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Like Lokiel, I have just installed a TSE radiator, and replaced the single 2 factory fans (coolant and A/C) with a single SPAL fan (13" curved puller). Wired the two relays together, so that whether the AC or the radiator fan relay triggers, the SPAL comes on. Holy mackerel, this thing draws some current when it turns on - nearly stalls the car at idle, the specs mention 70A in-rush current!

BUT - when it is triggered by the ECU, it only comes on for about 2 seconds, then goes off again, so it is caught in an on-off-on-off loop - 2secs on, 2 secs off, until it overheats.

We let the car cool, and then re-started, and turned the AC on to trigger the fan by the AC relay - and the fan comes on for longer, but still gets caught in an on-off loop - about 30 secs on, 30 secs off. The car has a Microtech LS-8 controlling spark, fuel and I think the coolant fan, and the factory ECU controls the AC fan. so there are my questions:

1. Anyone know what the hell is going on?
2. Just so I can get it to the track, is there any harm in just wiring in a manual switch, and leaving the fan switched on permanently?
3. I have got a 130A relay, should I use that instead of pairing the stock coolant and AC fan relays (which add up to approx 12A)?
4. Anyone know what the hell is going on?

The fan is powerful so is very effective at dropping the temperature with the big-arse TSE radiator; your ECU is triggering the fan to come on when it gets to the "ON" temperature but cools rapidly so the ECU turns it off when it reaches the "OFF" temperature, hence the cycle.

Simple solution is a manual switch to keep it ON the whole time, others have done this on track cars.

When the car is moving though you'll have air flowing through the radiator so you wont see the dramatic ON-OFF cycle you see when the car is standing still.

Thanks Lokiel - did just that - put a manual switch in. Number 1 son (doing engineering at Uni) had a look at the circuit, and we had joined the fans before the relay like this:
Microtech ECU output > Mazda ECU out put > joined (with diodes to prevent backward load between ECUs) > to relays > to fan motor.

The trick to fixing it was to join trigger signals for the Coolant and AC fans AFTER the relays, instead of before, like this:
Microtech ECU > Mazda ECU > Relays > Coolant and AC fan trigger outputs joined > fan motor.

Now it all runs fine, and the single fan comes on regardless of whether the AC or the coolant calls for it .This info would only be useful is there is someone out there with an older piggyback ECU setup who is wanting to go to s single SPAL fan - but you never know .

Does it have a separate "Fan on" and "fan off" temperature settings? You could put these further apart.

Yeah good point Okibi, have been putting off getting the $150 microtech dongle, think I'll get one to have a peek inside the black box.