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post #1 of 21 (permalink) Old 03-24-2019, 04:22 PM Thread Starter
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Trac fault light

I wonder if anyone can assist regarding a Traction Control problem I have been experiencing. The car is a Continental R 1998. First problem signs were that the car started cutting out at 70 mph ish and at the same time the TRAC light would illuminate by flashing on as the car did one of its cut out jolts.
I was surprised and relieved to find that by switching TRAC OFF I eliminated the problem. Although several 'experts' have told me that this should not have made a difference, it definitely eliminated the problem
An independent checked the car and advised that there was a fault on the NSF sensor and advised changing the sensor and the phonic gear wheel from which it takes its signal. How this was diagnosed I have no idea but I followed the advice and replaced the sensor. I did not replace the phonic gear wheel as it seems odd that a part which has no mechanical or electrical function and is simply a piece of machined steel could be faulty if appearing 100% OK.
For a short while I drove the car without the sensor and found that the speedo did not function nor the mileometer. This seemed understandable if that sensor provides the engine management system with the wheel speed and thus the speed to the speedometer. However, for the first time the TRAC light changed reading to TRAC FAULT.
I have now replaced the sensor and the speedo etc is all back to normal and after several road tests it seems that the cutting out has ceased but the TRAC FAULT light is still illuminated and now I am due to MOT the car so I need that light off.
Does anyone know if this is a fault which registers in the management system and then stays illuminated until a dealer plugs in a diagnostic and clears the error ? or whether it should have gone away if the new sensor had cured the problem.


As always any advice gratefully received.
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post #2 of 21 (permalink) Old 03-24-2019, 04:42 PM
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I'm not familiar with ECUs or electronics as all my cars are pre ECU but I think your light may extinguish if you disconnect the batteries for a while but I could be wrong. It's worth a try.
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post #3 of 21 (permalink) Old 03-25-2019, 02:43 AM
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Hi

If its like standard ECU faults the light will stay on until the fault has been cleared by diagnostic unit, looks like a trip to your local RR shop

Rolls Royce
Silver Spirit
1991
chassis 34285
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post #4 of 21 (permalink) Old 03-27-2019, 06:17 AM Thread Starter
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Just to keep information up to date..... one reputable Indy advised me that the TRAC FAULT cannot be removed with a diagnostic kit and that it was an ECU problem and that I would have to remove the ECU and have it bench tested as it should have cleared when the sensor was replaced.
Another Indy ( I am lucky to have 2 very good shops equidistant from me - 45 mins ) told me to bring the car over, they plugged in the diagnostic kit in the glovebox plug and cleared the fault in about 2 minutes.
Very depressing to find that such a difference of diagnosis exists even amongst the 'experts'.
The post script is that as soon as the fault was cleared the traction cut out symptom started to kick in again at motorway speeds. However, the TRAC FAULT signal did not reappear and I am able to switch off the TRAC again from the centre console.
The shop that cleared the fault were adamant that they have seen the problem numerous times ( and they are Continental R, T etc specialists ) and that the cure is to replace the 2 front phonic wheels to ensure that a clear equal strength signal is coming from both front wheels. One new phonic wheel , they say, can still have an imbalance in signal and thus cause the traction control system to assume a problem has occurred.
I will set about that job now, which apparently entails removal of hubs, knocking out the 'pressed in' phonic wheel, replacement of bearing seals which will have been disturbed by the work, pressing in new phonic wheels and hopefully - hey presto - problem cured.
For those interested I will report back when the work is completed. For those not interested, I sincerely hope you do not experience this issue as it is awful when driving on motorways.
Motorman Thank you for quick reaction advice from first query. As an aside, is the diagnostic plug specific to Bentley or can we use other diagnostic kit on these cars ?
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post #5 of 21 (permalink) Old 03-27-2019, 07:38 AM
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I question the replacement of the phonic wheel (unless it is distorted). The pickup that reads the wheel is the sensor that sends the pulses. The gap distance and electrical termination would be your most likely problem.

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post #6 of 21 (permalink) Old 03-27-2019, 07:48 AM
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post #7 of 21 (permalink) Old 03-27-2019, 12:18 PM Thread Starter
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Many thanks Wraithman. Yes logic ( not to be ignored , even with old Bentleys ! ) would suggest that the chance of the phonic wheels being the cause of the problem as remote when they are visually in perfect condition-no rust, maybe a small amount of surface oil but no shape distortion or damage to castellations I.e.nothing that should break the 0 1 reading that the sensor should be picking up.
That is why I only replaced the sensor which did look knackered after 20 years and which put up a tremendous struggle to remove as the sensor housing had rust welded itself to the locating steel. I am thinking more clearly after reading the link that you kindly provided and I am going to measure the signal from the sensors and see if they look consistent as my next test. Not to mention checking voltage etc to ensure that there is no chafing or connection problem in the wiring. many thanks and I will report back.
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post #8 of 21 (permalink) Old 03-27-2019, 01:05 PM
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Surprising, indeed.

Have you put exactly the same sensors ? Where do they come from ?
Are the plugs and wires in excellent condition ? No crack, no rust ?

What is the simplest and cheapest ? Have the ECU test by a specialist or have the phonic replaced ?
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post #9 of 21 (permalink) Old 03-29-2019, 03:58 PM Thread Starter
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The sensor was from Introspares who obtain it from Crewe, so no worries there. Wraithman's suggested route of finding out whether the signals are the same on each side is definitely the best first step in my opinion. I will report back on that. Also Jean you are correct I have not closely examined wiring which could be brittle and thus giving irregular connection - good advice.
I have a business interest in an ECU, ABS PAS Reman company in the UK, so that will be on the menu if the signals look correct, however, I have not yet asked if they could use any of the test rigs to check the ECU - clearly not a job that will have come across the counter very often, if ever.
I find the lack of factory technical data, which is available on issues such as this, very difficult and I am always worried that misguided enthusiasm from an operative could fry an irreplaceable ECU, so caution is the catchword.
Thanks for all advice, it is good to have a sanity check when listening to so much advice from Indys, who sound clued up but can often be just guessing.
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post #10 of 21 (permalink) Old 03-29-2019, 05:21 PM
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The Bentley Heritage site has been posting alot of technical bulletins and service papers recently.

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