Testing the power connection to the fans and fan operation itself
The fan's can easily be tested for operation and power, but it is bit tricky since you may not make any mistakes and mot touch ground with a hot (12v) wire. Remember that you are pulling 25amps or more upon connecting the Fan.
On the fan controller power stage (the one with the fins in the nose of the car) you see a round connector with numbers 1-8 on them. Disconnect and turn upside down. You see the holes that match the pins on the power stage. Ok, on the connector the 12v arrives for fan1 and for fan2, and power out is wired to power the Fan1 or Fan2. The Fans are connected to ground elsewhere. So the idea is top bridge 12v for Fan1 to power out for Fan1, and the same for Fan2.
Bridge 1-8 on the connector of the power stage which makes the Fan2 run, and is the right side fan
Bridge 4-5 on the connector of the power stage which makes the Fan1 run, and is the left side fan
Left and right are seen in the engine
bay looking from front to rear of the car
Here is a picture that helps understanding:
The bridge makes both fans run at full power, bypassing the controller and power stage. It takes 12v from battery via the 30A fuses.
For testing this, I'd take off the two connectors from the controller (and isolate them to avoid contacts to ground) just to a avoid damaging the controller when making a wrong connection.
Be very careful doing this. A shortcut to ground will make a bang and big spark, even if fused with a 30A fuse. I'd connect a voltmeter at the bridge to see what voltage is at the fan terminal. There should be 12v or maybe a bit less, but both fans should have similar reading, so not read 9v and 10v. That's not ok.... In case you have just one voltmeter and want to test both fans simultaneously: connect the voltmeter between the two bridges. If all is ok, i.e. both fans have the same power, the voltmeter will show a voltage very close to 0v. Anything like 0.5 or 1 volt is wrong.
Maybe best to disconnect the battery, connect the bridge, connect the voltmeter, and then connect power at the battery,. The fans can fun a few minutes on just battery power.
When both fans run without problems with similar rpm and when the fuse is not getting hot while testing, I'd say the power is ok.
Keep an eye on the battery. Draining too much hurts the battery life. Charge when needed.
It is also possible that the ground connection of the fans is oxidized or disconnected and causes trouble. The fans have their ground connection at GPII (for left hand drive cars). GPII is at the very front of the side wall in the engine bay, right side, at about radiator height. Look for a brown wire connection point with two thick brown wires and some smaller ones.
And if all looks ok, check the connector at the fan itself. Sometimes oxidation can cause intermitted problems.
regards
Theo
1992 928GTS midnight blue
The Netherlands