted high speed to be a 'dumb' failsafe - so it uses a temp sensor in the radiator and simply grounds when things get really hot (220 or so). Totally independent of the low-speed circuit, doesn't rely on MS being on, and for that matter, the key doesn't even need to be on (a few cool-down fan blows are not a bad thing).

So if high speed ever kicks on while the engine is running, low speed is almost certainly also on.

Cool. From the above diagram, it looks like when the high speed kicks on, it actually breaks the circuit for the low speed.

Broke4speed said:

Cool. From the above diagram, it looks like when the high speed kicks on, it actually breaks the circuit for the low speed.

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I think you're right, which would explain John's experience.

That's nice. Makes wiring the fan much easier.

You can ground both pins at the same time on the factory relay. That's how it is done from the factory.

could use two outputs with the oem relay, and set it up that way.

OEM 940 relay on my 240. You ground both for high-speed, and one only for low speed. You could either use a temp switch or the MS outputs.

PS: I do recall some weird issue I was having with my old MS1 setup where the fan relay was back feeding power into the MS ECU and keeping it (at least partially) powered up when the key was off. Which caused some weird behaviors (since the ECU needs to be power cycled to make certain changes stick).

So I had to isolate the ECU from the fan relay with a small cube relay.

JohnMc said:

PS: I do recall some weird issue I was having with my old MS1 setup where the fan relay was back feeding power into the MS ECU and keeping it (at least partially) powered up when the key was off. Which caused some weird behaviors (since the ECU needs to be power cycled to make certain changes stick).

So I had to isolate the ECU from the fan relay with a small cube relay.

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With the OEM Volvo 940 relay? I don't see how that can be with a ground switched circuit.

I think it's because the fan relay is powered all the time, even with the key off. So even when MS wasn't grounding that fan circuit to turn it on, enough current was flowing through the relay to send a small amount of 12V+ to MS.

Probably, MS shouldn't have been doing anything with that 12V+, and perhaps something wasn't quite 100% perfect inside my MS box, but with 12V+ present on the fan wire, the ECU wouldn't power down with the key off. Unplug the fan wire, and it would go dead as it should.

I just used a cube relay so that MS could ground against switched 12V+ to turn the relay on, and the relay itself was grounding the fan relay. Thus key off = no 12V+ present on the fin circuit in MS.

Gotcha. I wonder if that is an issue on ms3 or not.

JohnMc said:

I think it's because the fan relay is powered all the time, even with the key off. So even when MS wasn't grounding that fan circuit to turn it on, enough current was flowing through the relay to send a small amount of 12V+ to MS.

Probably, MS shouldn't have been doing anything with that 12V+, and perhaps something wasn't quite 100% perfect inside my MS box, but with 12V+ present on the fan wire, the ECU wouldn't power down with the key off. Unplug the fan wire, and it would go dead as it should.

I just used a cube relay so that MS could ground against switched 12V+ to turn the relay on, and the relay itself was grounding the fan relay. Thus key off = no 12V+ present on the fin circuit in MS.

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This could be a function of how you are switching the relay. In particular if you are using a recirculation diode in the control circuit. If the Volvo fan relay is supplied via unswitched +12v, depending on how / where you have the recirculation diode connected (the reverse biased diode across the relay coil), the unswitched +12v on the relay would forward bias that diode and could supply +12v to the MS Board keeping it alive. With the relay coil and recirculation diode in series, its a poor quality supply probably limited to less than 50 ma before the board voltage drops below 5 v. The voltage would probably collapse if you tried to start the car; but, its probably adequate to keep the MS alive.

This problem could occur if the recirculation diode is connected back to the sanctioned 12 v on the MS board. Moving the diode out to the relay so that you don't have a sneak circuit back to the sanctioned 12v on the board should eliminate the problem as would your solution or moving the fan relay to a switched 12v supply or use the same circuit as B&G use for the fuel pump control which eliminates the recirculation diode.

Gotcha. I wonder if that is an issue on ms3 or not.

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I am not particularly familiar with MS3. I think it depends on how the relay switching circuit is set up in MS3.

This is true with Ms3, even the Pro. All relays need switched 12v coil source

yeah there is that. I may have a relay inline to the other relay on the 940, it's been two years since I wired all that junk up.

good to know. stack up those relays