john_p_b
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 posted on 9/4/06 at 11:42 AM |
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vdo rev counter reading half speed
changed my ignition set up a couple of weeks back from points to electronic but there's still the good old coil there that the tacho connects
to. sinse changing the coil and dizzy my tacho is now only reading half revs.
when i set my igintion the other night the timing light was reading 997rpm and the tacho was reading just under 500rpm so it's literally half!
any ideas as to why this is happening?
built a car, built a home, had a family. lost the family, lost the home, still got the car.
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mandbsheldon
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| posted on 9/4/06 at 11:46 AM |
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May be it's turning into a BEC
Leigh
Sorry nothing constructive to say
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john_p_b
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| posted on 9/4/06 at 11:57 AM |
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no such luck there leigh, i can get myself in enough trouble with the great lump of cast iron up the front so i think make it much faster and my dad
would be getting his garage back and a spare room in the house
built a car, built a home, had a family. lost the family, lost the home, still got the car.
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flak monkey
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| posted on 9/4/06 at 01:24 PM |
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What tacho have you got John?
You might just need to change the number of pulses per revolution that the tacho is counting if possible. Its physically not changed obviously
(assuming all 4 cylinders are firing...i.e. getting a spark) but for some reason your tacho isnt seeing the right number of pulses.
David
Sera
http://www.motosera.com
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mookaloid
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| posted on 9/4/06 at 01:24 PM |
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It might help if we knew what sort of ignition you have fitted
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auzziejim
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| posted on 9/4/06 at 02:21 PM |
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could it be anything to do with wasted spark?
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john_p_b
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| posted on 9/4/06 at 03:27 PM |
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it's a bestek electronic system.
the name of the type of distributor has completely gone from my head at the moment (need some sleep after last night) no points if that helps?!
before i just had coil and dizzy
now i've got coil, amp and dizzy.
it's a the gauge is from a vdo cockpit vision set 0 - 7000rpm
built a car, built a home, had a family. lost the family, lost the home, still got the car.
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auzziejim
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| posted on 9/4/06 at 03:49 PM |
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i would check if the bestek system runs a wasted spark system i believe that would cause a 50% reading on a tacho
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Jon Ison
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| posted on 9/4/06 at 03:52 PM |
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Wouldn't that make the tacho read higher though ?
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rusty nuts
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| posted on 9/4/06 at 04:30 PM |
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Twice as many sparks . Do the VDO gauges need switches set to calibrate for type of ignition systems ? Sure my Greengauges have to
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john_p_b
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| posted on 9/4/06 at 04:44 PM |
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yeah the gauge has dip switches on it but they only vary between 4 - 6 - 8 cyl engines or for if it's running off a alternator (diesel
engine)
the pick up for the tacho is still coming off the negative from the coil like it always has and there's no chance anything on the tacho has
changed as i'd have to strip the dash back out to get at the switches on it.
built a car, built a home, had a family. lost the family, lost the home, still got the car.
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vintagebuilder
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| posted on 9/4/06 at 04:44 PM |
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Hi,
Had exactly the same problem with my Greengauge tacho. It turned out that I had incorrectly set one of the switches to suit the correct Pinto ignition
module.
Regards
David
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john_p_b
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| posted on 9/4/06 at 04:47 PM |
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why can i see me ending up pulling the dash back out
built a car, built a home, had a family. lost the family, lost the home, still got the car.
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rusty nuts
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| posted on 9/4/06 at 06:24 PM |
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Can you not reach behind and release tacho fixings to allow tacho to come out and get to switches
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john_p_b
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| posted on 9/4/06 at 07:12 PM |
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paneled under the dash to make it look tidy.
if i take the battery tray/ scuttle out i'll get to it that way.
didnt take into account removing things when i fitted them. must be too used to working on renaults
built a car, built a home, had a family. lost the family, lost the home, still got the car.
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Bob C
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| posted on 9/4/06 at 07:36 PM |
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Hi John,
sounds weird to me. I had a (sort of) opposite problem when I went to electronic ign on my V8 - at low revs the tacho read high, but it "went
right" at over 2500revs. I fixed this by splicing a low pass filter (RC) into the signal at the coil end. I guess the point I'm making is
that it sounds as though it might have been reading wrong in the past, so was set wrong (2 wrongs making a right) & is now reading right, if you
know what I mean.
However when the V8 was reading wrong the needle used to wander a bit... it was obviously not right!
Displaying too high revs was (for me) caused by the 'improved performance' (with the amp in) causing each spark to be noticed twice or
more by the tacho - hence it thinking the revs were higher.
Another possibility is that the points system has goosed the tacho - but that doesn't sound very likely to me.
Best of luck!
Bob
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john_p_b
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| posted on 9/4/06 at 08:11 PM |
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the tacho always used to wander around 200rpm either side of what it was actually running, sinse the ignition change although it's only reading
half engine speed it is extremely stable!
i always used to put it down to a earthing problem.
built a car, built a home, had a family. lost the family, lost the home, still got the car.
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Bob C
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| posted on 9/4/06 at 10:00 PM |
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Sounds like that might have been your problem then - now you just have to set the tacho up like it always should have been!!!!!
ATB
Bob
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