Professional Dwell - Tachometer Restoration! 

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For those wondering why 'dwell' matters in old cars with breaker point ignitions: The dwell angle is the number of degrees of crankshaft rotation in which the points are closed. During this period, the magnetic field in the primary winding of the ignition coil is building. When the points open, the magnetic field collapses quickly generating a brief high voltage, which is stepped up even more in the secondary coil eventually arcing across the spark plug gap. The longer the dwell angle (i.e. points closed), the more time there is to build a magnetic field in the primary - an increase in this magnetic fields leads to a higher voltage at breakdown and a better spark. The dwell angle is set by adjusting the breaker point gap - i.e. how far apart the points are when they're pushed open by the cam lobe. A larger gap implies a smaller dwell (inverse relationship) because the breaker points in the small dwell case are adjusted such that they open sooner on the cam lobe and are ultimately pushed farther apart at the top of the lobe. Because breaker points can be pitted due to arcing, adjusting the points gap distance (with feeler gauges) is relatively inaccurate (as the points faces may not be flat) and having a dwell meter does a much better job at getting a proper adjustment. Incidentally, I believe vintage race cars occasionally used dual point systems (and possibly additional coils ?) to maximize coil charging at high rpms - a scheme where every other ignition cycle would take place on the alternate coil/points set that would be fully charging during the previous cycle. I may have the details of this last part wrong. Anyway, I hope this is interesting/helpful to someone out there :-)

This is unique, always surprised by how your acumen in electronics applies to so many different fields.

I wonder how can someone dislike a video with this level of knowlege and care. Congrats, Mr. Carlson. Your content and you are simply awesome.

My old man taught me this, on a Craftsman in the late 70's early 80's..He's missed. Thanks for this

I have a Hawk hand held dwell tach meter and the casing is exactly the same shape as that circuit board so, mystery solved.

Knowing that you have moved the labs all around, it feels a little funny to see the old main lab in the intro and outro! Sears always had major manufacturers make their house brand devices. It is possible that the design really is a SUN product though it really does not matter. Nice job as is your normal! Even my "old" truck (1973) has electronic ignition so the dwell function to me is not needed. So many people today look down on the old way to do things. A needle is much easier to use when doing things like setting RPM as it is the way our brain works, analog not digital. Great job on a device I had totally forgotten about! NOW TO THE UFO device!!! :D

Thanks for the video. Sears tune up equipment was always very accurate OoB. I owned several over the years. In the 60's, the name they were sold under was Penske, as in Penske Racing Working on small sports cars, the most important readings were usually for low idle speeds. High speeds were not used as much, except for maybe checking your tachometer. The dwell meter was especially useful as that was the function that indicated the quality of your distributor bearings and points-springs. I was kind of hoping you were going to have to build in some calibration controls if this didn't have any originally. Please set aside some time in the future, to do one of those too. Thanks again.

Great video. Thanks.. The wedge shaped PCB is probably because the same board was used in a hand-held device variety of the same meter. Just a guess..

You have to remember that this meter was made during a time when Sears cared about making products well instead of today's habit of making things cheap and customers having to come back to replace it much quicker.

I work 6 mechanics. Not a single one knows what a dwell meter does. I had to teach all of them how to set points with a feeler gauge. The younger generation. Yikes!

love the video but i might suggest that a brief description of how the circuit converts hz to meter movement for rpm and duty cycle is converted for dwell

This is like apprenticing to Gandalf. All the old knowledge, spells, but most importantly, wisdom. You cannot buy this anywhere.

Hi Paul. You have just made lot's of shade-tree mechanic's and hometown racing team's Very Happy !!!

I think the last time I used one of these was probably 1983 and yes, it was from Sears! That and the stroboscopic timing light. Fun times.

Speaking of Sears Roebuck and Co, some decades ago you could buy from them model T spark coils. We bought one as kids to use for making a Tesla coil. That was around 70 years ago.

How ironic that you did this video as I just recently found a really nice Sears unit in someone's trash that does all these tests plus it has all the plug in test cables with it. Unit is in great shape. I'll probably never use it other than maybe the voltmeter and ampmeter. Looks good for the bench though. Also got a near mint complete old g.e. beige plastic portable record player. Can't wait to check it out.

There is no "big thumbs icon" so I will let you know in write I REALLY ENJOY YOUR VIDEOS !!!! thanks a lot for taking the time with this great, educational and quality produced videos!!!

I wish I knew half of what he forgot in electronics.

Hi Paul, Had the same unit. Saw less use when the HEI distributors started making an appearance. Don't know where my unit is now. I sold my shop when I retired in 2018 and closed down my trucking company. Enjoyed the video. You and the family stay safe. 73 WJ3U

When I started doing mechanic work over 50 years ago that was the tool to have to get your car is running great. I really enjoyed this video. It’s because of videos like this that I became a patron sometime ago. The only thing I can use my old dwell tachometer on is the tractor.🤣👍 I look forward to all your videos. Keep safe Paul.

The reason for the wedge shaped circuit board? My guess is that the manufacturer used the same board for the handheld model where the board is mounted in the handle with the meter movement above it.

That is the cleanest dwell meter I've ever seen in my life lol didn't know it can go out of spec sitting in a box.

I had a Sears VOM back in the early eighties and it was a pretty good meter. Not Simpson quality but plenty good for general use. I don't know if it's still spot on anymore but I'm not surprised your meter came back around with a little tlc.

Excellent. I have a Dwell/Tach meter that is much cheaper than this lovely thing and have used it since the 90s. Now I need a function generator to test it! There's always something 😄

It might be interesting to also have the complimentary video: How to Tune an Engine with an Oscilloscope. : )

Fine work, Paul. I thought, this thing will show/ give bad numbers by its nature of construction. But i was proven wrong. May I ask you something? Can you imagine to layout an build a special tool nobody ever made available , but a lot people need? I talk about an instruction or tool to check and calibrate wattmeters für HF and maybe VHF, too. Something to check an align in the 1W to 10W range, 100W will be a problem i think...? That will be a real great thing worldwide.. 73 de Olaf, Cologne-Germany

Wedge shape: probably a board from a hand-held model.

On a 4-stroke engine, each cylinder fires every 720°. The ignition system fires every 720°/(# of cylinders); Let's call this the "firing angle". A 4-cylinder has a firing angle of 180°, a 6-cylinder has a firing angle of 120°, and an 8-cylinder has a firing angle of 90°. The dwell angle is the rotational angle of the distributor the points are closed and the coil is storing up energy (magnetic flux) for the next spark. The tricky thing to remember, is that the distributor (and cam shaft) spins at 1/2 the speed of the crankshaft. The voltage on the coil's low terminal is small (near ground) during the dwell time because the points connect the low side of the coil to ground, or ground through a low valued resistor. For calibrating a Dwell Meter with a pulsed signal of variable duty cycle, the dwell angle is the fraction of the distributor rotation angle where the waveform is "low" (near 0V). Distributor Rotation Angle = 1/2 Crankshaft Rotation Angle Dwell Angle = (1/2)*(Firing angle)*(1-Duty_Cycle) For the 6-cylinder case with a 50% duty cycle, the Expected Dwell Angle displayed on the meter would be Dwell Angle = (1/2)*(120°)*(1-0.5) = (60°)*(0.5) = 30° It's not 50*(0.6) gets you the same answer, but I think you switched the decimal points around on the two numbers. I thought your viewers might appreciate a more detailed explanation. The dwell angle can be controlled by adjusting the "points gap". Side Note: PWM methods (frequency to voltage conversion, and duty cycle measurements) generally have excellent linearity.

I just calibrated a somewhat similar unit to the one in the video (a hand held style) that was also sold by Sears (Made in Japan). It could only read up to ~1000 RPM on the V8 scale even at the highest setting of the calibration potentiometer. I opened it and found a single electrolytic capacitor (47uF/10V) which is laughable since the unit normally run in a 12V-14V environment. I replaced the capacitor with a 47uf/50V and it worked beautifully and calibrated accurately throughout the whole range. Thought of sharing this for anyone who may contemplate such calibration.

Back in the day we used to buy open frame 1mA movements from Farnell (IIRC), and they'd arrive in all sorts of states!.....requiring spindle adjustment top and bottom to keep the coil level as well as adjusting the freeness of the movement. Yes, it was an art to get it all adjusted perfectly and linear.

Ive been looking for how to do this for ages. I have 2 of these. One is accurate and the other not and I didn't know how to fix it. I'm impressed with how easy this is to do. Thanks.

IS THIS REALLY FAIR! for a old school car guy this is like hiding peas in mashed potato's. twisting electronics with automotive, I love your videos this one made me think of all the times working on cars with a meter like that.

Sears is a way better department store then CA ever had! The first Amazon. Where could you purchase a kit house in the 1920 in CA?

Wow ,I haven't opened a Sears Dwell meter , I didn't realize how much adjustment was avalible,Great video,

You always impress me. There are times I laugh. You are a very good technician. As a retired Broadcast Engineer who cut my teeth on valve technology, I feel that you are a little bit excessive on the safety of high voltage circuits. It is still better to be safe. On transmitters we ALWAYS did several things. Remove all jewelry. Keep one hand in a pocket. Us the shorting stick to verify the power supply is discharged. Good work! Excellent audio! As always, nice production.

I was going to congratulate you on 300k subscribers last week and didn't get around to it. So, congratulations on 301k subscribers! Thanks for your great videos!

Mr. Carlson, I’m sure that you are aware that companies are not going to spend the time and the extra 5 cents to solider connections correctly. They are all about PROFIT. They make things now as cheap as possible and sell them for as much as possible. Products like this are made on a assembly line as fast as possible and as cheap as possible.

with these d'arsonval needles I also check needle balance {weight at end of needle}. This is done in horizontal and vertical position of meter to check that 'zero' position doesn't change. AAAHHHH memories.

Hey Paul. Now that you are using my kind of precisin tools do you need a new ball pein hammer? Hi Hi 73 W5DN

FYI, one would use a dwell meter while cranking the engine, usually with plugs remove, and set the gap while watching the meter. This also avoids getting oil on the point faces since you are not using feelers. Nice vid man

I love Mr Carlson's Lab!

If you think about this cheap supermarket little instrument, actually it is applied analog computing.

Please don't keep us in the dark. What kind of car are you working on? I have a 1933 Plymouth, a 1950 Ford and a 1968 Corvette. So I am very interested in vintage cars.

I rebuilt my serious exhaust gas analyzer and got It working flawlessly once again

Impressed is somewhat miss-understated, I really did not expect this 'Pro'! meter to be anywhere near as accurate, (so much is labelled 'Pro' now days and it means crap really) hats off to the designer and manufacturer, they really came up with something quite special, obviously the meter movement is of quite high quality not something you would get today unless you pay 'think of a number and double it' price, the old school stuff wins again, thanks very much for showing this Paul it was VERY interesting to watch and see it truly shine at the end!

The circuit board shape prob comes from its use in a hand held usage as I am sure one of the big tool brands did a hand held dwell meter with a smaller case but same gauge and function.

I can understand your skepticism working on a Sears branded piece of equipment. It works the opposite way, also. 25 years ago, my cousin handed me a top of the line Carver receiver to repair. I was excited to get my hands on it. It turns out, the receiver was actually made by Pioneer, which was a bit of a let down. I did get it repaired and back to my cousin.

Being a car mechanic , its been a very very long time i checked ther dwell angle on a vehicle, ahh the good old days.....

Learned a few things on this video. Strangely not about electronics but motors!

It has a thin angular circuit board because the same circuit board was used in a smaller handheld version that the circuitry was in the handle and above it was the meter gauge.

I liked this Vid I Have a Sears Tack/Dwell meter it is a dual meter it also has a ohm/volts meter

Hi Paul. Many years ago now, I used to be employed as supervisor of a small group of calibrators of mechanical meters. The device should really be calibrated and used in the same way, that is, if it is to be used standing up, it should be calibrated so, or if used lying on its back, then calibrate it in that position. With a good meter, any difference should be very small, but nevertheless when we issued a calibration certificate it would only guarantee the stated accuracy if used the same way it was calibrated.

You Or Genius Sir Very Detail Explain & nice Video Sir.

Great video! I just bought a vintage Snap-on tach-dwell meter. It needs re-wired, but your video will definitely help if I need to adjust it. Thanks for sharing.

Good timing! I use my old Sears unit (later model than this one) but just bought a Heathkit Automotive Scope to test the dwell accuracy thus calibrate - I should have just waited for this video!!

Paul, I look forward to learning from your adapter for this dwell/tach meter for it to work with 6V vehicles. Please show enough detail so that I can adopt _that_ to an 8V vehicle (i.e., a 6V Willys that starts a _whole_ lot better on 8V than it does on its original 6V! ;). I've got an "old" Sears dwell/tach test tool that I purchased in the mid-'70s that I'm sure needs a similar calibration like you've shown here.

Nice job. Good to stray out of radios and such. Maybe work on a cool old timing light next?

I've been a backyard mechanic since the 1970s. Over the years, I've collected several of these - '50s sun diagnostic analyzer, '60s Allen diagnostic analyzer, '70s Marquette diagnostic analyzer, '60s craftsman tach/dwell, '50s allstate tach/dwell, '70s sun tach/dwell, '80s sears charging system/tach/dwell. Wish I could post pictures here. Long term goal is to clean them all up & have them on display. Never even thought about checking their accuracy. Starting to get into arduinos, and have picked up some low cost electronics equipment, including the xr2206 function/signal generator kit. Would that work for this process?

Sorry Paul, I tried to follow your maths on dwelling, starting with 60°, and then.... 50% x .6 = 30°. I lost track of the way you use these numbers. I could understand 50% of 60° = 30°. The .6 totally escapes me. Maybe it is the USA school way of teaching percentages ?

I'll be honest, as much as I love older motors and older electro-mechanical systems, this stuff is why I'd much prefer to drive a vehicle with an electronic ignition haha. Great tutorial though, I'm sure this will come in handy due to my fetish for old automobiles!

Thanks Paul! Got out my old ArcherKit Engine Analyzer to check the calibration and sure enough it was off! That kit had you do the CAL of the tach using a line cord with a few 56K resistors in series with the Hot and Neutral. You plug it in the wall socket and attach your test leads to the ends of the resistors to get 60Hz (ish) line frequency. Got you close, thanks for the video and math to check the other RPMs!

Hi Paul, I was a little surprised when you soldered the first electrolytic cap, sans rosin flux. I learned from you how important flux is for every joint. Mmmm, just asking? I hope don't unfriend me for this. BTW I love flux and how well adds to the performance for soldering on anything that will take solder. Still love your videos Paul. Thanks for all you do. Gary ;-)

The tapered PCB is probably the same as used in the meters that had a tapered handle with meter display at the top.

How about fixing an old a/c spark plug oscilloscope

Thanks mr. Carlson. Interesting as always. That studio mike is a gem, excellent sound quality too. I'd love to see the electronics of that.

...Sticky movement or static electricity on plastic lens. Easily removed but some moisture, soapy water, dryer sheet, etc. Thumbs up!!

In the old service station I worked in back in the day, we had a big "SUN" cart with several types of portable meters you could select. The only one I ever saw any mechanic use was the Tach/Dwell meter. I was well worn while all the others looked brand new. It took a lot of punishment. No idea how accurate it actually was. LOL!

Yet another great video and Paul your knowledge knows no bounds !!

I have an old, different Sears Tach-Dwell meter too. I think I'll be doing this now. Thank You!

I learned smth new again 😊👍 TY Paul!

i had at one time a hand held one with the same board . that's why the board is not square so it con fit in the handle

All of the timing on my race car is done in the ignition box. My Dad and brother have a race pak in their car and we can change the timing during the run. I do have a dwell meter and a timing light in the garage in a box. I should dig in out and give it a go through. Congratulations on 300K Paul!!

Great topic. I have a vintage SUN dwell/tach meter and I just didn't feel like digging in the math to figure out how to simulate pulse train that will simulate engine RPMs or dwell timing. It is pristine except for some slight surface scuff in the paint. Internally it is totally clean. Not sign of any battery corrosion at all ever

Really a great educational video Mr C. on this type gear! I have a lot of Snap On electronic gear from my fathers garage that could use an alinement and refurbishment!

Always excited to watch another video of you!

My father had a tach/dwell meter, (which is still (probably) in his garage ;-), that he used to make fine adjustments to the points on his 69 Imperial. They used to call me "Mic Eye" when I was younger cause I could adjust the points without a feeler gauge. (I couldn't really tell the gap, but I had done it so much that I could get it REALLY close, so close that dad rarely had to adjust them once I'd set them) Thanks for the trip back in time Mr Carlson!!! I sure do miss my dad!!!

Nice video, does the square wave swing from positive to negative or just positive to zero volts? thanks.

Great video! Once again I learned a lot from your videos. Thanks, Mr. Carlson!

Great video, thanks for sharing!

My lifetime career was in electronics! 3 years at Westinghouse, 18 at Kodak and 12 at Rochester Institute of Technology! Loved my life’s work! Over those years I collected dumpster dive test instruments bound for the recycler! Most need repairs!! Want to fix them! Ha ha! Thanks for the videos!

Had one in the early 80's with the wedge shaped circuit board. It was also a Sears but was a hand held unit. The circuit board was in the handle and the top was a meter. Looks like when they made that model you had they could not be bothered making up a new board for it. Great video as always Paul.

Excellent video! I am really looking forward to the analog instrument movement.

Just finished fixing up a Canadian made Tuffy Tool model 1813 Tach-Dwell meter , thanks to this video. Having access to a function generator was able to calibrate rpm and dwell accurately. Was out by a bit due to out of tolerance resistors , linearity is perfect across the whole span of the meter. I use this for my Canadian ' 66 Malibu SS . Still running points as the car still has its original 283. Great Job Paul !

Please do a reverse-engineering video on this. I’m especially interested in how the “dwell” function works. I would have expected an integrator to convert PWM->analog voltage, but that would make the low end (15 degrees) to be at the left of the meter scale, and the high end (45 degrees) to be at the right end (full deflection) of the meter scale.

I always ask "if it says professional, Is it?"

Congrats on the 300k! Very much deserved. Best electronics channel in "RU-vid town"

Very interesting and a very good video, Paul; much enjoyed.

Now now, Mr. Carlson, there's no need to mock the dead... though it's pretty funny. 🤣 RIP Sears, the once-great retailer bean-countered to death. Do you have a vehicle with a 6-volt electrical system? 👍

i have a snap on thats built identical inside

Nice job again Paul, I have a quite large Dwell/Tacho meter in the cupboard for the last probably 30 years since last use, I might drag it out for a play. Someone might be able to use it, thanks 🙂

Just curious why you would say this would not work on 6 volts? Just the nature of the way it works I would not think that it would be very voltage sensitive. I have a Niehoff T-70 Dwell/Tach meter that I use on my 48 ford pickup that is still 6 volt and it seems to work just fine. I will be checking the calibration on it now. However it did appear to be working pretty accurately. You do have to wrap your head about the positive ground aspect of it when you connect it up however :-)

I haven't owned a car without an engine computer in 25 years but I still have my old Sears handheld tach/dwell meter from way back when. The timing light it came with has long since disappeared, but maybe now I can open up the meter and see if I can align it. Not sure what I'd use it for though. Any ideas for alternate uses? Thanks for the look inside these, Paul. Very interesting!

Sears...poor Sears...*Taps plays*

Also.... Being that this meter is so accurate, Is there any way to mod it ? So that you could use it for 1,2,3 and 4 cylinder engines ? And to also increase the max rpm to 24,000 for motorcycle mechanics ?? It would be a great tool for finding or setting up the powerband, because there isn't much if any lag-time ! Love your channel !

This brought me back to the 80s watching the technician calibrate our Allen Test scope. Great explanation.

In the day, I always wanted one. Now I have one for my 68 Karmann Ghia, Now that I know; I should align it. Thank you!

very cool meter - and really nicely described. Now... will you do a reverse of the circuit?

Would you know a good source for the schematics for these old meters? I have 2 old Fox Valley meters, the 965 electrical system analyzer, tach, dwell, volts, amps & ohms. The other is just a tack & dwell. I need to re-soldier some of the leads.

Glad I don't use points anymore, ie, gained 40km per tank-full switching to electronic. Now, how can we test if our cheap timing-lights have lag, ie, read late - lol -.