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Old 03-02-2008, 12:20   #1
aero
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cam position sensor

I can't start my diesel 75. I suspect the cam sensor as if it will start (it does occassionally) it will then run perfectly OK. The cam sensor is only used for starting.

Its not the fuel pumps or the relay, they are all known to be working.

The engine will turn but as the fuel pumps aren't coming on it doesn't start.

I repeat this is not related to fuel pumps!

I measured the cam sensor's resistance and any combination of the pins is infinate. (open circuit)

Is this normal, or what values should I get?
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Old 04-02-2008, 19:28   #2
AllanI
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It does sound like classic cam position sensor failure. I don't know whether checking the contacts can say if it's failed or not because it is a "Hall effect" sensor so has to see movement to give a signal. At "only" 54 it is one of the cheapest things to try. Been there done that and mucked about for too long before changing it. Although having said that I could still hear the pumps whining away even though the engine wouldn't start.
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Old 04-02-2008, 20:57   #3
kelvin
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I recall sitting through an apparently interminable lecture in 1967 and I always knew that learning about the Hall Effect would come in handy one day!
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Old 04-02-2008, 22:26   #4
T-Cut
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Quote:
Originally Posted by aero View Post
Is this normal, or what values should I get?
I think it's more about magnetic field strength than electrical resistance. I've read that you can get the field strength up a little by freezing the sensor for a few hours. This has been known to get the engine to start. It's little more than a diagnostic test really. If it works, you need a new one.

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Old 05-02-2008, 14:26   #5
ColinW
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Quote:
Originally Posted by aero View Post
I can't start my diesel 75. I suspect the cam sensor as if it will start (it does occassionally) it will then run perfectly OK. The cam sensor is only used for starting.

Its not the fuel pumps or the relay, they are all known to be working.

The engine will turn but as the fuel pumps aren't coming on it doesn't start.

I repeat this is not related to fuel pumps!

I measured the cam sensor's resistance and any combination of the pins is infinate. (open circuit)

Is this normal, or what values should I get?

There is a very complex chip in that particular sensor, whose inputs and outputs are high impedance, so any meanigful resistance measurement is not possible.

If you are interested, what goes on in that sensor is almost unbelievable. The actual Hall detector is but a small part of the processing which goes on in there. Because of the profile of the cam, it is not possible just to sense when it has got to a particular position. So there are lots of checks, hence the complexity of the system, which I have reproduced below.

The ABS and crankshaft sensors are a little simpler, but they all have a chip in there, to present the engine ECU with a good signal.



So not only the little magnet in there can get weaker, but the functionality of the processing can be affected by temperature.


Just a thought thought. A little fridge magnet placed on top, may just provide a little more magnetic flux to make it work.

Colin
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Old 06-02-2008, 12:22   #6
aero
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The magnet theory is b0ll0x
Sorry guys I just bought a new sensor, it sticks very strongy to the wing.

The old sensor doesn't.

But I still can't start the engine with the new sensor fitted, its exactly the same.

I found another Rover 75 round the corner, and the guy that owns it very kindly let me fit my old one to it, guess what the engine roared into life the first time.

So the magnetic test is INVALID!

I've given up now, I think I have an ECU fault. AT least now its a hard fault rather then than the intermittant fault I've been suffering for the last 3 months.

I'm beginning to think a good car is being let down by a ****e ECU system. If it is it will be the second time this ECU has gone tits. Its never been swimming either. Anyhow I need to find someone with a T4 scanner and a good deal of electrical know how - in or near Cambridge.

Any one want a low mileage Rover 75 diesel...I'm after a Golf I think

Last edited by aero; 06-02-2008 at 12:30.
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Old 06-02-2008, 12:56   #7
kelvin
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Have you tried a battery off reboot?
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Old 29-03-2008, 20:45   #8
roveressex
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Rover 75 starting problems

Are you sure its not cranshaft sensor as well

Can you get it going with easy start sprayed into the air intake
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Old 29-03-2008, 20:53   #9
roveressex
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rover 75 satarting problems

Just read again the fuel pumps are not coming on

They should really come on with cam or crank shaft sensor fault for about 30 seconds to 1 minute then switch off

But is does sound like fuel pumps fault maybe not the pumps that have packed up but something electrical not sure if they fused or relay but do know they both on same electrical circuit
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