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Old 20-02-2011, 19:35   #1
djkprojects
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Question HRW, Cooling Fan, Relay - problem

Hello guys,

I recently bought a Rover 420 '99 TD and having an issue with the HRW not working and also I suspect that the engine cooling fan is not working either.

Engine cooling fan - to be honest I'm not sure how to check if it comes on when the temperature is high but I'm 100% sure that the fan itself is working. How do I know that? There are two cables plugged into the radiator which I unplugged and connected together and the fan was working. The strange thing is that I took the the yellow cooling fan relay out of the fuse box and the fun was still on ??? According to the Haynes book diagram the power supply goes through the relay so I have no idea why it was still on. Any ideas ?

HRW issue - not working at all. The light on the switch doesn't come on either so I suppose it's something to do with the power supply (blue/yellow cable). I have disassembled it and measured voltage between ground (black cable) and blue/yellow and there is no power. One of the sugesstions I found here was to check if the oil pressure light goes off once the engine is on. I checked that and yes it does so I believe that MFU is working. I then took the relay out and connected T30 with T87 together in the fuse box and to my surprise the light on the switch came on and I could see the HRW working (sprayed the window with water ). I've checked voltage on T85 and T86 and it's 12V so this would mean that the relay is broken though I have tried Cooling fan one (which is exactly the same) but the HRW still didn't work. Would that mean that they are both broken ? Is there something else that I should check before buying new relays ?

As I'm a very curious person I took the housing of both (cooling fan and HRW) relays to see how they look like inside then measured resistance between T85/T86 which was almost zero (coil fine ?) and T30/T87 switch (also fine) which made me think that maybe it's something else.

Can you please advise ?

Thanks
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Old 20-02-2011, 21:06   #2
technozen
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According to the MG-R wiring diagrams I would agree that the cooling fan should not work if the relay is removed.

Is there any evidence of things being re-wired?


The HRW relay is controlled via the MFU, the 86 terminal should have a permanent ground connection with the MFU supplying 12 volts to the 85 terminal when the switch is pressed.

Is fuse M (20 Amp) in the engine bay fusebox ok?
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Old 20-02-2011, 21:28   #3
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Quote:
Originally Posted by technozen View Post
According to the MG-R wiring diagrams I would agree that the cooling fan should not work if the relay is removed.

Is there any evidence of things being re-wired?


The HRW relay is controlled via the MFU, the 86 terminal should have a permanent ground connection with the MFU supplying 12 volts to the 85 terminal when the switch is pressed.

Is fuse M (20 Amp) in the engine bay fusebox ok?

Hello Technozen,

With the cooling fan - I will have a look tomorrow.

With the HRW - I was checking the voltage coming out of the fuse box on 86/86 and I'm pretty sure it was constant 12V - can't remember though if I had the ignition and the switch on whilst testing - will try again tomorrow.

BTW - is this on/off or impulse type of HRW switch. When I was testing the it itself I noticed that it grounds yellow/white cable that comes out of the MFU only when I pressed and held the button. At that point I assumed that the switch grounds y/w cable which then triggers an impulse within MFU to turn the HRW on or off. Is that correct ?

Another thing is clock - I can see the backlight is on but there is nothing on the screen. I'm going to test connections but not sure how to take the clock module out Do I really need to take rear and centre console fittings off to get it out ?

With the M fuse - why would I need to check it? Isn't it Stop Lights fuse ? If so, then I'm sure they work fine.

Thanks
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Old 20-02-2011, 23:50   #4
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Pin 86 of the HRW relay should not have 12 volts on it, it's a ground for the relay coil.

Pin 85 should only have 12 volts present when the MFU switches the relay on.

The HRW switch is a momentary type, it pulses the MFU input to ground which toggles the relay on and off at each press of the switch, a timer within the MFU switches the relay off after 15 minutes of operation.

This switch input also means that the HRW cannot be left switched on accidentally.

Fuse M also provides the MFU with it's 12 volt operating voltage.
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Old 21-02-2011, 11:25   #5
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Quote:
Originally Posted by technozen View Post
Pin 86 of the HRW relay should not have 12 volts on it, it's a ground for the relay coil.

Pin 85 should only have 12 volts present when the MFU switches the relay on.

The HRW switch is a momentary type, it pulses the MFU input to ground which toggles the relay on and off at each press of the switch, a timer within the MFU switches the relay off after 15 minutes of operation.

This switch input also means that the HRW cannot be left switched on accidentally.

Fuse M also provides the MFU with it's 12 volt operating voltage.
Hello,

By Pin 86/86 I actually meant 85/86 sorry I've just checked the voltage again with the ignition off and there is 12V between these two ?? Would that mean that the MFU is not working ? The strange thing is that if there is 12V then with the relay plugged in the HRW would be on all the time but it's not :|

How can I check MFU ? Where is it anyway? Behind the passenger fuse box ?

Thanks
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Old 21-02-2011, 13:38   #6
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Hello again,

I've just discovered a very odd thing. I have measured the voltage between 85/86 in the fuse box with engine on and there is 12V. When I press HRW switch it drops to 6V, press again and it's back to 12V. It looks like the circuit in the car is fine.

I then plugged the relay in the fuse box without the yellow housing and measured the voltage on 85/86 inside the relay and there is no voltage at all. I took the relay back home to check it, plugged it into 15V computer scanner adapter and... it works !!!! There is a voltage on 85/86 inside the relay as well.

I'm going to clean the connectors and see what happens.

That is very odd

EDIT: cleaned the connectors and still not working

Last edited by djkprojects; 21-02-2011 at 14:36.
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Old 21-02-2011, 19:21   #7
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OK, I have tested the relay with the heater blower circuit and it definitely works. Also I have checked the current going through it and it's around 150mA which is enough to trigger the switch 30/87: 12V/80Ohm = 0.15A (80Ohm is the resistance of the relay).

Now when I plug the same relay to HRW circuit the current is 2mA and the voltage on 30/87 drops to almost zero (160mV) which would be correct 80Ohm * 2mA = 160mV. This means that there is not enough current to trigger the switch. Without the relay plugged in voltage is around 12V.

Any ideas why is that please ?

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Old 08-03-2011, 19:13   #8
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Hello,

In case somebody has a similar problem the faulty MFU was the reason. Replaced it and works fine now.

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