another dumb electrical question

[eurobiketrash]

My 66 3000 has been sitting quite a few years and I have just got it almost road worthy. Upon first start up we found the tachometer is not working. I remember now that when I bought it the prior owner had someone install a battery in positive ground mode after it had been switched to negative ground !!! The wiring under the dash has a few dodgey spots in it so.............. Is there anyway that anyone might know to try and rewire/test the tachometer outboard of the wiring harness?

 

[KCC]

The tachometer is one of only a couple of components that are sensitive to ground and they are usually marked on the dial to show whether they are "positive" or "negative" earth. A negative earth tacho will not work with a positive earth car set up and vice versa. Worth checking this first.

 

[Johnny]

You could convert it back to negative earth and start the car. Or you could read about converting a car to negative earth then check the back of the tach to see which way the diode is wired.

 

[pkmh]

My tach is still set up for positive while my battery is negative ground. I don't suppose there is a nice picture displaying what is needed to be done to convert the tach from positive to negative?

 

[Patrick67BJ8]

My tach is still set up for positive while my battery is negative ground. I don't suppose there is a nice picture displaying what is needed to be done to convert the tach from positive to negative?

I reversed the two wires next to the loop going to the tach by cutting them and reversing them(crossing the circuit).
No problems with the tach.

 

[pkmh]

Thanks for the info. I will prepare photos/sketches to confirm this procedure at some point and post here again in the near future.

 

[HealeyRick]

If you're not originality retentive, you might convert everything to negative ground. It will allow you to run accessories like radios, gps, etc, a lot easier. Here's the instructions for a BJ8, including how to do up the tach:
https://www.hendrixwirewheel.com/polarity.html

 

[Keoke]

Keep the car at negative ground Google "The MG Experience" and it will tell you all you need to know about the conversion processes.

 

[busybrit]

I've recently changed out a wiring harness on a customers BJ8. One of the issues was teh looped wire was melted in several places, but the tach worked when it wanted to!

After the rewire the tach worked no longer. I opened up the tach to see a blown capacitor and a smaller electronic thingy that was melted. Suggest you do the same.