Pre-war (up to 1942) Tach/Dwell/Timing Light recommendations for Positive Ground Vehicles

[J. Russo]

Hello,

Can anyone recommend a good modern Tach/Dwell meter and timing light for positive ground vehicles?

I have a very old dwell meter, but am unsure how accurate it is. I have a modern timing light that works but is very faint.

I have a modern multi-meter but it doesn't work for positive ground.

I bought a used Sun CP-7501 timing light that doesn't work. I also bought a used Sun 7601 Tach/Dwell meter but the measurements don't match my old meter.

Any recommendations are greatly appreciated.

[J. Gomez]

Hello,

Can anyone recommend a good modern Tach/Dwell meter and timing light for positive ground vehicles?

I have a very old dwell meter, but am unsure how accurate it is. I have a modern timing light that works but is very faint.

I have a modern multi-meter but it doesn't work for positive ground.

I bought a used Sun CP-7501 timing light that doesn't work. I also bought a used Sun 7601 Tach/Dwell meter but the measurements don't match my old meter.

Any recommendations are greatly appreciated.

John,

Since you have a 6v car the Sun CP-7501 will not work if you have it connected to the car battery, you could use a 12V external battery or a 12V power supply to power the timing light w/no issues, and use the pick-up clip as normal.

As for the Sun 7601 Tach/Dwell since you have a positive ground have you tried reversing the connections to it at the engine, meaning the red (will be chassis) and black to the ignition coil?

Any multi-meter (volt/amp/ohm) analog type does not care if you have a positive or negative ground to check voltages/amp as long as the test clips are reversed, with a digital type it should not care as it will read either as 6V (positive) or -6V (negative)

I have both (as a set) and they both works fine but again I'm talking on a 12V negative ground car.   

HTH

[TJ Hopland]

Timing light makes sense since its expecting 12v to run the internal electronics and running them off a 12v source is how I have done it.  It don't take much.  Do you have a trailer with electric with electric brakes?  Those have a little sealed lead acid 12v battery that is easy to move around and clip on to.  How about 12v tools?  You will need something to jam into the terminal slots. A jump pack could also work except its got to be one of the older dumb ones.  Those fancy compact new ones are too smart to be tricked into being a power supply.

The tach is interesting.  Is the dwell reading being taken between the green and red? Black not used?  IF so you should be able to stick the red to the battery - in this case.  I should go out and see if I can access the one machine I have around the house at the moment with a conventional coil and see how it responds with the different leads disconnected. 

[TJ Hopland]

The 6v P rig I have at home currently got converted to 12v N a few years ago so I can't do a real test but I was curious if you needed all 3 leads connected and you do so there must be some sort of voltage divider thing going on in those things vs a straight up voltage reading with some special calibration.

This all has me really curious how these thing actually work now. I know I have taken readings before on this rig before I converted it and other than needing the 12v battery for the timing light thought it just worked.  My meter is a Craftsman unit likely from the 70's or maybe early 80's but I don't think any of em were doing anything really different.

[J. Russo]

Thanks for the info. I'll have to get a 12 volt power supply for the Sun Timing light. I didn't realize I needed a 12 volt battery for it to work.

As for the SUN 7601 Tach/Dwell meter, since my Cadillac is positive ground I did connect the red clamp to a ground and the black clamp to the coil. The dwell reading on the SUN matches the dwell on my old unit. The RPMs do not match.

My modern multimeter measures voltage no problem. I'll have to double check the other functions like Tach/Dwell but I remember contacting the manufacturer a couple years ago and they said it does not support 6 volts and I believe positive ground.

I was mainly wondering what devices others might be using these days when tuning their prewar cars.

[TJ Hopland]

Ahhh...  this meter only has 2 wires?   I was assuming it had 3. I'm not sure I have used one with just 2.  So my tests don't mean anything since mine has 3. 

Found the manual:
https://www.scribd.com/document/616111827/SUN-Dwell-Tach


Says positive ground no problem and looks like you were correct with red to the frame and black to the coil.  No mention of 6v so that must be why the RPM doesn't work correctly.

Interesting looking at all the tests you can perform with this tool, no cloud subscription required.