6-cylinder Tach

[sphinx]

An Idea

Like most of us with straight sixes, I do not have a tachometer in the gauge cluster.  I have explored several options from replacing the gauge cluster with one that has a tach; swapping out the fuel gauge with a donor tach/fuel combo from another (big) truck; adding a small 2” tach in one of the idiot light holes;  adding an aftermarket  tach in the storage hole in the dash (I don’t have AC either);  mounting a gauge pod with aftermarket tach; and installing a digital multi-segment tach.  I’m sure I had other fleeting ideas that I just can’t remember them now.

All of these had some drawbacks and advantages. 

Replacing the complete gauge cluster with have the best look and feel but would require some rewiring especially if I used the oil, temp, fuel, clock, etc as I only have idiot lights in my cluster.  Then there would be the calibration problem as most tach clusters are set-up for 8-cylinder engines.  I’ve seen full gauge clusters with tachs on ebay with various options in the $100 to $300 range.  I could get a tach converter box or wire-up my own to make it work with a 6-cylinder.

Just swapping out the fuel gauge with a tach from a big truck (the ones with the little fuel gauge at the bottom) would be the easy to wire and look good, but would still have the calibration problem.  Again on ebay in the $100 to $300 range as these usually come with the rest of the cluster.

The 2” tach in one of the idiot light holes would be easy to wire just plug-n-play. I could relocate one of the indicators lamps (use an LED?) to somewhere else, but the small 2” tach just doesn’t appeal to me due to its small size.  Being able to actually see the tach is the whole reason for puting one on a stock 6  . . . I want to watch the needle go up and down (takes a lot to amaze me doesn’t it?).  This would not give me any real value as running this engine to the red-line  (around 4500 assuming it will even turn that fast!)  is just blowing gas out the tail pipe.  A 2” tack cost between $30 to $60 depending on which tach.

Putting an aftermarket in the storage hole would not be hard, but would not look too good either unless I modified the whole dash bezel to match.  Wiring would be plug-n-play and cost about $30 to $60 depending on which tach plus materials to modify the dash bezel.

Using gauge pod properly would be one of easiest as I could mount it anywhere convenient and it would be plug-n-play as well.  I think using one of these looks like I’ve got a heavily modified truck, so if I had 400HP with an induction hood it would look right at home.  But not on this weekend ride. 

I like digital readouts, they are clean, easy to mount and don’t require lots of space.  They just don’t look right with the rest of analog gauges to me.  I’m even going to take out the aftermarket Pioneer Stereo player and put a simple Delco FM stereo unit back-in the kind with real knobs and an analog station pointer. (I’ll add a aux input and tuck away the MP3 jack in the ash tray, shhhh . . . don’t tell). 

Of these ideas, I liked replacing the fuel gauge with a OEM tach/fuel gauge combo the best because I would not have to modify the dash other than a little rewiring for the fuel  guage and powering the tack.  But since I did not want to buy a full dash cluster just for a tach I try another option.

I decided to explore the possibility of modifying a fuel gage carrier, the metal face, to hold the “guts” from a modern aftermarket tach and lowering the fuel gauge meter to make a tack/fuel gauge combo similar to those found on big trucks.  This may make be a big waste of time and energy as may not work, but I’ll have some fun and learn some things along the way too.  I have spare basic gauge cluster from a donor truck similar to mine to experiment with.  Thoughts, comments, been there done that examples are all welcome!

[sphinx]

Experimental Tach

I went to my favorite junk store - ebay – to look for an inexpensive tach that I could cannibalize.  I found this one for $10 plus shipping.  It was about the right size at 3 1/4” with “universal” application that reads up to 7000 RPM; way more than I needed, but that was okay.

This unit is marked AutoLoc which per my internet search is part of the Hoffman Group which makes several gauge products under the Aurora brand.  This must have been an older model or a special production run as I could not find this model this unit on their web site. It looks like a decently built, modern stepper motor design.  Older tachs used conventional spring-loaded analog meter movements with some type of frequency to voltage conversion circuit to display the RPMs.  This one appears to uses a frequency counter and controller to drive a 4-pole stepper motor that moves the pointer to the desired position.

When I received it, it did not include instructions.  A quick email to the seller sent them to my inbox.  Even though there are 8 pin positions on the back connector only 6 of them have terminals and then only 3 of those are actually used.   Pins 2, 3 & 4.  Pin 2 is the pulse signal from the coil, 3 is ground, and 4 is for 12 volt positive.  There are also connections for powering the 2 internal lamps as well.  Before the instructions arrived, I went ahead and opened up the tach and traced out the connections on pins 2, 3 & 4 (you didn’t think I was going actually read the instruction first did you?) but was not sure about the 3 dip switches that I assumed set the number of cylinders.  Oh well I read them anyway - the instructions said that in addition to setting the number cylinders the dip switches could be set to read the alternator charging pulses on pin 2 when connected to the B+ on the alternator.   So, if you don’t have a tach connection on you distributor/ignition system you can connect this to the alternator as well (I assumed this is how to use it on a diesel engine). 

As I only need the tach to read up to about 5000 RPM, the remaining 2000 (5000 to 7000), about 20% or so of the indicator area, will be unused.  This being a universal tach, I could leave the tach set for a 4-cylinder operation and feed it a 6-cylinder signal.  As 6-cylinder ignition pulse is 1 ½ times the rate of a 4- cylinder pulse the tach would display a 5000 RPM 6-cylinder signal as 7500 RPM on the 4-cylinder setting which is slightly over the indicated max RPM buy may work.  To verify that this idea would work, I needed to test the tach with a 6-cylinder ignition pulse and make a re-mapped display face showing 5000 RPM as the maximum.

[sphinx]

Simulated Ignition

To test the tach’s ability to display a reasonably accurate RPM when reading the “wrong” number of cylinders I need a controllable ignition pulse.  I thought about hooking it up to the truck engine but how would I know the engine’s true RPM to verify that the tach was working correctly?  What I need is an adjustable signal that looks like an ignition pulse that can be adjusted from about 10 to about 300 pulses per second.  This would cover the 0 to 5000 RPM range of a 6-cylinder engine.  Here is a table of ignition pulse rates for given RPMs for 4, 6 and 8-cylinder engines using the following formula:

Number of cylinders ÷ 2   => this gives the number of ignition pulses per RPM for a 4-stroke engine, if you have an fancy ignition that fires on exhaust stoke as will omit this first step.
Times the desired RPM => this gives the number of pulses per Minute

Divided by 60 => this gives the number of pluses per Second or Hertz (Hz)

Example for a 4-cylinder engine at 3000 RPM:

4 ÷ 2 x 3000 ÷ 60 = 100 Hz

 Ignition pluses per second - Hz
RPM   4 cyl   6 cyl   8 cyl
500     17     25      33
1000    33    50      67
1500    50    75      100
2000    67    100    133
2500    83    125    167
3000   100   150    200
3500   117   175    233
4000   133   200    267
4500   150   225    300
5000   167   250    333
5500   183   275    367
6000   200   300    400

Notice that the 6-cylinder is 150% of the 4-ylinder and 75% of the 8-cylinder.  If you use an OEM 8-cylinder tach with a 6-cylinder engine, it will read 25% too low.  Example: a 6-cylinder engine at 4000 RPM (200 Hz) on a 8-cylinder tach would show 3000 RPM or 25% too low.  However, if connected to a 4-cylinder tach it would read 6000 RPM or 33% too high.

To generate an ignition pulse, I dug out my Engineer's Notebook 1 (Radio Shack, 1979) by Forrest Mims III.  I have used this old book many times over the years to build simple and useful circuits to support my many other hobbies.  By taking a 555 timer IC and a few other spare parts a simple pulse generator can be assembled that can make a pulse in the range of the table above.  I just happen to have enough parts in my junk box left from other projects to build this.  If you don’t have the parts they can be purchased from any electronics supplier or on-line for around $10.

I won’t bore you with electronic theory or complex timer circuit operation formulas because I don’t know any.  What I do have is some practical hobby experience and a lot of broken parts to show you what can go wrong.  So be warned!

Attached is a circuit drawing.
   

To make this pulse generator you need the following parts:

555 timer IC (a.k.a LM555 or LM7555, very common part)

12 volt DC power supply – this will also power the tach.  I used a leftover 12 volt 500 ma “wall-wart” adaptor from some other project that is long gone.

1 Timer capacitor (C) – value explained below

2 timer resisters (R1 & R2) – values explained below

1 filer capacitor – use a 0.01uf see drawing for placement.

Hook-up wire, “bread board”, alligator clips or whatever is available to hold the parts together.  They could be soldered directly to the IC if you choose; I used a breadboard and short jumper wire with alligator clips.

I also used an electronic multi-tester to verify the pulse rate, but this not required to make a reasonably accurate pulse generator.

This simply works by charging up a capacitor with voltage from the supply, then triggering the out-put pulse at about 12 volts when charged, dumping the capacitor charge and starting over again and again.   The timing capacitor and timing resisters set the time it takes for the cycle to complete and thus the out-put pulse rate or Hertz.


The timing rate is set by this formula:

Time in Hz or pulses per second = (1.44 ÷ ((R1 + R2 + RC)^C)) x 1000.

Don’t let this formula scare you as with the values selected for R2 = 2.2k ohms and C = 1uf the formula becomes 1.44 ÷ (R1 + 4.4) x 1000.

So with a value of R1 = 10k (or 10,000 ohms) we get 1.44 ÷ 14.4 x 1000 or 100 Hz.  If we look back at the ignition pulse table we see that a 100 Hz pulse = 3000 RPM on a 4-cyl, 2000 RPM on a 6-cyl and 1500 RPM on a 8-cyl.  Now we have an ignition pulse simulator.

[sphinx]

Controlling the Rev’s

So what values of R1 should be used for the various RPM’s?

One option is to use a variable resister or potentiometer (a “pot”) for R1.  A 100K linier pot would give an adjustable range from 0 to +300 that should cover the pulses needed to simulate an ignition system in a 4, 6, or 8-cylinder engine. This will work quite well if you want to view the movement of the needle as you can run the pulse rate up and down.  I did this several times and always given the since of awe that actually works.  I did not use the pot for the actual test because I want to change the pulse rates for comparison with different tach settings and did not want to have to “dial in” the desired pulse frequency every time.  By using fixed value resisters, the pulse rate should not vary when I swap out one resister value for another and back again.  If you are going to test several tachs with this method, I suggest you place each resister on a switch so you can simply “turn on” the desired pulse rate.

Here is a table showing the pulse rates using various “common” values of R1 when R2 = 2.2k and C = 1uf.  I picked common values to get a close as possible to standard tach reading spacing of 1000, 1500, 2000, 2500, etc. RPM.  As these are common resister values, there will be a slight variance in the actual pulse produced.  For example: a 5.6k resister may produce a pulse of 158Hz which is slightly more than 150Hz pulse at 3000 RPM on a 6-cylinder engine.

R1    Expected     Measured      Error
KΩ       Hz                        Hz              
∞          0           0            0
100       14           13.8         -1%
50        26           26.8          3%
23.5      52            53           2%
15        74            76           3%
10       100           102           2%
7.5      121           124           2%
5.6      144           147           2%
4.7      158           162           3%
3.9      173           178           3%
2.8      200           205           2%
2.2      218           224           3%
1.5      244           250           2%
1        267           272           2%
0.47     296           299           1%

The pulse rate is rounded to the nearest whole pulse.  So if you use the formula above to verify the table is will be slightly off for some of the values but still close enough for this experiment.  Note that the 23.5K and the 7.5K are 2 56K and 2 15K resisters in parallel, thus giving half the resistance.  I did not have anything close in my spare parts box so I improvised for these.

Resisters are not perfect electronic devices and neither is my circuit construction ability, so, there will be some variances in the expected pulse rate and the actual rate produced by the circuit.  I used my multi-tester to verify the actual pulse rate from the generator with each selected R1 value.

It looks like the average error is between 2 and 3 percent high for the pulse generator.  If you do not have a frequency meter, you can use these values and get a reasonably accurate ignition pulse with these values.

The infinite measurement in the first row is here to show that without a R1 in the circuit it produces no pulses.  When I first hooked up the circuit to the tach it read no lower than 2000 RPM no matter what I did.  Turns out I had a “leak” in my power supply letting the 60hz AC signal into the timer circuit.  This triggered the tach on both the up and down swing of the AC line voltage (2 x 60) causing a 120Hz signal on the output.  I put a large electrolytic 2200uf filter capacitor across the power leads from the powered supply to filter out the AC ripple voltage and the tach settled down to read 0 with no R1 in the circuit.  If you are using a sensitive tach ( I guess this one is as it can read alternator charging pulse signal) you may need to do the same if you are not using battery power.  Be sure to observe the +/- polarity of the electrolytic capacitorif you add one.  Strange things can happen when thay are hooked up backwards – the magic smoke inside can leak out causing it not to work anymore.  I even had some to go “BANG” when hooking up LED lights for a hot tub – but that is another story.

[sphinx]

How accurate is this tach?

New we have an adjustable ignition pulse.  Let’s hook it up to the tach and see if the RPM reading we get are close to what we expect them to be based on the pulses from the signal generator.  Attached is a pic of the results table.  I set the tach to both 4-cylinder and 6-cylinder operation. Note that the in 4-cylinder mode, with a 1K an 0.47K R1 the tach is pushed beyond the 7000 RPM scale.

The results were not too bad.  It looks like most of the error occurs at low RPM where the pulse rate is slow and the most “noise” can affect the reading.   The adjusted error in the table is a combination of the circuit timing error and the reading error in the tach.  I “eye balled” the observed RPMs so this is not scientific test.

[sphinx]

Put’n on a new face

I measured the “degrees” between the 1000 RPM segments on the factory tach face and found that are about 42 degrees apart.  Using the center line of the face a reference point and the 0, 3000, and 6000 RPM points as guides (0 will be 0 no matter what the cylinder setting) 3,000 RPM will become 2000 RPM and 6000 RPM will become 4000 RPM when using the set for 4- cylinders on a 6-cylinder engine.  So the for the new face the distance between each 1000 RPM should be 63 degrees.  6000 RPM minus 3000 RPM = 3000 RPM times @ 42 degrees per 1000 = 126 degrees between 3000 and 6000 RPM.  If 3000 to 6000 becomes 2000 to 4000 then 4000 – 2000 = 2000@126 degrees so each 1000 RPM = 63 degrees.  With this information and my CAD drawing package I made a new test face to experiment.  I tore the whole for the pointer rest when placing the face on.  I’ll have to measure it a better when I make a final one.

Here are the 6-Cylinder readings using he 4-cylinder setting and a new face:

Tach set @ 4 Cyl
R1     Expt     Obs     Obs
KΩ     6 Cyl    6 Cyl    Error
∞       0           0         0
100    276       100      -64%
50      536       450      -16%
23.5   1060     1200     13%
15      1520     1600     5%
10      2040     2050     0%
7.5     2480     2500     1%
5.6     2940     3000     2%
4.7     3240     3300     2%
3.9     3560     3525    -1%
2.8     4100     4050    -1%
2.2     4480     4300    -4%
1.5     5000     4800    -4%
1        5440     4900    -10%
0.47   5980     4900    -18%

[thirsty]

That's quite a write up. A lot of work to for a tach. Good job and thanks for sharing that info.

[sphinx]

Thanks thirsty

I usually don't make any notes when I'm trying ideas, then later I can't remember what I've done.  This forum is giving me a good place to document my "tinkering" as my wife calls it.

[sphinx]

Face Lift

After testing it looks like I some error in the lower RPM range and above 4500.   The upper end is not a problem as there will be little activity up there with my truck.  For the low end I would like it read idle speed correctly, so I marked up the replacement face with the actual positions from 500 to 2000 RPMs then adjusted the numbers on the face drawing to read correctly.  That caused a big gap between 500 and 1000 with smaller gap between 1000 and 1500 that looked odd.  So, I added a mark around 700 (I’ll call it “idle line”) on the indicator sweep this kind of tricks the eye into not noticing the different spacing between 500 and 1000.

[Jason S]

Curious investigation you have going. So you are going to keep the 4 cylinder setting on the tach and try to modify the 6 cylinder signal to the tach to adjust the reading at the tach? If so, is there an advantage to doing that versus setting the switches on the back of the tach to a 6 cylinder setting?

[sphinx]

Jason s

Yes using the 4-cylinder setting lets me use the full scale of the tach.  If i leave it set for 6-cylinder operation Ill lose about 1/3 of the RPM because my engine will never go over 4500 RPM and the tach reads to 7000 RPM.

[sphinx]

I saved a copy of my TurboCAD drawing of the new calibrated face to a .pdf file so I could print it on a color laser printer using an adhesive shipping label.  My color printer at home is an HP Inkjet and they uses water solvable inks, that would make a big mess if the tack got damp.  (FYI, if you have an older Epson Stylus printer they used non-water based inks.  I loved mine for making decals  models trains, rockets and airplanes [yep another hobby] but it died)  When I printed the .pdf on the color laser printer, all the color just came out gray even though it shows color when I view the .pdf.    I wanted to use the .pdf format because it will preserve the image size setting relative to the paper.  If I just save a .bmp or .jpg and print it will change the image size depending on the printer resolution ans probably wont fit the tach face.  So, back to the drawing board - literately.   

[sphinx]

Just a quick update.  I havent abandoned this project.  I need to go out to the donor truck and take the dash cluster out to see how Im going to retrofit the tach in the dash.  It has rained so much here in central Alabama that my donor truck is buried in mud.

[sphinx]

I had a slight change in direction.  I ran across this article by Kevin at http://www.howtoalmanac.com/kevin/projects/automotive/tachfixtruck.htm on how to re-calibrate a malfunctioning tach and it gave me an idea . . . what if the calibration of a GM 8 cylinder tack could be adjusted using Kevin's method so that it reads correct for a 6 cylinder engine?  so I visited my favorite junk store and came up with a tack from a dash cluster of an 8 cylinder truck for less than $20.   I used Kevin's method of adding an adjustable resister to replace the fixed value one on the tach circuit board. I wont go into the details as Kevin do a good job of explaining how to do this in his write-up. Then hooked it up to the pulse generator I used to simulate ignition pules for the other tach testing  (posted earlier).  I adjusted the variable resister until I got a stable reading at 2000 RPM using a 100Hz ignition pulse as if on a 6 cylinder engine.  Presto! As I changed the ignition pulse rate the tach followed accordingly.  It was a little bit off in the upper and lower ranges but that is to be expected with this old of a tach.  When I measured the value that the variable resister saw set at it was 322K ohms quite a bit more than the 260K factory value used for the V8, but it seemed to work consistently.

I decided to replace the variable resister with a fixed value one or the same value at permanently attache it to the circuit board.  I could have made the exact value of 322K by using sever resisters in series but that would take up more space on the board, so I had a 330K in my parts box, that seemed close enough to 322K so it should not make that much difference (< 2.5%) in this application.

Here are some pictures:

[sphinx]

Now all I have to do hack-up the instrument cluster carrier to fit the tack.  It will fit in the same location as the fuel gauge on my truck, but I have to cut out the back to allow for the wiring harness to fit the back of the tach.  Another problem will be figuring out what to do for a fuel gauge.  I could install a small LED bar graph in the seat-belt warring indicator hole in the lower part of the tack, but I like analog gauges.  Maybe I could retro fit the existing fuel gauge to fit like in the one in the C60's.

Here is the tach all back together with the new resister in place.   [Updated pic, as I had the wrong one attached, oops!]