Dino Tacho and MSD 6A ignition

No I don't suppose it is the type that needs a high voltage given that your car probably came with a CDI originally? You could try switching to the emergency coil and running with straight Kettering with the brown wire from the tach connected to the points or the coil negative (same point electrically in that case). If your tach is kaput, the high voltage pulse probably won't do it any more harm anyway, but like I mentioned before, I haven't been able to break any tachs I've tried yet with the high voltage pulse of the Kettering when they were intended for a 12V square wave only. I'm sure Adrian LeHanne could answer these questions much more precisely than I can. Fred

 

I found this on the internet when searching "how to test an electronic tachometer":

bench test for tachometer - Chevelle Tech

From the forum:

"How to Check Your Tachometer"

I was doubting my tach and was wondering how to test it to see if it is actually accurate. On the alt.autos.rod-n-custom newsgroup from 2001 (used www.dejanews.com to search), I found a discussion that talked about using a 12 volt battery charger to check the tach. So what the heck, I tried it.

The theory goes that a 120 Vac 60 Hz battery charger puts out an imperfect DC voltage. It is actually a pulsating DC voltage that corresponds to

* 1800 rpm for an 8 cylinder,
* 2400 for a 6 cylinder
* 3600 for a 4 cylinder

Well it works! When I hooked it up to my tach's input (+ve to tach +ve and -ve to gnd), it showed exactly 1800 rpm. The technical theory goes like this. The battery charger doesn't put out a DC signal, it is a 120 Hz full rectified waveform. When a battery charger is connected to a battery, the battery acts like a huge capacitor and smoothes out the waveform (simple explanation). I used a frequency counter to verify that the battery charger was putting out 120 Hz.

Here's the math:

120 Hz = 120 cycles/sec

120 cycles/sec * 60 = 7200 cycles/min

There are 8 cylinders, so 7200 / 8 = 900 cycles/min

but it is a 4 stroke engine which only fires on 1 firing stroke (combustion)

It fires once every 2 cycles

So 900 cycles/min *( 4 strokes/1 firing stroke) * (1 firing stroke/2 cycles) = 1800 cycles/min or 1800 rpm

 

Brilliant. Will try that. We are 50 hertz here but that ok. Will work out the maths ...

 

The battery charger idea is brilliant. However, it only works on tachs that can accept a low voltage signal. I've used it to calibrate Smiths RVI current sensing tachometers too. The signal, depending on the battery charger may only be 1/2 the expected frequency depending on how sensitive the tachometer is and how lopsided the chopped output signal of the battery charger is. However, since this method won't work on tachs that require a high voltage, and if we are trying to find out if that type of tachometer is working, hooking up the charger won't help. I tried this yesterday on my VDO Porsche 911 tachometer (I need to buy the remaining parts of the car and I might own a Porsche someday) and no joy from the battery charger. So after reading Freeman's post, I put on my thinking cap this morning and found a way to make the Porsche tachometer respond with the charger. I needed to find an inductor that could be switched rapidly on and off. I found a spare car horn and made that the load on the battery charger, and put the sensing wire from the tach to the positive side of the charger. The experiment was rather loud, but the tachometer responded at the frequency of the horn. No good for calibration, but the horn did provide enough inductive kick to raise the sensing voltage sufficiently for the tachometer to respond, proving that it works. That method is also worth a try and should work equally well with a Veglia tachometer that requires the higher sensing voltage. More food for thought! Fred

 

I did try the half wave battery charger method but it didnt work....and as Fred says maybe the half wave isnt big enough...finally ADRIAN said maybe he could help with a new PCB (most kind as this is not his business) ....and then as a last shot I changed both transistors...still did not work....and by luck I saw the input resistor ( 2.2K on the pulse input PCB track) was slightly tarnished and then when I touched it, it fell to pieces, cracked through the middle....
got excited and replaced it ...connected 12v and Earth and M (Pulse input) direct to points.....fired her up ...NOTHING......frustrated ...watched the rugby.....

reconnected the M to the grey output of the Ignition system ..(Brown wire through to Tacho in the car) ......SUCCESS...... it now works for the first time in 20 years ....

end of (happy) story...

 

That's great news. It's always satisfying to repair something and keep it original if possible. Since the tach wouldn't trigger from the points directly after you repaired it, tells me that MSD has lost some tribal knowledge and is limiting the current through the points to the extent that the tach won't trigger. They should know, or should have known, that it's important to run a minimum amount of current through the points when they are closed to prevent them from crudding up with insulating deposits. Not all cars are susceptible to this. VW and Porsche applications are mostly immune, but many British and American cars absolutely need it. It would be interesting to measure the current through the breaker points when they are closed with your MSD unit, but my guess is that it's pretty small. Since most people trigger those units magnetically, only a few customers will experience problems. If you are lucky, the Dino distributor points are one of the systems that is immune. The problem 'back in the day', was thought to be a combination of oil fumes and ozone inside the distributor cap that created the grey insulating paste. As I mentioned before, about 400mA is needed to keep the points clean in all applications. Anyway, I'm happy you have your tach repaired, Fred

 

Fantastic!

Freeman

 

Just saw this thread, sounds like someone wired your tacho to coil minus somewhere in the past, that would have damaged the 2.2K inline resistor. Grey wire is the correct one for electronic Dino tach's, it supplies a 12V square wave.

 

Thanks everybody....been interesting..

 

Congratulations.

How did you crimp the chrome trim ring of the casing? Very tough to do without changing the reflective surfaces

 

I just gently worked it back on until it clicked into place ...I had it off a few times and back on without a problem....well almost....I have the needle on wrong this time and it reads 300 rpm high....but Im too lazy to take it all to bits again...maybe another month...