I have a 2002 LS1 in an FC RX-7. I am currently setting up an aftermarket electric fan to be triggered by the PCM switched ground pin for slow speed primary fan. The relay I will be using for turning the fan on is not the same relay that this fan uses for power. This is merely for signaling the fan on, therefore, only a very small amount of current (~5 mA) will pass through the relay.

I am not using the oem relay, but I want to know the resistance of it. The relay I am using was taken from my RX-7. It is 75 ohms, and I am not sure if that is too low for the driver circuitry. I don't want my PCM to sink more current through the driver than it was designed for. There are probably protective measures implemented into it, but I don't want to take any chances. Can anyone quickly give me the resistance of cooling fan relay #1?

Thanks,
Bob

 

Do the math, I=V/R so 12V/75ohm=0.16Amps which isn't much. Relays take high current initially to pull in and require lower current to hold. One of the real issues is to use a relay that requires too low of a hold current as this could be equal to the leakage current of the switching transister that is in the PCM. This leakage current is approximately 3ma or .003 Amps so if a relays holding current is right at 3ma then it will never shut off even after the transistor in the PCM is commanded off.

If you are concerned about that specific relay then go get a different automotive relay and just use it and trust the manufacture knows what they are doing. Another alternative is to find a relay that you know is used in a automotive computer controlled application and then use it, that way it will give you piece of mind.

Transistors are tough and since it is a automotive computer that is designed to take heat I think it can control pretty much any small automotive relay out there.

 

Ah, well thank you for clarifying that, but I don't think leakage current is going to present a problem since this relay is used in a computer controlled application in the RX-7. I am just going to try the relay I have.

I was just measuring the resistance of the ground switching pin (Pin 42 dk grn on Blu connector of PCM), and I noticed that there is 95 ohms to ground when the ignition is on and the engine is completely cold (this is causing my relay to actuate). It was my understanding that the slow speed fan only turns on at around 230 degrees, but it seems like it would be turning it on all the time. Should I just go with the blue wire used for full speed fans instead? I only want my fan to come on when the engine is warm..

Thanks again for the info,
Bob

 

I used the #1 fan relay control (in my '89 RX7). It turns on at 126 degrees, and I have a 180 * thermostat.If you are going to get your PCM tuned (who doesn't) you can have the fan turn on at any temp you want. When I get mine tuned I am going to have both fan controls set at 195*, so if one control output should crap out I'll have another.

 

That is the way I thought it should work, but for some reason both of my wires for fan control are connecting to ground even when my coolant temperatures are completely cold. I'm at a loss. Maybe my water temp sensor is broken and the PCM thinks the temp is pegged? I havn't noticed any rich condition..

~Bob

 

if you're checking them with a meter, they always read slightly grounded... the PCM always has a hanging ground there....

 

Cool! Ignoring the meter and just hooking it up, it works!!

Thanks