Checking Dwell with a multimeter?
Thread Starter
Team Owner
Joined: Jun 2001
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From: Virginia USA
Checking Dwell with a multimeter?
My cousin has a 1972 Chevelle with a points distributor. He has a miss under light to heavy load and of course I have to fix it. Anyway I was going to rebuild his dizzy but I don't have a dwell meter. I'm thinking I can use the duty cycle settings on my Fluke. This is not your ordinary Fluke meter it's the $1000 Fluke that I use at work (robots and semiconductor manufacturing systems). Duty cycle is what dwell is, how long the points are conducting and how long they are not. I found the duty cycle instructions in my Fluke meter manual:
My question is where would I connect the DMM leads in the ignition system to get the percent reading then I can just convert that to degrees................or just get a dwell meter :lol My thinking is ground and the ignition side of the coil? Thoughts?
Thanks
How to measure duty cycle
- Set the digital multimeter (DMM) to measure frequency. The steps can vary by meter. Usually a multimeter’s dial will be turned to dc V (
) and the Hz button is pressed. The DMM is ready to measure duty cycle when a percent sign (%) appears in the right side of the multimeter’s display. - First insert the black test lead into the COM jack.
- Then insert the red lead into the V Ω jack. When finished, remove the leads in reverse order: red first, then black.
- Connect the test leads to the circuit to be tested.
- Read the measurement in the display. A positive symbol (+) indicates POSITIVE time percent voltage measurement. A negative symbol (-) indicates NEGATIVE time percent voltage measurement.Note: A positive reading typically indicates a circuit’s ON time and a negative reading its OFF time. On occasion a negative portion of the signal can create an ON signal.
- Press the beeper button (
) to toggle between POSITIVE time and NEGATIVE time percent voltage measurement. Note: The button used varies by digital multimeter. Refer to you model’s user manual for specific instructions.
Duty cycle basics
- Duty cycle is the ratio of time a load or circuit is ON to the time a load or circuit is OFF. A load that is turned ON and OFF several times per second has a duty cycle.
- Why do this? Many loads are rapidly cycled on and off by a fast-acting electronic switch to accurately control output power at the load. Lamp brightness, heating element outputs and magnetic strength of a coil can be controlled by duty cycle.
- Duty cycle is measured in percentage of ON time. Example: A 60% duty cycle is a signal that is on 60% of the time and off 40% of the time.
- An alternate way to measure duty cycle is dwell, measured in degrees instead of percent.
- When measuring duty cycle, a digital multimeter displays the amount of time the input signal is above or below a fixed trigger level – the fixed level at which the multimeter counter is triggered to record frequency. Slope is the waveform edge on which the trigger level is selected.
- The percent of time above the trigger level is displayed if the positive trigger slope is selected. Conversely, the percent of time below the trigger level is displayed if the negative trigger slope is selected. The slope selected is indicated by a positive (+) or negative (-) symbol in the display. Most multimeters default to display the positive trigger slope; the negative trigger slope is usually selected by pressing an additional button. Refer to a DMM’s user manual for specifics.
Thanks
Thread Starter
Team Owner
Joined: Jun 2001
Posts: 23,363
Likes: 1,059
From: Virginia USA
Update: The miss was caused by the coil breaking down after getting hot. My cousin told me when I got to his house he also had a stumble going from idle to accelerating, like from a stop light. I took it out for a drive to warm in up and sure enough when you press on the gas about 1/2 throttle it would buck and bogg then pick up and accelerate normally.
Got it up to temp and went to the garage. Pulled the cap and rotor and they looked good. Checked the points gap and it was set to .018 so I didn't change it. Put it back together and checked timing. I pulled the vac advance hose off and asked him if he had a rubber cap to cover the carb vac source (manifold vac) and he asked what I was doing that for. That's when it dawned on me that he set the timing himself with the vac can connected. I explained to him how the timing works, he knows how to turn a wrench just doesn't know cars that well. We proceeded to time his engine the right way. We ended up with 8 BDC and 34 degrees all in at 2800RPM. Took her for a drive and he had a big smile on his face. Now the thing will spin the tire (non-posi) and the stumble was gone. Still had the off idle miss a little though. Went back to the garage and I had a spare coil I brought just in case so we installed it just to see if it would help before going into the plug wires and plugs since it is so quick and easy to change. That fixed it. Now the thing runs real good.
No dwell meter needed!
I also fixed his windshield wipers and his radio. All in all a good day working on an 1972 Chevelle convertible. The only thing that was frustrating me is how far I had to lean over the fenders to get to stuff and the front opening hood. lol
Got it up to temp and went to the garage. Pulled the cap and rotor and they looked good. Checked the points gap and it was set to .018 so I didn't change it. Put it back together and checked timing. I pulled the vac advance hose off and asked him if he had a rubber cap to cover the carb vac source (manifold vac) and he asked what I was doing that for. That's when it dawned on me that he set the timing himself with the vac can connected. I explained to him how the timing works, he knows how to turn a wrench just doesn't know cars that well. We proceeded to time his engine the right way. We ended up with 8 BDC and 34 degrees all in at 2800RPM. Took her for a drive and he had a big smile on his face. Now the thing will spin the tire (non-posi) and the stumble was gone. Still had the off idle miss a little though. Went back to the garage and I had a spare coil I brought just in case so we installed it just to see if it would help before going into the plug wires and plugs since it is so quick and easy to change. That fixed it. Now the thing runs real good.
No dwell meter needed!
I also fixed his windshield wipers and his radio. All in all a good day working on an 1972 Chevelle convertible. The only thing that was frustrating me is how far I had to lean over the fenders to get to stuff and the front opening hood. lol
