75ZT Community

I have over the last week or so been looking at the Rover 75 fans on our cars ie the 3 speed four wires,two speed 3 wires and the two speed with silver resistor and thought it may help if a thread was stated so other members could also see and maybe add what they know about the cooling fans on our cars below you will find photos i have taken of the different fans their wires and control boxes some with PCB (Printed Circuit Board) board others with just relays.

To test if your fan is working start engine switch on demist and looking through the front grill to see if fan is spinning or not, if not the low speed could have stopped working on your fan if your a three speed fan with four wires going to the fan motor or if you have a two speed fan with three wires going to the fan no silver resistor visible through the grill behind the fan blades at 2.00pm, if you have a silver resistor and the fan is not working it is most likely the resistor as packed up and needs changing for a gold resistor.


Gold resistor




In the photo the left fan is a two speed fan with two wires from the motor and a silver resistor, the right fan two speed fan with three wires no resistor this is a original two speed.


Front view of the fans with a resistor and with out the front fan is off a diesel the cowling as a different shape from the petrol fan.

Hi Sorry for the delay been busy and have only found time today to bring this up to date, below you will see photos of the wiring and control boxes of fans our cars use some with four wires which is a 3 speed fan, some with three wires and is a 2 speed fan without a resistor, then a fan with two wires a 2 speed fan with silver resistor. Its these silver resistors that need to be changed to a gold resistor which is higher rated.

First pic Fig 1 is of two fans the back one is a two speed with silver resistor, the front one is a three speed fan four wires and no resistor these tend to brake down after about 60000-75000 miles, and need to be changed for the two speed with resistor but uprated to a GOLD resistor.
1

Fig 2 close up of the fans
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fig 3 is a fan from a 1.8 which is a 2 speed fan with 3 wires and no resistor these also tend to break down after a certain mileage ? and need to be change for a two speed fan with Gold resistor
3

Fig 4 is the control box and its wires of the 2 speed 3 wires fan above.
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fig 5 is showing a three speed fan with four wires this is from a diesel M47 again these tend to break down after a certain mileage and need to be changed for two speed with GOLD resistor,
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Fig 6 is the control box of the above three speed diesel fan this control box as a PCB board
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Fig 7 shows a two speed fan with silver resistor which is the weak link of this fan and needs to be changed if the fan is not working, change to an higher rated GOLD resistor 95% of the time when the fan as stopped working on these two speed two wire fans its this resistor.
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Fig 8 is the control box of the two speed two wire fan with silver resistor as above
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Fig 9 is the two speed fan with two wires which had the silver resistor but now as the higher rated GOLD resistor fitted this should last the life of the car
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Fig 10 the control box of the two speed 2 wire fan as above
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Fig 11 the same fans 2 speed with two wires left with silver resistor right with the GOLD resistor a before and after shot


Fig 12 both fans now fitted with the GOLD resistors and ready to fit rear view to either a 1.8 or 2.0ltr and replaces both the 3 speed four wire fan and the three wire 2 speed fan with no resistors just plug and play.
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Fig 13 front view of above
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If the two speed fan with GOLD resistor was to be used for a diesel the wiring in and to the control box needs to be changed and therefore is not just a plug and play as above.

Fig 14 control box of my diesel with wires being changed ready to to fit two speed fan with two wires and GOLD resistor
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Fig 15 I extended the wires so my control box could be fitted in the engine bay and not behind the fan in case anything went wrong in the control box in the future
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Fig 16/17 once all the wires had been changed with the GOLD resistor in place waiting to be fitted to my car
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Close up shot
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Fig 19 Fan fitted to the car the next day
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Fig 20 fan using the fast speed note the GOLD resistor in place behind the fan at 2.00pm
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Fig 21 the slow speed of the fan being used
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Fig 22/23 the control box now relocated in the engine bay next to the fuse box the reason for me making the wires longer than usual other wise you can just refit behind the cowling.
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23

To check if your fan is working either it be a four speed four wire two speed three wire or even a two speed with silver resistor start car a press demist on the air control unit and fan should kick in if not your slow speed is not working on the fan and if later of the the first two above needs to be changed, if fan with silver resistor 95% of the time only the resistor will need changing check them now the warmer weather is on its way cheers Arctic.

Hi Arctic,

that's a fantastic article you have put together - very informative. I need to change the fan on my 2002 MG ZT-T 2.0 CDTI to a two wire system with gold resistor. You mention that the control box requires modification in order to do this, would you mind providing more information on what mods are required please? Looking at the photo's I was kind of assuming that the Purple and Black wires must be the feeds from the car electrics and that if I got a 2 speed 2 wire fan complete with cowling and control box then I would just need to re-connect these two wires. Am I mis-guided?

Thanks again

Steve

Hi Steve.
if you get a two speed to wire fan with silver resistor from a diesel all you would need to do would be change the resistor, if you use a petrol two speed two wire fan with resistor again change the resistor but also the wire need to be changed in the control box i think you will find Dave (takestock) as wrote a little note on this somewhere on here.

When i first attempeted mine mine i pestered him something rotten but he still kept me in tune all the way to the end we have now just started to do a few at the nano's together most enjoyable indeed. Arctic.

If you are changing the fan and relay box together then it is plug and play, making sure you upgrade the resistor

If you are just changing the fan motor and using the existing relay box you need to follow this diagram and the footnote


NB all the switching is done on the negative side on the fan assy

That's great, thanks for the info guy's, I know where to source one complete with relay bbox from a diesel so I'll go for that and replace the lot.

Cheers

Steve

Hello Great article !

however on my 1.8T I have this controller.
And single spead fan with + and -. (like the kenlowe)
What to connect ? to the plus an minus of my new fan ?
Wich one is the slow speed relais an wich one the fast speed ?
thanks in advance, Remko

Hi Remko
the 1.8 will either have three wires entering the fan motor itself or two. if it has two then the black one ( neg ) should split into 2 and one of the leads should go to a resistor.
Before we go any further I could do with seeing a picture of the fan motor and wiring.

Hi takestock;

I used a aftermarket fan with only plus and minus....
Moved the controlbox next to the accu compartiment...
And closed the gap of the old control box with (dount laugh..) a TFT monotor foot, the plastic part.
Looks very airodynamic trough.

No resistor I was hoping to use the old control box. Now wired brown 12v + to the plus of the fan, and connected minus to the black wire of the control box.
On the right relay in my photo the left one.
Works fine but soens't stop rotating runs continiusly.
(p.s. excuse my bad English as I am a Dutch MG fanatic.)

funny thing is the TFT foot matched my car color perfectly .....

UPDATE ! I think I I've got it right...
Please advise ! connected the brown (+12v) one not comming from the control box to the + of the fan.
And the black one (as stated before) from the control box (the left one) tho the - of the fan.
Now the following happens :
when I start my car; no fan (with normal temperture set on the airco system e.g. 20 celcius)
After a few seconds the fan kicks in and the temperature gauge keeps on a normal level.
When I put the airco/heater on untill max the fan stops....
The engine temperature doen not get above 093 degrees (on the display in test/diagnostic mode).
When I put the heater from HI to 28 Degrees the fan kicks in directly.
Could not get my enigine over 93 degrees is this acceptible ?
And does my MG ZT replacement fan the right job a'm I missing something ?
please help/advise Thanks in advance !!!

OK the brown wire from the plug should go directly to the Positive of the fan. the negative coming from the fan motor should split into 2. one of the wires should go straight to the smaller of the two relays, the second should go to a 100watt resistor such as this
http://www.ebay.co.uk/itm/Cooling-Fan-1 ... 5824wt_913

And then go to the larger relay.

How it works:

Fan is live from start up and switching is done on the negative side. When the engine temp reaches 100 deg the larger relay is earther out by the BCU (Body Control Unit) allowing current through the 100watt resistor and the fan operates at a reduced rate. the fan will stay on untill the engine temp reaches 96 deg.

If the engine temp continues to rise at 106 deg the second relay will be activated and allowing the fan to run at full speed.

In addition to this the fan works in conjunction with the air conditioning. Start the car and set the A/c to Demist and the fan should run continuous on a slow speed.

It has also been known for relays to stick on or off as a secondary fault.

I have come across the situation where the relays have been seen to operate the opposite way round, if this happens IE the high speed is operated first, simply swap the two negatives over

hope the above helps

Added edition 2014

To get us started i am hoping or presuming you have the original diesel contol box and wiring as below here.

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Take note of the purple wire as this will be obsolete later and cut out

2



Also take note of the thick brown wire top right in the control box this to will be cut out later.

3



Here we have the petrol two wire 2 speed fan with red and black wires coming from the fan motor.

4



This is a quick snap shot of what we are aiming to achieve once all the wiring as be changed and cut etc.

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The red wire form the fan will join up with the thick brown wire from the plug end this will become clear later.

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As you can see here i have left a little of the brown wire so we know what it needs to attach to later, i dont always do this but on this occasion i have.

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Now we turn our attension to the black wire coming from the fan motor as here below.

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The black wire further down the line from the fan motor splits into two as here in this picture, note where i have made the cut after the joint.

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The adjoining black wire will go to one end of the gold resistor picutre below.

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The next step is we now pay attention the the diesel control box from our car remove the thick brown wire from the right relay the one the grey wire is also attached to, disregard the black wire for the moment as i am infront of myself in the photo

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With the thick brown wire removed from the male spade follow it back and through the rubber bung in the control box and cut the wire.

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This wire will now be joined to the thick red wire coming from the fan motor as here below.

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Now you know why i left the little brown wire coming from the red wire earlier on.

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Crimp or solder these wires together making sure you have added heat shrink before doing so. it can then cover the join and help with water proofing it.

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The other thick brown wire in the control box whcih is attached to the large relay will become our black wire later on follow it through the rubber seal in the control box wall.

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keep following it until you come to the split joint all this is much clearer as you would have removed the out sheath covering all these wires

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At the joint/split cut the wire close to the join, you can see from the photo below i have added a little black insulation tape so we now know this wire will become a black wire very soon.

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We now go back to the contorl box and deal with the thick black wire with a thin black wire joined to it, pull this off the male spade and move it to the next relay the one with the grey wire on and fit it to that male spade and relay, htis is here we moved the first thick brown wire from, at the same time take this oppotunity to cut out and remove the purple wire as this is obsolate, cut it close to the rubber seal as possible

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Now follow the black wire back to where it joins and splits into two on the main loom.

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The loose black wire which was originally attached to your old fan motor one of the four, you cut this but leave at least 4" showing from the joint.

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This will now be joined to the thick brown wire which had the little bit of black insulation tape one and the brwon wire which is connected to the large relay in the control box.

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Remembering to add the heat shrink before crimping or soldering the joint this as now turned the brown wire into a black wire in the control box on the large relay.

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Now look at the red wire coming from the contol box trace it through the rubber seal.

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Cut it to length and join it to the thick black wire that splits into two one going to the fan motor the other going to the resistor not forgetting the heat shrink as always.

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The last wire to deal with is the grey wire this joins to the other end of the gold resistor'

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This as i say joins to the gold resistor i have added a bit of black wire coming from the resistor as in these photo's below.

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The final job in the control box is to remove the first thick brown wire we cut as here the one not joined to any relay in the control box.

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Take care and make sure you are cutting the right wire cut close to the rubber seal.

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Your diesel control box should now look like this job done.

33



All that remains now is you refit the sheathing refit the fan to the cowling plug it into the main loom, then test it by switch the ignition to II and press the demist button, low speed should run continually.



Any questions please let us know cheers Arctic.

Hi Arctic,
You said some time ago..."First pic Fig 1 is of two fans the back one is a two speed with silver resistor the front one is a four speed fan four wires and no resistor these tend to brake down after about 60000-75000 miles. and need to be changed for the two speed with resistor but uprated to a GOLD resistor" Would you please clarify if typical failures meant total fan failure or low speed failure.
Also, do you know if a 3 or 4 wire fan without resistor could be wired using the full current wiring to operate at low speed with a resistor?

Hi Grant.
Most of the time if the four wire 3 speed fan fails on the low speed it will take the medium speed with it, until the motor is opened you will not know the damage caused, if the slow speed brush as been worn right down it may have damaged the commutator, ie ground a deep line into it thus making it no good, some also can burn out.

Most of the time it is better to replace with a two 2 speed with gold resistor, the later of your question I have not seen this done

Hi Artic, Thank you for that reply. I got distracted through the winter and now having a go...
I bought a 2 speed cdti fan with a cdti box as in fig 4 above and installed a gold resistor in it. (I can't use my old fan as it is broken)
I have just got around to fitting it. On removing my old fan, I found it has a control Box with pcb similar to the one in fig 20 with only one low speed relay connected.
Under the pub is a processor.
I note that the wiring connection in the LHf wing uses a different pin on the low speed signal.
The basic relay box in fig 4 would have a simple 12vdc switching from presumably the ecu, whereas the pcb controller may utilise a modulated low voltage signal from the ecu.
Do you (or anyone) know what type of signals go to the pcb type controller and if it's compatible with the relay box if I rewire the plug?
Thanks in advance, Grant

Hi Grant.
I note that Simon is dealing with this on the other channel

Yes thanks. Just been reading it.
I now need to know now how to test a box and the ecu output!