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Re-calibrating gauges

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vping

vping

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At some point in my cars life it was changed over to negative ground.
Firstly kinda know what is means but do know what it entailed. Did all grounds have to be changed?
Why does my Tach not work propery & what else might not work because of this?
What needs to happen to the tach in order to re-calibrate it?
 
It's for the GT. At very low idle it is at 1500rpm and if I give it some gas it easily shoots up to 7000 rpm.
What do you mean that it is "fried" and what is your guess on a rebuild cost?
 
If it works at all then it isn't "fried". You may just need a new thingie inside it. /ubbthreads/images/graemlins/wink.gif

There is a resistor or capacitor, I forget which, that can be affected by heat and read way off. A full rebuild would probably cost around $100.
 
Look on the face of the gauge. Do you see the letters RVI or RVC? The RVI tachs are current sensing and don't work well with electronic ignition. If you've got electronic ignition (Crane, Pertronix, etc) and an RVI, don't waste your time trying to resolve this, replace the tach with an RVC type. If you have an RVI tach and points, start by looking at the back of the gauge. There should be a loop of wire on the back. Look at the direction of the loop. If it's clockwise, rewind it counterclockwise and try again. There are methods to calibrate RVI tachs on the bench. How comfortable are you with delving into your tach's electronics?

If the tach says RVC on the face, look for an access hole on the back of its case. Some have them, others don't. If you find a hole covered with tape, pull the tape off and look inside with a small light. If you're lucky you'll see a screwdriver slot on a white nylon wheel. This is part of a trim-pot used to calibrate the gauge. You can use your car as the pulse generator. You need a second reference tach to look at while you tweak the calibration (trim-pot) of the one in your car.
 
She a "Positive Earth" "RVI2401/00B".
I am not in fear of trying anything once as long as I understand it.

I have a tach from one of my midgets I can use. Will that help?
 
[ QUOTE ]
There are methods to calibrate RVI tachs on the bench. How comfortable are you with delving into your tach's electronics?


[/ QUOTE ]

Stock ignition & I am now comfy & ready to delve.
 
Would be careful of the fuel pump also. If the PO properly re-gounded the car, he either replaced the pump with a neg ground or rebuilt it. Seems there's a thingee in the fuel pump circuitry that will foul the points if current is passed the wrond way. I converted a '67 and lost the pump points 4 minths later.
I you aren't a purist, you can remove the guts of a '68 and later (neg ground) tach and put them in the case of your pos ground. Unfortunately, the '68 and late tach is of too small a diameter for a direct replacement.
Other than the pump and tach, though, ground pos or neg really doesn't matter.
 
I had the pump running for about an hour to drain the tank with NP. Later than week after I cleaned out the tank and was able to start it, the pump held it's own & was running fine. Only the Tach seems to be off.
 
Nissonger charges about $150 to do the conversion and calibration now. Had one done a few months back.
 
Sorry to have failed to re-post on the thread earlier.

Since the tach works (of sorts), you probably haven't let the white smoke out of anything. The previous owner may have already changed the necessary internal components. Since it's an RVI tach, start with the simple stuff. There should be a white wire loop on the back of the gauge. Reverse the direction of this loop and try your car again.

The following links may help a little:
https://www.classictiger.com/techtips/motach.html
https://www.lotus-cortina.com/electric/convert.htm and
https://www.speedycables.com/faq7.htm

There are trim pots inside most RVI tachs but not all cases have access holes in them to adjust the assembled gauge.

Dr., what conversion did Nisonger do for you? Ground conversion or other?
 
Yup, flipped polarity and cleaned/serviced/calibrated. From a bud's '67 MGB.
 
I have to try this after I get it back up & running. I have the Classic tiger info printed for a later read.
 
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