Radiator fans not working auto

[CitizenK]

Hi everyone,
I drive a 1991 405 GL with air-con. a few years ago I had to change my engine with an identical one. However, ever since the new engine was fixed, the radiator cooling fan system does not work anymore.

I will admit that prior to the engine change I took little interest in these things, mostly because everything was working well. At first I noticed the temperature gauge was not working. After some research I located 3 single-pin sensors behind the thermostat head of which one was connected. I finally found out that the lowermost one read the temperature. However this did not rectify the fan issue and an electrician connected them 'direct', i.e. a wire was run from the ignition to the leftmost (seen from the driver) fan relay, however, he ran it to the power side of the relay, which sometimes led to the wire becoming very hot.
Also the fan was running once I turned the ignition key once, and it was the hi-speed, so it weakened my battery considerably. I eventually changed it myself, being an electrician (though not auto) and fixed a switch under the steering column which I activate when the engine temp nears 90 degrees.
This proves to me that the fan circuit is intact, because the power to the relays is now run through the two green 30A fuses in the small box under the bonnet. However, I am still at a loss as to find the trigger for the fans, more so, because I have air-con in my car, albeit inoperative for some years now because of a leak which I am told is under the dashboard. I have therefore not fixed the compressor yet or done any repairs to the air-con system.

However mainly because I want to fix it and because I am only human and can forget to activate the fan switch, I want to fix the system and have the fans run automatically. I have read a lot of differing facts concerning the fan switch.
Some cars seem to have it in the radiator (which I never had). Others seem to have the sensors in the thermostat head on the engine, however those seem to be double pin, whereas mine are all single.

Another thread I came across last week seems to suggest that non air-con vehicles had their fan trigger sitting on the right-hand lower side of the radiator, in what was essentially a low-coolant level sensor, but which also triggered the fan circuit.
This seemed to have been changed if the car had air-con, but was later re-instated (allegedly) even in air-con vehicles. (I have that coolant level sensor, and it works, when I put two contacts together, the warning light appears on the dashboard, however I cannot tell whether this controlled the fans previously or not).
The Haynes manual is not very clear about this and speaks only of a Bitron temp control. I have a control box sitting above the steering column under the dashboard and it says Bitron on it, but I have also read that the Bitron may have a component in the engine area under the nearside headlight (I have found nothing like that in my car).

I know this is all very confusing, but the last thing I want to present is two components which were delivered to my mother when the engine was changed, but were never fixed. Can anyone tell me what they are for and where they are fixed on the engine? I believe they have something to do with this problem.

[spider]

Those should be around the thermostat housing on most models.

You post is a bit long and confusing for me to read (my fault, not yours)

The single pin one can be either gauge or the 'overheating' as some models had these separately at that age, my 205TD did it had two single pins fitted in the housing, one for just the gauge one for just the warning light.

The twin pin is probably the bitron sensor. This controls the fans via two or three (later models I think) relays, One for each fan and an overheat relay. It connects them in series or parallel to be able to give a low and high speed. Not all models had twin speeds though.

On models with the 'overheat' protection, unplugging the wire to the sender should cause the fans to operate after a few seconds anyway, the bitron notices the missing signal and as a precaution signals the fans to operate. This may take a few seconds after removing the sensor plug before anything happens, but they should come on after say 10 seconds approx in most cases.

I wonder if when you had the engine replaced if the donor (assuming a used unit) came with a thermostat housing / sensors already fitted and they just plugged in what they had.

[CitizenK]

Spider thanks, and btw, I do know that my post was a little long! I was trying to incorporate all the snippets of info I have come across trying to fix this thing.
I have three relays and I know they ran on both speeds in the past. Yes the engine came with a thermostat housing. But as I said, what confuses me is that I have 3 single pin sensors in there, and no double pin, and all I have left to connect back there are double pole plugs. I have even tried to get some photo of an early 405 GL's engine, specifically the back which is so difficult to see when it is already fixed - unless you have a pit. I assume these sensors can be removed/replaced? Does water flow along them, and will it leak when I remove them?
Does anyone have a photo of the back of the thermostat housing?

[spider]

Water will leak when you remove them, and then the cooling system will need careful bleeding via its bleed points.

With a cold (must be cold) engine you can unscrew them and place a thumb etc over the opening then screw the replacement in, if you are careful / quick you will not lose any coolant nor take in air.

As far as I know the bitron is a two pin sensor.

Is the car carb or fuel injected ? A sensor for FI would be fitted too to monitor the coolant temp.

Diesel / carb units without AC normally have two single pin sensors fitted if I remember correctly, not sure why there are three present. Is it possible to obtain the old thermostat housing ?

[CitizenK]

Ok, as for the bleeding I've done it before a few times, so I know the procedure. The car is carb; unfortunately I can't obtain the old housing anymore (well, I can try anyway, maybe they kept it).
Is it possible to test the Bitron sensor, I mean like connecting it to the plug and dipping it in, say 100 degrees hot water? Would the fans kick in then?

[spider]

Not tried it that way but it should work in theory yes.

The fans should run all the time the AC is on too regardless of engine temperature.

I'm not sure about the low / high fan speed although on the gauge (on most models if they have centigrade markings) the fan should come on at 90c approx (probably low speed) and high speed a little hotter (guess about 97c) if needed.

I don't have any resistance readings for the bitron unfortunately.

[CitizenK]

Thanks, I read that about the fans too that they should be on low speed as soon as the air-con switch is pressed. If not they should kick in at 90degrees (yes centigrade, it's a long time I heard this word!) and then switch to hi-speed around 97.
Thanks for the info, I will try doing this either if I get home early or at the weekend and give feedback! In the meantime, suggestions, information from anyone else is highly welcome!