Wales
by Joske Vermeule, September 1
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Jun 14th, 2026
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Joined: Jul 2011
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Hello Folks,
When I had the car, '98 NAS +8, on a dyno last April, the tech mentioned that the tach was reading high but, since I was preoccupied with the process, I did not follow up.
Today I used my Faultmate Nano to check the tach against. With the engine running steady at 3,000 rpm by the tach, the Nano showed 2,640 +/- 4or 6 rpm. Seems quite an offset. Has anyone else run into this issue?
Does the tach signal come from the alternator?
Thanks for your comments, Skip #12034
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Joined: May 2010
Posts: 220
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I'm not sure, but I believe it might be from the crankshaft position sensor? David Poole (author of the GEMS treatise) would probably know, or maybe a call to Dennis Glavis at Morgan West.
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Joined: Aug 2009
Posts: 152
L - Learner Plates On
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L - Learner Plates On
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Hi, If I well remember, this signal comes from the alternator. I have exactly the same trouble (is it really one ?), I checked with my Rovacom and found that at the max RPM the tach is up by 500rpm. I am not sure that this can be corrected, so now I know that I can go up to 6000rpm.  BR. Daniel.
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Joined: Jul 2011
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True, it is not much of a problem. It is useful to know.
Another item I noticed yesterday is that the tach and speedo are rather loose in their holes in the dash. There does not seem to be any clip or tab holding these meters in place. Is this normal MMC construction?
As for the big offset in rpm shown on the tach, as noted above, I have more room to run up the engine than I thought.
Cheers, Skip
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Joined: Aug 2009
Posts: 152
L - Learner Plates On
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L - Learner Plates On
Joined: Aug 2009
Posts: 152 |
True, it is not much of a problem. It is useful to know.
Another item I noticed yesterday is that the tach and speedo are rather loose in their holes in the dash. There does not seem to be any clip or tab holding these meters in place. Is this normal MMC construction?
As for the big offset in rpm shown on the tach, as noted above, I have more room to run up the engine than I thought.
Cheers, Skip Hi, Normally tach and speedo are fixed like the others gauges: you have a kind of big calliper with two plastic nuts which keep the gauges fastened. BR. Daniel.
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Joined: Jul 2011
Posts: 15
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Danial,
Thanks for the clue. I was able to grasp the nut and tighten. Probably just worked loose over the years.
Skip
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Joined: Aug 2009
Posts: 152
L - Learner Plates On
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Danial,
Thanks for the clue. I was able to grasp the nut and tighten. Probably just worked loose over the years.
Skip You are welcome, Skip. BR. Daniel.
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Joined: Jul 2007
Posts: 28,900 Likes: 352
Salty Sea Dog Member of the Inner Circle
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Salty Sea Dog Member of the Inner Circle
Joined: Jul 2007
Posts: 28,900 Likes: 352 |
They can work loose. On my way to the Midland Section Driving Tests the other Sunday I felt something drop on my leg. Couldn't see anything, until I got to the event, then found the brass(? mine's a much older car - no plastic nuts!)) knurled knut on the floor. Knew it was off the back of a gauge - just needed to find the right one, which didn't take long! Have been along all the clocks (not that there's many in my car) and tightened the rest up.  Cheers,
Graham (G4FUJ)
Sold L44FOR 4/4 Giallo Fly
'11 MINI Countryman Cooper D All4
'90 LR 90 SW
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Joined: Jul 2007
Posts: 27,814 Likes: 749
Member of the Inner Circle
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Member of the Inner Circle
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Graham,
yes that happens to mine from time to time too.
I resorted to lock tight and so far so good.
John
JohnV6
2022 CX Plus Four
2025 Renault Scenic Alpine etech "Yvette"
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Joined: Aug 2009
Posts: 152
L - Learner Plates On
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L - Learner Plates On
Joined: Aug 2009
Posts: 152 |
They can work loose. On my way to the Midland Section Driving Tests the other Sunday I felt something drop on my leg. Couldn't see anything, until I got to the event, then found the brass(? mine's a much older car - no plastic nuts!)) knurled knut on the floor. Knew it was off the back of a gauge - just needed to find the right one, which didn't take long! Have been along all the clocks (not that there's many in my car) and tightened the rest up. Cheers, Hi , Graham, as you cannot tighten hard these nuts, normally you have to fit special washers with a profile like a "S". They maintain a certain pressure and avoid the nuts to loose. BR. Daniel.
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