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David Towers
Prolific User
Username: xtriple

Post Number: 190
Registered: 6-2010
Posted on Tuesday, 31 October, 2017 - 02:19 am:   Edit PostDelete PostView Post/Check IP

I have asked over on the 'other' forum but I'm asking here as well 'cos it's my birthday and I can so be nice to me! :-)
Out today and the rev counter (tachometer) was randomly jumping by a few hundred RPM with no change in engine speed - in fact, no RR built V8 could change revs that fast, ever! It was also doing it at idle, jumping from 550-600 rpm up to a 1000 and back in quick time. It was a merry dance...

Any ideas why? On other makes of car I have noted that tis sort of behaviour is often indicative of other problems (ECU, crank position sensor etc) but the car was running fine. Later on in the day, it was behaving properly and I did watch it like a hawk I swear, only had about three close calls with other road users due to watching the clocks instead of the road...

Please tell me this is a really easy fix that will cost peanuts!
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Mark Luft
Prolific User
Username: bentleyman1993

Post Number: 124
Registered: 10-2016
Posted on Tuesday, 31 October, 2017 - 04:09 am:   Edit PostDelete PostView Post/Check IP

Happy Birthday David!! 29 again?

I have had this happen on other cars with the culprit usually being a bad ground. A (hopefully) EZ fix.
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David Towers
Prolific User
Username: xtriple

Post Number: 191
Registered: 6-2010
Posted on Tuesday, 31 October, 2017 - 08:13 pm:   Edit PostDelete PostView Post/Check IP

Thanks Mark, no mate, I will admit to 30 this year :-)

Could you point me in the direction of whatever ground you refer to? When it comes to electrics/electronics my brain melts and all I am left with is a soggy grey goo pouring from my ears!
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Mark Luft
Prolific User
Username: bentleyman1993

Post Number: 125
Registered: 10-2016
Posted on Tuesday, 31 October, 2017 - 11:53 pm:   Edit PostDelete PostView Post/Check IP

The instruments need to be grounded. Check all panel grounds, make sure all connectors are tight and pushed home. Using a contact cleaner is not a bad idea.

Best of luck!
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Omar M. Shams
Grand Master
Username: omar

Post Number: 1349
Registered: 4-2009
Posted on Wednesday, 01 November, 2017 - 02:45 am:   Edit PostDelete PostView Post/Check IP

I have the same problem with my 96 Continental R David.

I am told it is the instrument itself. I am living with it for now. It doesn't always happen - only sometimes.
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David Gore
Moderator
Username: david_gore

Post Number: 2717
Registered: 4-2003
Posted on Wednesday, 01 November, 2017 - 07:24 am:   Edit PostDelete PostView Post/Check IP

Omar,

Is it possible you are getting voltage spikes in the power feeds to the tachometer sensor and/or the tachometer itself?

? is the alternator voltage regulator misbehaving.

.

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