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tach issue
My 930 tacho dies between gear shifts , ie drops from say 5000 rpm to dead between shifts. It then comes alive again a few seconds later and hits what looks like correct revs. Then dies again at next shift.
I have checked alternator out put - 13.4 V at 2000 rpm. Is my tacho on the way out, or is it a symptom of something else? Thanks Alan
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-) |
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12-07-2011, 03:42 PM
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Fleabit peanut monkey
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Is the output steady of the alternator? Within a few hundredths of a volt? 13.8+ volts is more of what I see with working V-regs.
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12-07-2011, 03:48 PM
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Yes, seems stable - was not bouncing around. Seemed about the same at 1000 rpm too.
The other thing I should mention is the tach is quite happy idling at 1000 rpm and up to 2000 and back to 1000. It seems to die in the big drops in rpm. I wonder if some internal damping thing has failed and it just drops to the floor til it picks up a voltage again, or something. Alan
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-) |
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12-07-2011, 04:34 PM
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muck-raker
Join Date: Jan 2009
Location: Coastal PNW
Posts: 3,059
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I'd be inclined to say that your tach might be showing signs of failing, but I'd definitely be concerned about a voltage regulator starting to fail. Usually, a tach starting to "flip out" is a sure sign that a VR is starting to take a dump. I'd break out the voltmeter and start taking some readings to rule out the VR first.
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STONE '88 Cabriolet, using EP Slick 20w50 partial synthetic Snake Oil...just as Rommel intended.  Deny Everything; Admit Nothing; and Always Make Counter-accusations 
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12-07-2011, 06:06 PM
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How would you rule out the VR? I thought a stable 13.4 would be OK. I guess I could see if it is spiking at 5000, but not too keen on that. Could try revving it a bit higher - but the tach is reading OK at those revs, it is when I drop the revs that the problem occurs - but it is holding 13.4V at the lower rev end. I wonder if it is just falling free- fall and something in the mechanism is supposed to dampen that.
Alan
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-) |
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12-07-2011, 07:44 PM
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I would ask Racing Ray if he has a spare you could swap in for a quick fault elimination exercise. I think he has a bunch of guages left over from the leopard car. (RIP, that was the most exciting ride I ever had around Puke)
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1972 911T Coupe with a '73E MFI engine and 'S' pistons 10 year resto mostly completed, in original Albert Blue. ***If only I didn't know now what I didn't know then*** |
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12-07-2011, 09:15 PM
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What happened to the Leopard? Did I miss something?
Alan
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-) |
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12-07-2011, 09:51 PM
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I thought he broke it some years ago?, the Ivory Beast is still driving I believe.
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1972 911T Coupe with a '73E MFI engine and 'S' pistons 10 year resto mostly completed, in original Albert Blue. ***If only I didn't know now what I didn't know then*** |
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12-07-2011, 10:07 PM
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muck-raker
Join Date: Jan 2009
Location: Coastal PNW
Posts: 3,059
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as usual, I neglected to read the entire post....I just saw that you've already done a diagnosis. I'd probably rule out your VR. I apologize for my uninformed post (again).
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STONE '88 Cabriolet, using EP Slick 20w50 partial synthetic Snake Oil...just as Rommel intended. Deny Everything; Admit Nothing; and Always Make Counter-accusations
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12-08-2011, 08:58 AM
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I had a think about this problem and I do not think the tacho would be the likely cause. Its a simple pulse driven amp voltage output to a standard damped meter. Maybe try attaching one of those cheapo tacho/dwell meters you can buy at Jaycar and see what happens. I would be thinking about what could possibly disrupt the pulses from the distributor to the tach during shifts, presumably the motor is running strong. Is there a over rev or shift light wired in or some exotic electronic thingy on the wastegate?
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1972 911T Coupe with a '73E MFI engine and 'S' pistons 10 year resto mostly completed, in original Albert Blue. ***If only I didn't know now what I didn't know then*** |
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12-08-2011, 10:04 AM
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1981 Porsche 911 SC Targa
1972 Porsche 911T
