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How can I tell if I have a negative ground tach?

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J

John_Progess

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I am working on a 67 BJ8 which has been converted to negative ground and an alternator. How can I tell if the tach has been converted to negative ground? What will happen if it is still positive ground and I connect it to the negative ground system and try it? I finally got the engine started but I disconnected the tach for fear of damaging it. Thanks for your help and have a good day!

John
 
One the power wire going to it will show + 12 Volts from the coil
 
Keoke, if the car has already been converted to negative ground, then the power wire (i.e. the green one that comes from the fuse box via the fuel gauge) will show +12 volts when the positive lead of a voltmeter is connected to it and the ignition switch is on, and the white wire at the coil will show the same. But that doesn't indicate whether the tach itself has been converted. The only external change to the tach for negative ground is the direction of the loop in the white wire on the back of the tach. This can be a bit confusing, but if the tach has been modified for negative ground the loop should look like the photos below that I made after the conversion of my car. The one on the right shows the tach from under the dash.

The only other way to determine whether the tach has been converted is to open the case and look at the tach guts. There are only two connections that need to be reversed to convert from positive to negative ground, as shown in the other photo.

However, it doesn't cause any damage to the tach to connect it either way. It just won't work if the modifications haven't been made to the appropriate polarity or the loop is in the wrong direction.
 

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Keoke
This car is a partial restoration about a year ago. It has not been run since the partial restoration. The electrical was not completed so I am starting from scratch. If I hook it up as a negative ground system then the white wire from the ignition switch thru the rev counter and onto the coil will be +12 volts. Will this damage a tach that was originally in a positive ground car or will the tach just not work if it was wired for a positive ground system. I do not know if the tach was converted to negative ground. Thanks for your help and have a good day!

John
 
You may be able to tell by the loop in the rear of the tach. If the white wire (I think) has been cut and re attached, then the tach has been converted. This is the most common way to convert it.
TH
 
Keoke you really blew this one------:glee:
 
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