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Join Date: Nov 2005
Location: columbia md
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tach stops at 4500-5000 RPM when air temp is hot

Tach is operational through the entire range when the air temp is cool... when it is hot out ( 85-95 degrees) the tach will operate up to 4500-5000 rpm and then stops.. doesnt fall back just stays at the 4500 rpm level and when the engine falls back below that RPM it starts registering again.. Anyone experience this problem or know what the repair might be...other than sending it out for rebuild....Thanks
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1987 930 -Guards Red
2001 740I sport Blk
2011 F-150 supercrew Blk
2014 X3 2.8 white
"What goes around comes around..."
Old 06-21-2008, 01:21 PM
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Crotchety Old Bastard
 
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Not an uncommon problem in our aging cars. A couple of things cause that. One is the glass moving as it heats up. The tach needl will actually touch it and freeze. Another is diodes that get hot in the sun and quit until they cool off.
Typically the tach will begin working again once you get air moving throught the vents.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 06-21-2008, 02:07 PM
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thanks brian... the diodes you speak of.. are they in the cdi box or are they part of the tach gauges internal electronics?
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1987 930 -Guards Red
2001 740I sport Blk
2011 F-150 supercrew Blk
2014 X3 2.8 white
"What goes around comes around..."
Old 06-22-2008, 09:26 AM
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brian ... is this the diode in the cdi box ????
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1987 930 -Guards Red
2001 740I sport Blk
2011 F-150 supercrew Blk
2014 X3 2.8 white
"What goes around comes around..."
Old 06-22-2008, 09:33 AM
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Sorry ... this is the CDI box schematic

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1987 930 -Guards Red
2001 740I sport Blk
2011 F-150 supercrew Blk
2014 X3 2.8 white
"What goes around comes around..."
Old 06-22-2008, 09:39 AM
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Crotchety Old Bastard
 
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The tach itself. The dash gets hot in the sun which affects the old electronics.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 06-22-2008, 10:14 AM
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