300Tdi 1997 Disco thermostat housing

**edddo** · 19th September 2025, 01:30 PM

Thanks for this post.

How would I go about adding a manual overide ie a manual on/off switch for the thermo fans?

I have another engine temp gauge in cab which I monitor to decide if I want the fans on or not.

Originally Posted by Blknight.aus

first of do something useful with the haynes, give it to a pulp mill so it can be reprinted as a newspaper or used as toilet paper. I've been known to stack them and drill them together and use them for wheel chocks and anyone who turns up at my place with a vehicle with a problem that they've tried to fix "according to haynes" gets the price jacked up.

with that step completed goto the dave can do IT banner and then purchase the Rave CD suitable for your vehicle.

there are 3 senders in the same area

a small single wire jobby which is the temperature sender

the two on the elbow. one kicks the fans on under all circumstances if the engine begins to over heat (even if the AC is off providing that all the fuses and electricals are functional) the other cuts the AC out completely.

to work out which is which just probe them with a meter, the one that is NC with the engine at room temp is the one for the AC cut out and the one that is NO is the one for the fan override.

to test them set up a good fan in front of the engine bay, open and leave open the bonnet remove the shroud, remove the fan VC wind the windows down and run the engine up at high idle with the AC on. the first thing you should notice is the fans coming on an staying on (you may need to pull the AC fan relay/fuse) then the temp gauge should swing off of the normal position to just under the hot/overheat point and then the AC pump should cut out. as soon as the AC pump cuts out from the overheat protection re-enable the AC fans, set low idle and turn on the fan in front of the engine bay. leave the AC off.

ON EDIT....

you NEED a good infrared thermometer to monitor the temps at the senders if it gets past 120deg C carry out the cool off procedures of going to low idle and getting the air flowing through the radiator, you should also have a hose ready to spray gently onto the radiator. (the normal temperature sender should move from the middle to hot at about 105-110 and then begin to climb rapidly into the red overheat mark)

DO NOT just shut it down the heat soak in the block will be enough to boil off the coolant in the engine and may be enough to either force the cooling system to vent out the cap or cause a hose/tank/radiator failure.

**Blknight.aus** · 19th September 2025, 09:37 PM

Originally Posted by edddo

Thanks for this post.

How would I go about adding a manual overide ie a manual on/off switch for the thermo fans?

I have another engine temp gauge in cab which I monitor to decide if I want the fans on or not.

depends on if you want, off/auto/on.

If you want auto/on (which makes sense to me) using M+F spade crimp terminal, with some 2 core double insulated cable pick up off the 2 wires on the sender that turns the fans on on overheat and connect it to a single pole single throw latching N/O switch of your choice.

Off/Auto/On can also be achieved but it needs a little more work and doing it the simplest way will result in a switch that gives Auto/Off/On. To do it you're going to use the same basic principle but you need a double run of 2 core wire, the same joiners some standard joiners and a Double pole, double throw, latching center off switch. To wire it up....

As per the AUTO/on wire up one set of contacts to bridge the over heat fans on switch, for the other, theres a relay that drives the fans, cut that and wire it to the other set of contacts for the switch.

(on edit)

For the relay, cut the earth or signal wire not the main power wire to the fan or the main supply wire, the current will be too high for most switches

**edddo** · 20th September 2025, 03:59 PM

Thanks Dave

Originally Posted by Blknight.aus

depends on if you want, off/auto/on.

If you want auto/on (which makes sense to me) using M+F spade crimp terminal, with some 2 core double insulated cable pick up off the 2 wires on the sender that turns the fans on on overheat and connect it to a single pole single throw latching N/O switch of your choice.

Off/Auto/On can also be achieved but it needs a little more work and doing it the simplest way will result in a switch that gives Auto/Off/On. To do it you're going to use the same basic principle but you need a double run of 2 core wire, the same joiners some standard joiners and a Double pole, double throw, latching center off switch. To wire it up....

As per the AUTO/on wire up one set of contacts to bridge the over heat fans on switch, for the other, theres a relay that drives the fans, cut that and wire it to the other set of contacts for the switch.

(on edit)

For the relay, cut the earth or signal wire not the main power wire to the fan or the main supply wire, the current will be too high for most switches

**Blknight.aus** · 21st September 2025, 07:44 PM

Originally Posted by edddo

Thanks Dave

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