Thread: Radiator fans not turning on

I've got an issue with the fans not coming on, and I'm a bit stumped. Here's the story so far:

'91 SE-R ECU and wiring harness, temp sensors from original engine moved over. Coolant temp sensor (the one with two pins, red housing) seems to correctly read temps when I measure the resistance. I don't have a good temp sensor at home right now to get accurate temp readings though.

Radiator fans turn on when I jumper across the relay connections for the fan motors.

I have continuity from the relay coil line all the way back to the ECU.

Fans do NOT turn on when the coolant temp sensor is unplugged. My understanding is that this should have triggered a form of limp mode and kept the fans running?

Right now everything is leading me back to the ECU not turning on the fans for some reason. Other than the temp sensor, is there some other sensor or input at play here? I'm starting to suspect the ECU itself as the problem. I have a spare somewhere if I can find it...

I do have different fans from stock, P10 radiator w/fans, running through a new 30a dual pole relay. But that seems irrelevant since the fans work when I jumper the relay outputs to give them 12v. I can probably wire up a switch to manually activate them through a switch, but would rather let the ECU handle if possible.

Just put the 2nd ECU on, sensor still unplugged, no radiator fans with the engine running.

Did you check voltage at the relay coil?

Stupid, stupid, stupid... I checked my relay wiring twice, but I checked it wrong... The problem is my new relay wiring. I disconnected the cable going to the ECU and jumpers the blue wire to ground, I could hear the relay activate. I unplugged the relay and tries jumoering just to be sure I was activating the right relay, no sound, so that prt checked out.

I looked at the pinout shown on the relay, and found that the drawing on top didn't actually match the PIN numbers as shown on the bottom. I had battery voltage on the two lower pins, and fan wiring to the upper two pins. But I actually should have had battery on either the left or right pins, and the fans on the other side.

When the relay activated, I was effectively jumpers get the two battery lines together, and the two fan wires together, which does nothing.

Off to resolder connections.

Just an FYI for future google searches... I found in the FSM in I believe the EF & EC section some information on the ECU's failsafe modes, one of which covered if the temp sensor wasn't reading correctly. The computer would assume that the engine was cold at startup (regardless of actual temp), at some higher temp shortly after, and hot after 6 minutes. So reading this, the fans wouldn't necessarily kick on immediately with the sensor unplugged, but mine fans did kick on after about 2 minutes. Possibly sooner, but I noticed right at the 2 minute mark and wasn't expecting them sooner than 6 minutes. My sensor is plugged in again, and my fans are working normally after redoing the relay.