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MG TD TF 1500 - dwell rpm metering positiv ground

I have two meters but none gives a comprehensible result - no matter of changing polarity at the meter.

Which meter performs right if the battery is connected with plus to ground?

If you are talking about an rpm/dwell meter that only has two leads you probably want to connect the negative lead to ground and the other lead to the terminal of the coil that is attached to the distributor. If it's a meter with three leads I'd suggest that you the maker of the meter . Bud
Bud Krueger


I'd tried the first - no success.

You know that I'm not a native speaker. I don't understand " If it's a meter with three leads I'd suggest that you the maker of the meter" - is a word missing - perhaps "contact"? The meters I have are nearly half as old as my car ;-).

The missing word is 'Google'. Es tut mir leid.
What is the make and model of the meter? Bud
Bud Krueger


the meters which surely are cheap and have been bought in a hardware store are in a garage 20 miles away from my home. So I can't say immediatly.

My opinion is there are only two possibilitys for connecting the red and the black wire to ground and coil.

What kind of meters do you use for dwell and rpm if a car is grounded positiv?

If there are only two leads, red and black, it's a simple matter. Connect the black lead to the negative source. In this case, the distributor points connection. Connect the red lead to the positive source. In this case that goes to ground. The meter will be displaying the portion of time that there is voltage between the red and black leads. That is proportional to the dwell period.

I have such a dwell/tach meter. I'll see if I can find the instructions. Bud
Bud Krueger

There are two types of dwell meters: a 2 wire and a 3 wire. A 2 wire will work but you must connect the positive(red) lead to ground.
A 3 wire will not work, however you can make it work by using a second battery, to do so, connect the red wire to the positive of the second battery, the black to the negative of the second battery and the 3rd wire to the coil. You may be able to replace the second battery with a battery charger(depending on how smooth the output of the charger is).

J Scragg

Thank you

what Bud said was the first what I'd thought and what I'd tried because the value of the meter shall show the duty cycle which is the average between voltage an no voltage. But e.g. the presence of a diode inside can cause a malfunction.

One meter has two wires and the other has three.

The points are well seated and the engine runs - but the meter with two leads shows about 80 for 4 cylinders and after alternate polarity the pointer doesn't move. I can't test it with another car because all other cars around have an ECU. May be the meter is damaged caused by wrong polarity or it is too old. I'm not the only user of that meter.

I'll try the advise of John - the second meter has 3 wires. But I'm a little sceptic, because measuring the voltage at the coil with a second voltage source means measuring a floating voltage without a reference point.


Who is the maker and what is the model of the tester? Bud
Bud Krueger


the meters are in a garage 20 miles away from my home. So I can't say immediatly.

Sorry, I forgot that. Please let me know when you get the chance. Bud
Bud Krueger

This thread was discussed between 07/10/2017 and 08/10/2017

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