MR2 Owners Club

My situation - I have Rev2 Turbo, previous owner gutted AC, when my coolant reach over 90C my fans don't start and coolant starts boiling. This only happened once after returning to pits after track session. If I disconnect rad sensor plug fans spin at maximum speed.
My radiator sensor (https://toyota-europe.epc-data.com/mr2/ ... 01/88620C/ discontinued part) still might be fine, but I can't be bothered troubleshooting whole AC system - I have no AC anyway, so only need for fans to start once coolant is over 90C.

Solution - adapt radiator switch (https://toyota-europe.epc-data.com/mr2/ ... 401/89428/ from non AC cars readily available for £15 or even less) to work in AC car. Once coolant temperature reaches 90C rad switch breaks continuity, creating same situation as disconnecting plug, resulting in fans spinning at max speed.

Explanation of how I got to this approach
Original sensor uses resistance to report temperature:
Approx. 1.35k ohm at 85°C
Approx. 1.19k ohm at 90°C
Approx. 1.05k ohm at 95°C
Radiator switch has continuity until it reaches 90C, then it keeps circuit open until temperature drops to 85C

If you just plug radiator switch into AC car, fans will be spinning at maximum as car is thinking temperature is way over 95C as resistance is something like 0.001k ohm.
Trick is to make car thinking that coolant is not hot - as far as I can tell sensor is not used anywhere in engine management, only in controlling radiator fans and maybe something in AC which in my case doesn't matter.
So my solution was to add 2k ohm resistor onto one of the radiator sensor cables.

While coolant is below 90C sensor is showing 2k ohm - pretty much resistor value as radiator switch itself has negligible resistance. Car thinks coolant is cold and fans are not spinning
Coolant goes over 90C, radiator switch opens circuit breaking continuity and creating same situation as unplugging sensor. Fans are spinning at maximum
Coolant temperature drops below 85C, radiator switch closes circuit restoring continuity, car is reading 2k ohm again and fans stops



I've dry tested this with rad switch connected to the car, switch sitting in metal mug with water and mug being heated with a torch. As I still need to flush coolant and do other bits before taking car to MOT I will update this post later if I find something wrong with this setup.

If the wiring wasn't modified after the AC was removed, then the fans won't work as they should, they normally operate at 2 speeds, Slow (below 83 deg, both fans in series) and Fast (above 90deg, both fans in parallel)

jimi wrote: ↑18/12/21 23:51 If the wiring wasn't modified after the AC was removed, then the fans won't work as they should, they normally operate at 2 speeds, Slow (below 83 deg, both fans in series) and Fast (above 90deg, both fans in parallel)

This is for people who are not interested in AC working and just want rad fans to start once temperature goes over 90C.
I feel this is better, easier and cheaper solution than wiring manual switch to cabin or adding digital fan controller - both bypassing original circuits anyway.

My apologies if my post wasn't clear, this was never intended for people wanting to retain OEM behaviour.

No worries it's a good solution for anyone with a U/S temp sensor, if they can't source a good 2nd hand one.
If the sensor is good it's relatively easy to change the wiring to suit ( the AC being removed) the wiring diagrams show the differences, I did this for a friend many years ago, when the AC was removed from his MK2.
IIRC there's two links to go in at the AC triple pressure switch plug (engine bay) and a wire to move at the AC amplifier (behind the dash) and change the rad sensor to the rad switch. Might have been another wire to disconnect at the AC amp, but I can't remember for sure it was over 10 years ago.
The other way I've seen it done was with an adjustable temperature switch replacing the rad sensor, sensing bulb of the adjustable switch was strapped to the radiator hose. That worked ok as well and allowed some control over the fan cut in point.

Hi, just wondering if this method can be used for mr2 turbo with a/c?
Think the thermo sensor in the radiator has gone bad, as fans on constantly, and messed up the aircon function. I know the rad sensor is long discontinued, but could a standard switch be modified?

If not would a a/c rad sensor work from a lexus/celica or other model?

Modification I've described for the switch will trigger your radiator fans once coolant too hot. It just causes circuit to open, on AC car it's same as disconnecting radiator switch - this makes fans go full speed.

I have no idea how it would affect functionality of AC itself though

ryanaj79 wrote: ↑22/04/22 20:39 Hi, just wondering if this method can be used for mr2 turbo with a/c?
Think the thermo sensor in the radiator has gone bad, as fans on constantly, and messed up the aircon function. I know the rad sensor is long discontinued, but could a standard switch be modified?

No, the non AC one is just a switch, i.e. open or closed. The AC sensor is a thermistor i.e. a varying resistance

If not would a a/c rad sensor work from a lexus/celica or other model?

If it was the same fitment and the correct resistance value, then yes, but how do you find out. I doubt it, if there was an interchangeable one then the Toyota EPC would list it.