VCCA Home Forums TECHNICAL - GENERAL Electrical - 12 volt How to Calibrate Dwell meter

I have two old analog (needle) dwell meters. They have served me well, but it is a nuisance when I have to peer down closely to read where the needle is pointing. So I recently bought a new digital dwell meter (with large LED numerals) from PT. And when I hooked ALL THREE of them up at the same time, it was obvious that the NEW meter was consistently reading 4 degrees HIGHER than the two older meters.

I emailed the meter manufacturer and they said that they haven't had any complaints and that I should just return it to the store. So I did that. Brought the new one home and it reads EXACTLY the SAME - 4 degrees higher than the older meters. So, my dilimmena now is - which one to trust?

Can you think of any way to calibrate the new meter? And I've already looked at the points and it's a BEAR to try to measure the gap of them. This is on a 1966 327.

First I would check to see where the new meters are made. If overseas then I would suspect they MAY not be totally accurate. Next I would find someone that has a digital meter and compare readings. It's possible that the "old" meters are no longer accurate and you need to determine which ones are incorrect. I am always leary of foreign made items as sometimes they simply substitute what they have which is closest. For instance I purchased a cotter pin kit from HF this week as it contained 555 pieces for $3.83 . I only needed about a dozen 5/32"x1-1/14" pins but from the local hardware store (ACE) they were 30 cents each for a total of $3.60. Seemed like a no brainer as the HF kit contained 30 5/32 pins. The problem however was the HF 5/32 pins are .020 smaller than the ACE or correct pins. Maybe in some applications it would not matter bur seeing I was working on my brakes I wanted the correct ones.

I would say that your new digital dwell meter is probably more accurate than the analog dwell meters that you have.

I have several analog dwell meters and when compared to each other none of them read exactly the same. They are anywhere from 2 degrees to around 4 degrees off with each other.

The analog meters may be reading different due to slow needle action that the digital doesnt have. I would be more inclined to agree with JYD and believe the digital. I found similar when I first got my Snap on multimeter.
Tony

Yes, the response rate of the needle in the analog meter will be a factor.

There are some other simple checks you might want to make.

One is to see what each meter reads when you simply connect the test leads to each other. For all the meters the dwell reading should be the maximum based on the number of cylinders for that engine. A 6 cylinder should read 60 degrees because that means the point never open.

I have 2 older Sears analog meters. I have learned that I need to use the manual calibration adjustment before each use. The calibration will shift as the voltage of the internal battery declines with time. Plus there seems to be some sensitivity to how warm everything is.

I also calibrate them by connecting the leads together. I have determined that the "push to calibrate" button only shorts out the internal circuitry of the meter. That ignores any resistance in the leads.

Another point to remember is that on some meters you use the same calibration and meter scale for 4 and 8 cylinder engines. You have to double the reading to determine the dwell for a 4 cylinder engine. So a 2 degree error becomes 4.

Rusty,
Good comments. I've never thought about that before. I took my 3 dwell meters to our local "garage nite" on Wednesday. The host also had an identical Sears meter to mine. We hooked all four of them up to his 1966 Corvair. AGAIN my new digital meter was 4 degrees higher than ALL of the other three.

So I tried your suggestion. When I connect the leads together for the 3 analog (older) meters, nothing happens. That's because there is no battery in them.

When I connected the leads together for the new digital meter (which DOES have batteries in it), I get a reading of 65. You stated that I should see 60 on the scale. I assume that that is 360 degrees / 6 cylinder = 60 total degree window for each cylinder. But, it reads about 5 degrees high.

If I turn the dial to 8 cylinder, then the reading is 50. Again, 360/8 = 45. So I assume that it should read 45 but it's still 5 high.

Don't know if this is concrete evidence yet, so I'm still looking for a distributor tester that I can use to calibrate the new dwell meter.

Hi Bill,

Thanks for the follow-up testing and note. You are correct with the math for the maximum dwell for 6 and 8 cylinder engines.

My unfortunate experience with digital processors is that if the chip is not quite right there is nothing you can do to fix it.

About 3 years ago we did a kitchen remodel with including upper mid range cost appliances. The digital clock in the stove loses 1 to 2 minutes per month. We did not realize this until (of course) the warranty had expired.

Sounds like your meter has a defect. In this case it is an constant offset error which you can compensate for.

If you have access to a 1980's GM electric carb set meter on 6 cylinder scale and hook positive lead to green lead at carb and ground lead to a good ground. Jump A-B terminals on ALDL connector under dash on drivers side. Turn ignition to "ON" not start. Reading should be 30 degrees. The ECM is calibrated to give that reading with key on and engine not running.