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Posted by: brant Apr 4 2004, 12:40 AM

Ok..
I should know this but I don't ....
I spent the day getting a car running... not driving, just running on the jack stands...
I finally go the wiring sorted out, got the tach working and she starts right up...


problem is: at an idle of around 1200rpm.. the tach is reading 2500rpm...


WHY?

is this a dwell issue or something?
I really think I have the wiring straight.. so would it be the tach itself failing?
I dunno
(I'm so not used to having points and FI.... I'm learning a lot)
brant

Posted by: ArtechnikA Apr 4 2004, 07:28 AM

is it a constant 1200 rpm error, or is it a constant 2X error ?

is it an unmolested OEM tach? 'cause it sounds like it might have been modified for V8 use (V8's have twice as many ignition events per rev as we do ... -- except i'd think it'd read halfspeed, rather than 2X... -- so nevermind...)

if it's a constant additive-type error, could be wonky grounding. for a multiplicative error, could be that, i s'pose, or -- well, i don't know what else. there doesn't seem to be any lack of 914/4 tachs to check out with a known good one...

make sure the tach wire isn't running right past any plug wires - especially the coil wire - that can confuse them ... are your plug wires in really good shape ?

Posted by: brant Apr 4 2004, 08:35 AM

good idea about the plug wires...
I don't think it is a multiplication/constant error, when I blip the throttle it seems to work fine but is obviously reading higher than my ears told me was accurate....

the numbers I used were guesses... as the car was at a high idle but reading 2500rpm. The tach worked fine for all of these years and this is my decommisioned race car... Its sat for months now and I've re-installed a street motor (both 4cylinder motors).... The tach has been rebuilt about 10 years ago and always worked flawless..

one thing I molested was that I rebuilt the rear motor wiring harness off of the 12 pin connector... but the tach works so makes me think that the wiring is alright...

when the motor is off it drops right down to 0. I'll definitely go look at how close to the plug wires or even coil wire It is that could very likely be one problem

thanks for the input rich and all...
b