Dual-speed dual-fan circuit
Credit goes to Ian / Bull bar Cowboy from LR4x4 for designing this, I've copy-pasted it here for now.
Although this article applies to the Rover V8, you can easily adapt it to any engine if you know the normal operating temperatures.
When the V8 EFI was transplanted into the 90, it quickly became evident that the Rover standard fit of a junior propeller had to go and be replaced by an electrical fan arrangement.
The fans are Vauxhall Astra MkIII. The fans are laid face down and the triangular mounting parts of the fans are cut away, this then allows the two fans to be bolted to give a twin fan setup. The top and bottom of the fans are then joined using a short strip of stainless steel with a thickness of 1.5mm. This assembly is then mounted on the rear of the radiator using stainless steel “L” brackets. The built in cowls of each fan are in contact with the radiator thus providing a very good thermal air path. You can use any pair of fans you wish, though.
Control of the fans is achieved by the use of a dual thermal radiator switch (Intermotor 50091), which is mounted into the top hose. These switches are a standard fit item on many models and thus easily sourced in a variety of different temperature characteristics.
Note: This bit is kinda superseded by the X-Eng fan switch but I've included it for completeness:
The “splice” fitting for the top hose is constructed from a short length of 38mm diameter thick walled brass tube. The tube is machined (or drilled and filed) to take a standard 22mm temperature switch radiator fitting. These can be obtained very cheaply from most good radiator specialists. Alternatively a 16mm brass compression pipe fitting can be used as this has a 22mm internal thread (not to be confused with the standard 15mm pipe fitting that has a BSP thread).The fitting is then soldered into the brass tube which is mounted close to the radiator.
An alternative method is to use the thermostat housing from an export or “air-con” model as this is designed to take the thermal switch (the air con models have a 100°C switch fitted). However, the housing is in the order of £45 +VAT. As the coolant is some 6°C to 8°C cooler at the radiator entry point than the engine block , a switch fitted in the thermostat housing will need to reflect this difference (use a 50174).
The dual speed switching came about as it was found that both fans running at normal speed were too efficient - @ tickover, the fans were set to come on at 100°C and would cool the engine coolant to thermostat temperature in 15 seconds. Unfortunately that amount of heating / rapid cooling is bad news for the expansion and contraction stresses of the radiator and engine components. To overcome this it was decided to run the fans in series (a bit less than half speed) when the engine temperature rose to circa 95°C and to run them on full speed if the engine temperature further rose to 100°C. In practice the fans only come on when sitting in stationary traffic (the slow speed gently cools the engine down to 90°C and keeps it there) or when towing (on hills). Only once have I had the fans come onto full speed - the radiator was completely clogged with mud and I had driven from Wiltshire to Devon with the fans on slow most of the way, but at telegraph hill the engine temp rose to the magical 100°C. However the fans did the trick and the engine temp dropped back sufficiently.
Much of my work is in the design of Radio systems for Public Safety (mainly Fire and Ambulance related) where any single point of failure is designed out - hence two fans, dual switching etc - get the picture!
The master switch controls the supply to the thermal switch (s). In its off position it turns on one of the spare dash lights (amber, in the bottom right hand corner). Closure of the first thermal switch at 88°C (engine temp of 95°C - remember the 6°C - 8°C temperature difference between coolant in block and coolant in top hose) operates relay A. This completes the circuit of — supply to Fan 1, via normally closed relay D contacts to Fan 2. This connects the fans in series and under normal circumstances will give more than adequate cooling.
If you are giving the engine a hard time and the temperature rises to 100C the second thermal switch will close and operate relays B and D. Relay D contacts will change over and a supply via the relay B contacts will be directly applied to Fan 2 The closure of Relay B contacts causes relay C to operate and apply an earth to the negative side of Fan 1. Relay C also ensures an appropriate delay in applying the earth (if this feature was not included there was a possibility of intermittent fuse blowing dependant upon relay timing). This arrangement now allows the fans to run at full speed (parallel operation). A further feed is also taken back to the dash lights as a warning that the fans are on full (I used the spare green light - top row centre)
The supplies to the fans should come directly from the battery (Starter motor in most cases, as this in the shortest run). I run the feed to the main switch as a direct fused live, because having the fans remain on when the engine in switched off (and they then come on intermittently as thermo siphoning takes place in the cooling system) causes less long term engine bay problems due to heat soak.